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Section 1
(50 cards)
How can you set the n-th bit
Shift n times to the left and OR
How can you create a duplicate of an array range?
Arrays.copyOfRange
What is a marker interface?
A Marker interface, has no method. Serializable, Clonable are marker interfaces.
what is the size of float?
32 bits
How do you make an object serializable?
The class must implement the java.io.Serializable interface
How do you clone an object?
MyClone a = (MyClone) c.clone() - TypeCast is nessesary. - handle CloneNotSupportedException
What is Double-Checked Locking?
To reduce the overhead of acquiring a lock by first testing the locking criterion without actually acquiring the lock.
What is ~0 bitwise?
0
How can you find a value fast in a sorted array?
Arrays.binarysearch works on all primitives and on objects using a comparator
If you overwrite clone(), which 3 rules must this method obey?
1) the new object should be new: memory address should differ 2) Both should be an object of the same class 3) Both should be in the same state: a.clone().equals(a) == true
How can you sort with different rules on the same array?
use a comparator:
How can you isolate the rightmost 0-bit?
invert number AND (number + 1)
How do you check if a number is even?
check if number AND 1 is 0
How can you sort an array using parallel tasks
Arrays.parallelSort usesmergesort and ForkJoin common pool to execute any parallel tasks. works with all primitive types and comparables
what algorithm is Arrays.sort?
An improved quicksort with average nlog(n) runtime
how can you sort a range of an array in-place?
Arrays.sort(int[] a, int fromIndex, int toIndex) works with all primitive types and comparables
What is the size of char?
16 bits
How can you transform a List into an array?
Collections.toArray
What is the range of short?
-32,768 to 32,767
What is the size of short?
16 bits
what is the size of double?
64 bits
How can you unset the nth bit
Shift n times to the left and invert, then AND
What is the range of int?
-2,1 billion to 2,1billion
What is the largest number binary representation in a two complement?
...
How can you turn off the rightmost 1Bit?
AND with (number - 1)
Java object serialization is performed using which classes?
Java object serialization (writing) is done with the ObjectOutputStream and deserialization (reading) is done with the ObjectInputStream.
How do you recognize overflow in two complement?
Adding 2 positive results in negative and vice versa
What is the smallest number binary representation in two complement?
...
How can you compare arrays?
Arrays.equals : two arrays are equal if they contain the same elements in the same order
How is the comparable interface implemented?
public interface Comparable<T>{ int compareTo(T o) { return 1; //if this > that return 0; //if this == that return -1; //if this < that } }
How can you transform an array into a List?
Arrays.asList
What is the size of byte?
8 bits
How can you turn on the rightmost 0-bit.
number OR (number + 1)
How can you create a duplicate of an array?
Arrays.copyOf copies until a given length
How can you toggle the nth Bit?
Shift n times to the left and XOR
How can you also declare -x bitwise?
~x + 1
What is the range of byte
-128 to 127
What is escape analysis?
escape analysis is a method for determining the dynamic scope of pointers - where in the program a pointer can be accessed.
Why do we need to implement the Clonable interface?
At runtime it would throw the CloneNotSupportedException if we don't implement the Cloneable interface. A class implements the Cloneable interface to indicate to the Object.clone() method that it is legal for that method to make a field-for-field copy of instances of that class.
What is serialVersionUID?
In addition to implementing the Serializable interface, a class intended for serialization should also contain a private static final long variable named serialVersionUID. The serialVersionUID variable is used by Java's object serialization API to determine if a deserialized object was serialized (written) with the same version of the class, as it is now attempting to deserialize it into. If you make changes to the class that affect serialization, you should also change its serialVersionUID value.
How can you return only 1 or 0 for the rightmost 1Bit value?
number & ( - number)
How can you right propagate the rightmost 1-bit?
number OR (number - 1), does not work for 0
What can you use Arrays.parallelPrefix for?
cumulate values in an array
How do you test if the n-th bit is set?
Shift n times to the right and then AND
How can Double-checked locking improve performance?
By limiting synchronization to the rare case of computing the field's value or constructing a new instance for the field to reference and by foregoing synchronization during the common case of retrieving an already-created instance or value.
what is the size of long?
64 bits
What is -1 in twos complement?
1111 1111
How can you assign a specific value to each element of an array or subarray?
public static void Arrays.fill
Why can Annotations replace marker interfaces?
Annotations can convey metadata about the class to its consumers without creating a separate type for it. Annotations let you pass information to classes that "consume" it.
What is the size of int?
32 bits
Section 2
(50 cards)
Which package contains ForkAndJoinPool?
java.util.concurrent
What does Functional Parallelism mean?
It is a third concurrency model. Functions can be seen as "agents" or "actors" that send messages to each other (call each other).
When is volatile Enough?
n case only one thread reads and writes the value of a volatile variable and other threads only read the variable, then the reading threads are guaranteed to see the latest value written to the volatile variable. Without making the variable volatile, this would not be guaranteed. The volatile keyword is guaranteed to work on 32 bit and 64 variables.
What are the advantages of implementing Runnable or extending Thread?
Implementing Runnable handing an instance of the implementation to a Thread instance is easy with Threadpools
Name some advantages of event driven models!
No shared state between workers
Explain advantages of channel vs actor model!
In the channel model, workers do not communicate directly with each other. Instead they publish their messages (events) on different channels. Other workers can then listen for messages on these channels without the sender knowing who is listening.
What is the use of Streams in Java 8?
parallel streams which can help you parallelize the iteration of large collections.
How can you prevent race conditions?
Make sure that the critical section is executed as an atomic instruction. So that when a thread is executing it, no other can execute it until the first thread has left the critical section.
What is the Holder Class Idiom?
Solution to Singleton with the initialize-on-demand, holder class idiom that implicitly incorporates lazy initialization by declaring a static variable within a static Holder inner class: final class Foo { // Lazy initialization private static class Holder { static Helper helper = new Helper(); } public static Helper getInstance() { return Holder.helper; } }
Attempting to synchronize on a null object will
throw a NullPointerException.
What is a critical section?
A section of code that is executed by multiple threads and where the sequence of execution for the threads makes a difference in the result
Name some Concurrency Models
Parallel Workers
What is the volatile Happens-Before Guarantee?
If Thread A writes to a volatile variable and Thread B subsequently reads the same volatile variable, then all variables visible to Thread A before writing the volatile variable, will also be visible to Thread B after it has read the volatile variable. Instructions before and after can be reordered, but the volatile read or write cannot be mixed with these instructions. Whatever instructions follow a read or write of a volatile variable are guaranteed to happen after the read or write.
What is non-blocking IO?
...
Explain the consequences of workers NOT sharing state with each other!
They can be implemented without having to think about all the concurrency problems that may arise from concurrent access to shared state. This makes it much easier to implement workers. You implement a worker as if it was the only thread performing that work - essentially a singlethreaded implementation.
In which situation would you not use volatile?
When we want to read-update-write as an atomic operation (unless we're prepared to "miss an update"); Accessing a volatile variable never holds a lock.
What exactly is parallelism?
Application splits its tasks up into smaller subtasks which can be processed in parallel, for instance on multiple CPUs at the exact same time.
Explain the channel model!
Workers do not communicate directly with each other. Instead they publish their messages (events) on different channels. Other workers can then listen for messages on these channels without the sender knowing who is listening.
volatile is used to indicate that
a variable's value will be modified by different threads.
Explain the actor model!
Each worker is called an actor. Actors can send messages directly to each other. Messages are sent and processed asynchronously. Actors can be used to implement one or more job processing assembly lines, as described earlier.
What is the private lock object idiom?
This idiom uses the intrinsic lock associated with the instance of a private final java.lang.Object declared within the class instead of the intrinsic lock of the object itself. This idiom requires the use of synchronized blocks within the class's methods rather than the use of synchronized methods. Lock contention between the class's methods is prevented.
...
an because accessing a volatile variable never holds a lock, it is not suitable for cases where we want to read-update-write as an atomic operation (unless we're prepared to "miss an update");
How is the intrinsic lock used?
Methods declared as synchronized and blocks that synchronize on the this reference both use the object as monitor (that is, its intrinsic lock)
What exactly is concurrency?
An application is making progress on more than one task at the same time (concurrently). It does not completely finish one task before it begins the next.
What is the "Visibility Problem"?
The problem with threads not seeing the latest value of a variable because it has not yet been written back to main memory by another thread, is called a "visibility" problem. The updates of one thread are not visible to other threads. Solution: By declaring the counter variable volatile all writes to the counter variable will be written back to main memory immediately. Also, all reads of the counter variable will be read directly from main memory.
How do you create a Thread?
1) Extend Thread (java.lang.Thread) 2) Implement Runnable (java.lang.Runnable)
What is the most common mistake when starting a thread?
To call the run() method of the Thread instead of start(). in that case, the run() method is executed by the thread that created the thread.
Parallel Workers Disadvantages
- The shared workers often need access to some kind of shared data, either in memory or in a shared database. That creates complexity. Threads need to avoid race conditions, deadlock and many other shared state concurrency problems. - Part of the parallelization is lost when threads are waiting for each other when accessing the shared data structures. Many concurrent data structures are blocking, leading to contention and eventually serialization. - Shared state can be modified by other threads in the system. Therefore workers must re-read the state every time it needs it, to make sure it is working on the latest copy. This is true no matter whether the shared state is kept in memory or in an external database. A worker that does not keep state internally (but re-reads it every time it is needed) is called stateless . Re-reading data every time you need it can get slow. Especially if the state is stored in an external database.
Explain how Functional Parallelism avoids race conditions?
All parameters passed to the function are copied, so no entity outside the receiving function can manipulate the data. This copying is essential to avoiding race conditions on the shared data. This makes the function execution similar to an atomic operation. Each function call can be executed independently of any other function call.
What is the volatile Visibility Guarantee
The Java volatile keyword guarantees visibility of changes to variables across threads.
two ways to synchronize access to shared mutable variables:
method synchronization and block synchronization
Which concurrency model is used by reactive or event driven systems
Each worker only performs a part of the full job. When that part is finished the worker forwards the job to the next worker. Each worker is running in its own thread, and shares no state with other workers. This is also sometimes referred to as a shared nothing concurrency model. They are usually design to use non-blocking IO.
What is same-threading?
A concurrency model where a single-threaded systems are scaled out to N single-threaded systems. The result is N single-threaded systems running in parallel.
Declaring a volatile Java variable means:
1) The value of this variable will never be cached thread-locally: all reads and writes will go straight to "main memory"; 2) Access to the variable acts as though it is enclosed in a synchronized block, synchronized on itself.
Explain the consequences of workers having state.
If workers can be stateful, they have to be sure there are no other threads modify their data. They can keep their data in memory, only writing changes back the eventual external systems. A stateful worker can therefore often be faster than a stateless worker.
Are you holding a lock when you access a volatile variable?
access to a volatile variable never has the potential to block: we're only ever doing a simple read or write, so unlike a synchronized block we will never hold on to any lock;
Can you think of a case where you may have to implement Runnable as well as subclass Thread?
For instance, if creating a subclass of Thread that can execute more than one Runnable. This is typically the case when implementing a thread pool.
Describe the idea behind event driven concurrency!
The system's workers react to events occurring in the system, either received from the outside world or emitted by other workers.
Are you holding a lock when you access a synchronized method?
access to a volatile variable never has the potential to block: we're only ever doing a simple read or write, so unlike a synchronized block we will never hold on to any lock;
Parallel Workers Advantages
it is easy to understand. To increase the parallelization of the application you just add more workers.
May a volatile variable that is an object reference be null?
Yes! (because you're effectively synchronizing on the reference, not the actual object).
What is the difference between static nested and inner classes?
Nested classes that are declared static are called static nested classes. Non-static nested classes are called inner classes.
What is the ForkAndJoinPool?
...
What is a race condition?
When the result of multiple threads executing a critical section may differ depending on the sequence in which the threads execute.
What is the parallel worker concurrency model ?
A delegator distributes the incoming jobs to different workers. Each worker completes the full job. The workers work in parallel, running in different threads, and possibly on different CPUs.
Give an example of a race condition when using a volatile variable.
As soon as a thread needs to first read the value of a volatile variable, and based on that value generate a new value for the shared volatile variable, a volatile variable is no longer enough to guarantee correct visibility. The short time gap in between the reading of the volatile variable and the writing of its new value, creates an race condition where multiple threads might read the same value of the volatile variable, generate a new value for the variable, and when writing the value back to main memory - overwrite each other's values.
May a primitive variable may be declared volatile or synchronized?
Volatile, but not synchronized.
How can Functional Parallelism be performant?
When each function call can be executed independently, each function call can be executed on separate CPUs. That means, that an algorithm implemented functionally can be executed in parallel, on multiple CPUs.
How can you avoid race conditions?
By proper thread synchronization in critical sections.
What is Context Switching Overhead
When a CPU switches from executing one thread to executing another, the CPU needs to save the local data, program pointer etc. of the current thread, and load the local data, program pointer etc. of the next thread to execute. This switch is called a "context switch". The CPU switches from executing in the context of one thread to executing in the context of another. Context switching isn't cheap. You don't want to switch between threads more than necessary.
Section 3
(50 cards)
...
The Math.floor() function rounds a floating point value down to the nearest integer value.
...
The object versions of the primitive data types are immutable.
Race conditions occur only if ...
multiple threads are accessing the same resource, and one or more of the threads write to the resource. If multiple threads read the same resource race conditions do not occur.
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Java has a way to force all numbers in a calculation to be floating point variables. You suffix the numbers with either a capital F or D. Here is an example: 4F or 4D
Describe how write operation is done on hardware.
When the CPU needs to write the result of a computation back to main memory it will flush the value from its internal register to the cache memory, and at some point flush the value back to main memory.
Describe the Java memory model.
The Java memory model used internally in the JVM divides memory between thread stacks and the heap.
What is the role of the thread stack?
Each thread running in the Java virtual machine has its own thread stack. A thread can only access it's own thread stack.
Are object member variables thread safe?
No, object member variables (fields) are stored on the heap along with the object. Therefore, if two threads call a method on the same object instance and this method updates object member variables, the method is not thread safe.
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If you want to be sure that two String variables point to separate String objects, use the new operator like this: String myString1 = new String("Hello World");
shared objects can be made thread safe by ..
making sure that these objects are never updated by making them immutable.
Describe the problem of Visibility of Shared Objects
If two or more threads are sharing an object, without the proper use of either volatile declarations or synchronization, updates to the shared object made by one thread may not be visible to other threads. Solved by volatile
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The Math.round() method rounds a float or double to the nearest integer using normal math round rules .
When multithreading, local primitive variables are stored ..
In each thread's own stack. That means that local variables are never shared between threads. That also means that all local primitive variables are thread safe.
Which kinds of variables are fully stored on the thread stack?
All local variables of primitive types ( boolean, byte, short, char, int, long, float, double) are fully stored on the thread stack and are thus not visible to other threads.
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The Math.min() method returns the smallest of two values passed to it as parameter.
create an array using literals
int[] ints2 = new int[]{ 1,2,3,4,5,6,7,8,9,10 };
Where are heap and thread stack located in hardware?
On the hardware, both the thread stack and the heap are located in main memory. Parts of the thread stacks and heap may sometimes be present in CPU caches and in internal CPU registers.
Static class variables are also stored on the..
heap along with the class definition.
Which constructs can also be achieve thread synchronization?
Locks over critical sections or atomic variables like java.util.concurrent.atomic.AtomicInteger.
What are cache lines?
The values stored in the cache memory is typically flushed back to main memory when the CPU needs to store something else in the cache memory. The CPU cache can have data written to part of its memory at a time, and flush part of its memory at a time. It does not have to read / write the full cache each time it is updated. Typically the cache is updated in smaller memory blocks called "cache lines". One or more cache lines may be read into the cache memory, and one or mor cache lines may be flushed back to main memory again.
How can you create an immutable object?
Set a value in the constructor and expose only a getter.
Can you think of a guarantee to know if a given object is thread safe?
If an object created locally never escapes the method it was created in, it is thread safe.
...
More precisely, objects representing Java String literals are obtained from a constant String pool which the Java virtual machine keeps internally. That means, that even classes from different projects compiled separately, but which are used in the same application may share constant String objects. The sharing happens at runtime. It is not a compile time feature.
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The Math.max() method returns the largest of two values passed to it as parameter.
What kind of information is held on the thread stack?
- all methods the thread has called to reach the current point of execution. - all local variables for all methods on the call stack
How can a thread share a variable with another?
One thread may pass a copy of a primitive variable to another thread, but it cannot share the primitive local variable itself.
...
The Math.abs() function returns the absolute value of the parameter passed to it.
Name integer types that will truncate division.
Java integer types (byte, short, int and long)
What kind of information is contained on the heap?
- all objects created in your Java application, regardless of what thread created the object. - This includes the object versions of the primitive types (e.g. Byte, Integer, Long etc.). - It does not matter if an object was created and assigned to a local variable, or created as a member variable of another object, the object is still stored on the heap.
How can you perform operations on an immutable object?
Operate and create a new immutable object.
create a new array of strings
String[] stringArray = new String[10];
Describe the Java memory model
...
You can convert an Java array of primitive types to a String using the Arrays.toString() method. H
An object's member variables are stored on the ..
heap along with the object itself. That is true both when the member variable is of a primitive type, and if it is a reference to an object.
When objects and variables can be stored in various different memory areas in the computer, certain problems may occur. The two main problems are:
Visibility of thread updates (writes) to shared variables. Race conditions when reading, checking and writing shared variables.
...
Java's auto boxing features enables you to use primitive data types where the object version of that data type was normally required, and vice versa. There is one pitfall to keep in mind though. A variable of type object (a reference to an object) can point to null, meaning it points to nothing - no object. If you try to convert null to a primitive value you will get a NullPointerException
Describe how a read operation is done on hardware.
Typically, when a CPU needs to access main memory it will read part of main memory into its CPU cache. It may even read part of the cache into its internal registers and then perform operations on it.
how Do you calculate the reminder of 100 / 9 ?
int remainder = 100 % 9;
Where is the Math class located?
java.lang.Math
...
It is possible to have many different variables reference the same object. This is not possible with primitive data types.
...
The Math.floorDiv() method divides one integer (int or long) by another, and rounds the result down to the nearest integer value. What is the difference to regular int division?
...
The Math.random() method returns a random floating point number between 0 and 1.
Objects on the heap can be accessed by..
all threads that have a reference to the object. When a thread has access to an object, it can also get access to that object's member variables.
...
You can access the length of an array via its length field
What is the Thread Control Escape Rule
If a resource is created, used and disposed within the control of the same thread, and never escapes the control of this thread, the use of that resource is thread safe.
...
The Math.ceil() function rounds a floating point value up to the nearest integer value.
Would a local variable be stored on heap or stack?
- primitive type, it is totally kept on the thread stack - reference to an object. In that case the reference (the local variable) is stored on the thread stack, but the object itself if stored on the heap.
What does thread safe mean?
Code that is safe to call by multiple threads simultaneously
Describe how race conditions can occur.
If two or more threads share an object, and more than one thread updates variables in that shared object, race conditions may occur. Solved by Java synchronized block.
...
Binary search If more than one element exists in the array with the searched value, there is no guarantee about which element will be found. If no element is found with the given value, a negative number will be returned. The negative number will be the index at which the searched element would be inserted, and then minus one.
Section 4
(50 cards)
...
if a constructor is declared protected then only classes in the same package, or subclasses of that class can call that constructor.
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Fields and methods with default (package) access modifiers can be accessed by subclasses only if the subclass is located in the same package as the superclass. Private fields and methods of the superclass can never be referenced directly by subclasses.
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The substring() method takes two parameters.
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If you override a method in a subclass, and the method is all of a sudden removed or renamed or have its signature changed in the superclass, the method in the subclass no longer overrides the method in the superclass.
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You can also get the byte representation of the String method using the getBytes() method. Use the specific charset to do that.
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The Java access modifiers private and protected cannot be assigned to a class.
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fields cannot be overridden in a subclass. If you define a field in a subclass with the same name as a field in the superclass, the field in the subclass will hide (shadow) the field in the superclass.
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The trim() method does not modify the String instance. Instead it returns a new Java String object which is equal to the String object it was created from, but with the white space in the beginning and end of the String removed.
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The Java inheritance mechanism does not include constructors. In other words, constructors of a superclass are not inherited by subclasses. Subclasses can still call the constructors in the superclass using the super() contruct. In fact, a subclass constructor is required to call one of the constructors in the superclass as the very first action inside the constructor body.
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If, however, the subclass calls up into a method in the superclass, and that method accesses the field with the same name as in the subclass, it is the field in the superclass that is accessed.
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In Java you cannot override private methods from a superclass. If the superclass calls a private method internally from some other method, it will continue to call that method from the superclass, even if you create a private method in the subclass with the same signature.
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Now, for every iteration in this loop a new StringBuilder is created. Additionally, a String object is created by the toString() method. This results in a small object instantiation overhead per iteration: One StringBuilder object and one String object. This by itself is not the real performance killer though. But something else related to the creation of these objects is. Every time the new StringBuilder(result) code is executed, the StringBuilder constructor copies all characters from the result String into the StringBuilder. The more iterations the loop has, the bigger the result String grows. The bigger the result String grows, the longer it takes to copy the characters from it into a new StringBuilder, and again copy the characters from the StringBuilder into the temporary String created by the toString() method. In other words, the more iterations the slower each iteration becomes.
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A Java field can have be given an initial value. This value is assigned to the field when the field is created in the JVM. Static fields are created when the class is loaded. A class is loaded the first time it is referenced in your program. Non-static fields are created when the object owning them are created.
What is the result: Car car = new Car(); boolean isVehicle = car instanceof Vehicle;
...
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The default access modifier means that code inside the class itself as well as code inside classes in the same package as this class, can access the class, field, constructor or method which the default access modifier is assigned to. Therefore, the default access modifier is also sometimes referred to as the package access modifier.
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The Java String class contains a method called trim() which can trim a string object. By trim is meant to remove white space characters at the beginning and end of the string. White space characters include space, tab and new lines.
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Subclasses cannot access methods and member variables (fields) in the superclass, if they these methods and fields are marked with the default access modifier, unless the subclass is located in the same package as the superclass.
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A Java method parameter can be declared final, just like a variable. The value of a final parameter cannot be changed. That is, if the parameter is a reference to an object, the reference cannot be changed, but values inside the object can still be changed.
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If you override a method in a subclass, but still need to call the method defined in the superclass, you can do so using the super reference
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The String split() method exists in a version that takes a limit as a second parameter. Here is a Java String split() example using the limit parameter: String source = "A man drove with a car."; int limit = 2; String[] occurrences = source.split("a", limit); The limit parameter sets the maximum number of elements that can be in the returned array.
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It is possible to get a character at a certain index in a String using the charAt() method.
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The protected access modifier provides the same access as the default access modifier, with the addition that subclasses can access protected methods and member variables (fields) of the superclass. This is true even if the subclass is not located in the same package as the superclass.
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A private constructor can still get called from other constructors, or from static methods in the same class.
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You can obtain the length of a String using the length() method.
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When you call a constructor from inside another constructor, you use the this keyword to refer to the constructor
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When you create a subclass of some class, the methods in the subclass cannot have less accessible access modifiers assigned to them than they had in the superclass.
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A class that extends another class does not inherit its constructors. However, the subclass must call a constructor in the superclass inside one of the subclass constructors!
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Classes cannot be marked with the private access modifier. Therefore the private access modifier is not allowed for classes.
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When concatenating Strings you have to watch out for possible performance problems. Concatenating two Strings in Java will be translated by the Java compiler to something like this: String one = "Hello"; String two = " World"; String three = new StringBuilder(one).append(two).toString(); As you can see, a new StringBuilder is created, passing along the first String to its constructor, and the second String to its append() method, before finally calling the toString() method. This code actually creates two objects: A StringBuilder instance and a new String instance returned from the toString() method.
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loop over each string in the array: for(String aString : strings) { // }
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The Java String class contains a split() method which can be used to split a String into an array of String objects. The source String has been split on the a characters. The Strings returned do not contain the a characters. The a characters are considered delimiters to split the String by, and the delimiters are not returned in the resulting String array.
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Java access modifier assigned to a Java class takes precedence over any access modifiers assigned to fields, constructors and methods of that class. If the class is marked with the default access modifier, then no other class outside the same Java package can access that class, including its constructors, fields and methods.
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method named replace() which can replace characters in a String. The replace() method does not actually replace characters in the existing String. Rather, it returns a new String instance which is equal to the String instance it was created from, but with the given characters replaced.
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If a method or variable is marked as private (has the private access modifier assigned to it), then only code inside the same class can access the variable, or call the method. Code inside subclasses cannot access the variable or method, nor can code from any external class.
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Java classes where the subclass constructors did not seem to call the constructors in the superclass. Maybe the superclass did not even have a constructor. However, the subclass constructors have still called superclass constructors in those case.
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Using the keyword super refers to the superclass of the class using the super keyword. When super keyword is followed by parentheses like it is here, it refers to a constructor in the superclass.
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The Java String class also has a lastIndexOf() method which finds the last occurrence of a substring.
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The parameters mean "from - including, to - excluding".
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If the substring is not found within the string, the indexOf() method returns -1;
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The default Java access modifier is declared by not writing any access modifier at all.
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The indexOf() method returns the index of where the first character in the first matching substring is found.
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compareTo() method may not work correctly for Strings in different languages than English. To sort Strings correctly in a specific language, use a Collator.
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Nested classes which are declared private are not inherited. Nested classes with the default (package) access modifier are only accessible to subclasses if the subclass is located in the same package as the superclass. Nested classes with the protected or public access modifier are always inherited by subclasses.
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The compareTo() method compares the String to another String and returns an int telling whether this String is smaller, equal to or larger than the other String. If the String is earlier in sorting order than the other String, compareTo() returns a negative number. If the String is equal in sorting order to the other String, compareTo() returns 0. If the String is after the other String in sorting order, the compareTo() metod returns a positive number.
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In fact, since the constructor is now empty, we could leave it out and the Java compiler would insert it, and insert an implicit call to the no-arg constructor in the superclass.
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There is a version of the indexOf() method that takes an index from which the search is to start. That way you can search through a string to find more than one occurrence of a substring.
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The Java String class contains a set of overloaded static methods named valueOf() which can be used to convert a number to a String.
How can you check whether a tree is a valid binary tree or not?
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A final class cannot be extended. In other words, you cannot inherit from a final class in Java.
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The private access modifier means that only code inside the class itself can access this Java field. The package access modifier means that only code inside the class itself, or other classes in the same package, can access the field. You don't actually write the package modifier. By leaving out any access modifier, the access modifier defaults to package scope. The protected access modifier is like the package modifier, except subclasses of the class can also access the field, even if the subclass is not located in the same package. The public access modifier means that the field can be accessed by all classes in your application.
Section 5
(50 cards)
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The Template Method design pattern provides a partial implementation of some process, which subclasses can complete when extending the Template Method base class.
which package is Arrays ?
java.util.Arrays
Determine if a singly linked list contains a circle.
Use a fast and a slow iterator and check if they meet.
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The different Java nested class types are: Static nested classes Non-static nested classes Local classes Anonymous classes
Describe the logic that needs to be followed when implementing Comparator.
int compare(Object o1,Object o2) This method compares o1 and o2 objects. and returns a integer.Its value has following meaning. 1. positive - o1 is greater than o2 2. zero - o1 equals to o2 3. negative - o1 is less than o1
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You can check the size of a collection using the size() method. By "size" is meant the number of elements in the collection.
When is a function pure?
The outcome of a pure function depends only on the input parameters and they do not have any side effects.
Why would a pure function be better testable?
To test a pure function, it is sufficient to create the input parameter, run the "function under test" and compare the outcome. No mockups, no dependency injection, no complex setup, and no other techniques are necessary that take the fun out of testing.
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Regardless of what Collection subtype you are using there are a few standard methods to add and remove elements from a Collection. Adding and removing single elements is done like this:
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remove() removes the given element and returns true if the removed element was present in the Collection, and was removed. If the element was not present, the remove() method returns false.
Describe the purpose of the Comparator interface:
Class whose objects to be sorted do not need to implement this interface. Some third class can implement this interface to sort.
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If a Java inner class declares fields or methods with the same names as field or methods in its enclosing class, the inner fields or methods are said to shadow over the outer fields or methods.
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Local classes can only be accessed from inside the method or scope block in which they are defined. Local classes can access members (fields and methods) of its enclosing class just like regular inner classes. Local classes can also access local variables inside the same method or scope block, provided these variables are declared final.
How to use Comparator on a collection?
Collections.sort(List, Comparator) Here objects will be sorted on the basis of Compare method in Comparator
When implementing equals, how do you compare array fields
: use Arrays.equals
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The enum constructor must be either private or package scope (default). You cannot use public or protected constructors for a Java enum.
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You can obtain an array of all the possible values of a Java enum type by calling its static values() method.
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In Java nested classes are considered members of their enclosing class.
What is the implication of implementing comparable for collections and arrays alike?
If any class implements the comparable interface then collection of that object can be sorted automatically using Collection.sort() or Arrays.sort()
When implementing equals, how do you compare primitive fields other than float or double
use ==
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Matching a Java lambda expression against a functional interface is divided into these steps: Does the interface have only one method? Does the parameters of the lambda expression match the parameters of the single method? Does the return type of the lambda expression match the return type of the single method? If the answer is yes to these three questions, then the given lambda expression is matched successfully against the interface.
Write a method that compares Country objects.
@Override public int compareTo(Object arg0) { Country country=(Country) arg0; return (this.countryId < country.countryId ) ? -1: (this.countryId > country.countryId ) ? 1:0 ; }
When implementing equals, how do you compare objects?
object fields, including collections : use equals
Here's a string with numbers from 1-250 in random order, but it's missing one number. How will you find the missed number?
I'd XOR them.
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java.util.List interface is a subtype of the java.util.Collection interface. It represents an ordered list of objects, meaning you can access the elements of a List in a specific order, and by an index too. You can also add the same element more than once to a List.
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addAll() adds all elements found in the Collection passed as parameter to the method. The Collection object itself is not added. Only its elements.
Delete a node in a singly linked list in O(1) time.
Delete the next node instead: Copy the next node value into the current node, point next to next.next.
When implementing equals, how do you compare enums?
type-safe enumerations : use either equals or == (they amount to the same thing, in this case)
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An interface default method can contain a default implementation of that method. Classes that implement the interface but which contain no implementation for the default interface will then automatically get the default method implementation. You mark a method in an interface as a default method using the default keyword.
Take two posibly cyclic singly linked lists and determine if there is an element that belongs to both.
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When implementing equals, how do you compare possibly-null objects?
use both == and equals
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A class that implements the Iterable can be used with the new for-loop. Here is such an example: List list = new ArrayList(); for(Object o : list){ //do something o; }
Take two cycle free singly linked lists and determine if there is an element that belongs to both.
The lists would have the same tail element.
When implementing equals, how do you compare doubles
convert to long using Double.doubleToLongBits, then use ==
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The purpose of a nested class is to clearly group the nested class with its surrounding class, signaling that these two classes are to be used together. Or perhaps that the nested class is only to be used from inside its enclosing (owning) class.
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In Java a static nested class is essentially a normal class that has just been nested inside another class. Being static, a static nested class can only access instance variables of the enclosing class via a reference to an instance of the enclosing class.
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A Java lambda expression is thus a function which can be created without belonging to any class. A lambda expression can be passed around as if it was an object and executed on demand.
When implementing equals, how do you compare float
convert to int using Float.floatToIntBits, then use ==
Name an advantage of an object's immutability for collections
if an object is immutable, then hashCode is a candidate for caching and lazy initialization
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Let's assume our application, a todo-list, is already running for a while and the user presses a button to create a new entry in the todo-list. This will result in a button-clicked event in the DOM, which is captured by the DOM-Driver and forwarded to one of our ActionCreators. The ActionCreator takes the DOM-event and maps it to an action. Actions are an implementation of the Command Pattern, i.e. they describe what should be done, but do not modify anything themselves. In our example, we create an AddToDoItemAction and pass it to the Updater. The Updater contains the application logic. It keeps a reference to the current state of the application. Every time it receives an action from one of the ActionCreators, it generates the new state. In our example, if the current state contains three todo-items and we receive the AddToDoItemAction, the Updater will create a new state that contains the existing todo-items plus a new one. The state is passed to the View()-Function, which creates the so-called Virtual DOM. As the name suggests, the Virtual DOM is not the real DOM, but it is a data-structure that describes how the DOM should look like. The code snippet above shows an example of a Virtual DOM for a simple <div>. A later article will explain the Virtual DOM and its advantages in detail. The Virtual DOM is passed to the DOM-Driver which will update the DOM and wait for the next user input. With this, the cycle ends.
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The Template Method design pattern provides a partial implementation of some process, which subclasses can complete when extending the Template Method base class.
Describe the contract between hashcode and equals!
hashCode and equals are closely related : if you override equals, you must override hashCode. hashCode must generate equal values for equal objects. equals and hashCode must depend on the same set of "significant" fields.
How do you use Comparable on a collection?
Collections.sort(List) Here objects will be sorted on the basis of CompareTo method
Describe the logic that needs to be followed when implementing Comparable
int compareTo(Object o1) This method compares this object with o1 object and returns a integer. Its value has following meaning 1. positive - this object is greater than o1 2. zero - this object equals to o1 3. negative - this object is less than o1
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Java annotations are typically used for the following purposes: Compiler instructions Build-time instructions Runtime instructions
How would you implement the natural ordering of an entity?
Comparable interface: Class whose objects to be sorted must implement this interface
What is the implication for TreeMap or TreeSet
Objects which implement Comparable in java can be used as keys without implementing any other interface.
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add() adds the given element to the collection, and returns true if the Collection changed as a result of calling the add() method.
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Notice how you put new after the reference to the outer class in order to create an instance of the inner class. Non-static nested classes (inner classes) have access to the fields of the enclosing class, even if they are declared private.
Describe the contract that needs to be followed when implementing hashcode.
1) if a class overrides equals, it must override hashCode 2) equals and hashCode must use the same set of fields 3) if two objects are equal, then their hashCode values must be equal as well 4) if the object is immutable, then hashCode is a candidate for caching and lazy initialization
Section 6
(39 cards)