Java Interview Questions

Java Interview Questions

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It’s a no-brainer that Java is one of the leading programming options for bagging a lucrative job. After all, the class-based, general-purpose, object-oriented, programming language, is one of the most widely used programming languages in the world.

Java Interview Questions

With a plethora of great features, the programming language is preferred not only by the seasoned experts but also pursued by those new to the programming world. So, here are important java interview questions that will help you bag a Java job, or, at the very least, enhance your learning.

Question: What is Java?

Answer: Java is an object-oriented, high-level, general-purpose programming language originally designed by James Gosling and further developed by the Oracle Corporation. It is among one of the most popular programming languages in the world.

To know more about what is java, Click here and know all the details of Java, Features, and Component.

Question: What do you mean by Constructor?

Answer: A constructor is a method that has the same name as that of the class to which it belongs. As soon as a new object is created, a constructor corresponding to the class gets invoked.

Although the user can explicitly create a constructor, it is created on its own as soon as a class is created. This is known as the default constructor. Constructors can be overloaded.

Note: – If an explicitly-created constructor has a parameter then it is necessary to create another constructor without a parameter.

Question: What are the features of Java?

Answer: Following are the various features of the Java programming language:

  • High Performance Using a JIT (Just-In-Time) compiler allows high performance in Java. The JIT compiler converts the Java bytecode into machine language code, which then gets executed by the JVM
  • Multi-threading A thread is a flow of execution. The JVM creates a thread which is called the main thread. Java allows the creation of several threads by means of either extending the thread class or implementing the Runnable interface
  • OOPS Concepts Java follows various OOPS concepts, namely abstraction, encapsulation, inheritance, object-oriented, and polymorphism
  • Platform Independency Java makes use of the Java Virtual Machine or JVM which allows a single Java program to operate on multiple platforms without any modifications

Question: How does Java enable high performance?

Answer: In Just-in-Time compilation, the required code is executed at run time. Typically, it involves translating bytecode into machine code and then executing it directly. For enabling high performance, Java can make use of the Just-In-Time compilation.

The JIT compiler is enabled by default in Java and gets activated as soon as a method is called. It then compiles the bytecode of the Java method into native machine code. Thereafter, the JVM calls the compiled code directly instead of interpreting it. This grants a performance boost.

Question: What do you understand by Java IDEs?

Answer: A Java IDE is a software that allows Java developers to easily write as well as debug Java programs. It is basically a collection of various programming tools, accessible via a single interface, and several helpful features, such as code completion and syntax highlighting.

Codenvy, Eclipse, and NetBeans are some of the most popular Java IDEs.

Question: Please explain Local variables and Instance variables in Java.

Answer: Variables that are only accessible to the method or code block in which they are declared are known as local variables. Instance variables, on the other hand, are accessible to all methods in a class.

While local variables are declared inside a method or a code block, instance variables are declared inside a class but outside a method.

Even when not assigned, instance variables have a value that can be null, 0, 0.0, or false. This isn’t the case with local variables that need to be assigned a value, where failing to assign a value will yield an error.

Local variables are automatically created when a method is called and destroyed as soon as the method exits. For creating instance variables, the new keyword must be used.

Question: What is an Object?

Answer: An instance of a Java class is known as an object. Two important properties of a Java object are behavior and state. An object is created as soon as the JVM comes across the new keyword.

Question: Could you explain the Oops concepts?

Answer: Following are the various OOPS Concepts:

  • Abstraction Representing essential features without the need to give out background details. The technique is used for creating a new suitable data type for some specific application
  • Aggregation– All objects have their separate lifecycle but ownership is present. No child object can belong to some other object except for the parent object
  • Association– The relationship between two objects, where each object has their separate lifecycle. There is no ownership
  • Class A group of similar entities
  • Composition– Also called the death relationship, it is a specialized form of aggregation. Child objects don’t have a lifecycle. As such, they automatically get deleted if the associated parent object is deleted
  • Encapsulation Refers to the wrapping up of data and code into a single entity. Allows the variables of a class to be only accessible by the parent class and no other classes
  • Inheritance When an object acquires the properties of some other object, it is called inheritance. It results in the formation of a parent-child relationship amongst classes involved. Offers a robust and natural mechanism of organizing and structuring software
  • Object Denotes an instance of a class. Any class can have multiple instances. An object contains the data as well as the method that will operate on the data
  • Polymorphism– Refers to the ability of a method, object, or variable to assume several forms

Question: Please explain Method Overriding in Java?

Answer: Method Overriding in Java allows a subclass to offer a specific implementation of a method that has already provided by its parent, or super, class. Method overriding happens if the subclass method and the Superclass method have:

  • The same name
  • The same argument
  • The same return type

Question: What do you mean by Overloading?

Answer: Overloading is the phenomenon when two or more different methods (method overloading) or operators (operator overloading) has the same representation.

For example, the + operator adds two integer values but concatenates two strings. Similarly, an overloaded function called Add can be used for two purposes

  1. To add two integers
  2. To concatenate two strings

Unlike method overriding, method overloading requires two overloaded methods to have the same name but different arguments. The overloaded functions may or may not have different return types.

Question: Please explain the difference between String, String Builder, and String Buffer.

Answer: String variables are stored in a constant string pool. With the change in the string reference, it becomes impossible to delete the old value. For example, if a string has stored a value “Old” then adding the new value “New” will not delete the old value. It will still be there, however, in a dormant state.

In a String Buffer, values are stored in a stack. With the change in the string reference, the new value replaces the older value.

The String Buffer is synchronized (and therefore, thread-safe) and offers slower performance than the String Builder, which is also a String Buffer but is not synchronized. Hence, performance is fast in String Builder than the String Buffer.

Question: What is Set in Java? Also, explain its types in a Java Collections.

Answer: In Java, a Set is a collection of unique objects. It uses the equals() method to determine whether two objects are the same or not. Various types of Set in Java Collections are:

  1. Hash Set An unordered and unsorted set that uses the hash code of the object for adding values. Used when the order of the collection isn’t important
  2. Linked Hash Set This is an ordered version of the hash set that maintains a doubly-linked list of all the elements. Used when iteration order is mandatory. Insertion order is the same as that of the manner in which elements are added to the Set.
  3. Tree SetOne of the two sorted collections in Java, it uses Read-Black tree structure and ensures that the elements are present in the ascending order.

Question: Please draw a comparison between notify() and notifyAll() methods.

Answer: The notify() method is used for sending a signal to wake up a single thread in the waiting pool. Contrarily, the notifyAll() method is used for sending a signal to wake up all threads in a waiting pool.

Question: When is the Runnable interface preferred over thread class and vice-versa?

Answer: In Java, it is possible to extend only one class. Hence, the thread class is only extended when no other class needs to be extended. If it is required for a class to extend some other class than the thread class, then we need to use the Runnable interface.

Question: Please explain the various types of garbage collectors in Java?

Answer: The Java programming language has four types of garbage collectors:

  1. Serial Garbage Collector Using only a single thread for garbage collection, the serial garbage collector works by holding all the application threads. It is designed especially for single-threaded environments. Because serial garbage collector freezes all application threads while performing garbage collection, it is most suitable for command-line programs only. For using the serial garbage collector, one needs to turn on the -XX:+UseSerialGC JVM argument.
  2. Parallel Garbage Collector Also known as the throughput collector, the parallel garbage collector is the default garbage collector of the JVM. It uses multiple threads for garbage collection and like serial garbage collector freezes all application threads during garbage collection.
  3. CMS Garbage Collector Short for Concurrent Mark Sweep, CMS garbage collector uses multiple threads for scanning the heap memory for marking instances for eviction, followed by sweeping the marked instances. There are only two scenarios when the CMS garbage collector holds all the application threads:
    1. When marking the referenced objects in the tenured generation space
    2. If there is some change in the heap memory while performing the garbage collection CMS garbage collector ensures better application throughput over parallel garbage collector by using more CPU. For using the CMS garbage collector, the XX:+USeParNewGC JVM argument needs to be turned on.
  4. G1 Garbage Collector Used for large heap memory areas, G1 garbage collector works by separating the heap memory into multiple regions and then executing garbage collection in them in parallel. Unlike the CMS garbage collector that compacts the memory on STW (Stop The World) situations, G1 garbage collector compacts the free heap space right after reclaiming the memory. Also, the G1 garbage collector prioritizes the region with the most garbage. Turning on the –XX:+UseG1GC JVM argument is required for using the G1 garbage collector.

Question: How will you differentiate HashMap from HashTable?

Answer: HashMap in Java is a Map-based collection class, used for storing key & value pairs. It is denoted as:

HashMap<Key, Value> or HashMap<K, V>

HashTable is an array of a list, where each list is called a bucket. Values contained in a HashTable are unique and depend on the key.

Methods are not synchronized in HashMap while key methods are synchronized in HashTable. However, HashMap doesn’t have thread safety while HashTable has the same. For iterating values, HashMap uses iterator and HashTable uses enumerator.

HashTable doesn’t allow anything that is null while HashMap allows one null key and several null values. In terms of performance, HashTable is slow. Comparatively, HashMap is faster.

Question: What do you mean by Collections in Java? What are the constituents of Collections in Java?

Answer: A group of objects in Java is known as collections. Java.util package contains, along with date and time facilities, internationalization, legacy collection classes, etc., the various classes and interfaces for collecting.

Alternatively, collections can be considered as a framework designed for storing the objects and manipulating the design in which the objects are stored. You can use collections to perform the following operations on objects:

  • Deletion
  • Insertion
  • Manipulation
  • Searching
  • Sorting

Following are the various constituents of the collections framework:

  • Classes – Array List, Linked List, Lists, and Vector
  • Interfaces – Collection, List, Map, Queue, Set, Sorted Map, and Sorted Set
  • Maps – HashMap, HashTable, LinkedHashMap, and TreeMap
  • Queues – Priority Queue
  • Sets – Hash Set, Linked Hash Set, and Tree Set

Question: Please explain Map and their types in Java.

Answer: A Java Map is an object that maps keys to values. It can’t contain duplicate keys and each key can map to only one value. In order to determine whether two keys are the same or distinct, Map makes use of the equals() method. There are 4 types of Map in Java, described as follows:

  • HashMap – It is an unordered and unsorted map and hence, is a good choice when there is no emphasis on the order. A HashMap allows one null key and multiple null values and doesn’t maintain any insertion order.
  • HashTable – Doesn’t allow anything null and has methods that are synchronized. As it allows for thread safety, the performance is slow.
  • LinkedHashMap – Slower than a HashMap but maintains insertion order and has a faster iteration.
  • TreeMap – A sorted Map providing support for constructing a sort order using a constructor.

Question: What do you mean by Priority Queue in Java?

Answer: Priority queue, like a regular queue, is an abstract data type with the exception of having a priority associated with each element contained by it. The element with the high priority is served before the element with low priority in a priority queue.

Elements in a priority queue are ordered either according to the comparator or naturally. The order of the elements in a priority queue represents their relative priority.

Question: How is an Abstract class different from an Interface?

Answer: There are several differences between an Abstract class and an Interface in Java, summed up as follows:

  • Constituents – An abstract class contains instance variables, whereas an interface can contain only constants.
  • Constructor and Instantiation – While an interface has neither a constructor nor it can be instantiated, an abstract class can have a default constructor that is called whenever the concrete subclass is instantiated.
  • Implementation of Methods – All classes that implement the interface need to provide an implementation for all the methods contained by it. A class that extends the abstract class, however, doesn’t require implementing all the methods contained in it. Only abstract methods need to be implemented in the concrete subclass.
  • Type of Methods – Any abstract class has both abstract as well as non-abstract methods. Interface, on the other hand, has only a single abstract method.

Question: Could you explain various types of Exceptions in Java? Also, tell us about the different ways of handling them.

Answer: Java has provision for two types of exceptions:

  • Checked Exceptions – Classes which extend Throwable class, except Runtime exception and Error, are called checked exceptions. Such exceptions are checked by the compiler during the compile time. These type of exceptions must either have appropriate try/catch blocks or be declared using the throws keyword. ClassNotFoundException is a checked exception.
  • Unchecked Exceptions – Such exceptions aren’t checked by the compiler during the compile time. As such, the compiler doesn’t necessitate for handling unchecked exceptions. Arithmetic Exception and ArrayIndexOutOfBounds Exception are unchecked exceptions.

Exceptions in Java are handled in two ways:

  • Declaring the throws keyword – We can declare the exception using throws keyword at the end of the method. For example,
    class ExceptionCheck{
    public static void main(String[] args){
    add();
    }
    public void add() throws Exception{
    addition();
    }
    }
    
  • Using try/catch – Any code segment that is expected to yield an exception is surrounded by the try block. Upon the occurrence of the exception, it is caught by the catch block that follows the try block. For example,
    class ExceptionCheck{
    public static void main (String[] args) {
    add();
    }
    public void add(){
    try{
    addition();
    }catch(Exception e){
    e.printStacktrace();
    }
    }
    }
    

Question: Could you draw the Java Exception Hierarchy?

Answer:Java Exception Hierarchy

Question: What role does the final keyword play in Java? What impact does it have on a variable, method, and class?

Answer: The final keyword in Java is a non-access modifier that is applicable only to a class, method, or variable. It serves a different purpose based on the context where it is used.

  • With a class:

When a class is declared as final then it is disabled from being subclassed i.e. no class can extend the final class.

  • With a method:

Any method accompanying the final keyword is restricted from being overridden by the subclass.

  • With a variable:

A variable followed by the final keyword is not able to change the value that it holds during the program execution. So, it behaves like a constant.

Question: How do you make a thread in Java? Give examples.

Answer: In order to make a thread in Java, there are two options:

  • Extend the Thread Class The thread is available in the java.lang.Thread class. In order to make a thread, you need to extend a thread class and override the run method. For example,
public class Addition extends Thread {
public void run() {
}
}

A disadvantage of using the thread class is that it becomes impossible to extend any other classes. Nonetheless, it is possible to overload the run() method in the class

  • Implement Runnable Interface – Another way of making a thread in Java is by implementing a runnable interface. For doing so, there is the need to provide the implementation for the run() method that is defined in the
interface. For example,
public class Addition implements Runnable {
public void run() {
}
}

Question: Why do we use the yield() method?

Answer: The yield() method belongs to the thread class. It transfers the currently running thread to a runnable state and also allows the other threads to execute. In other words, it gives equal priority threads a chance to run. Because yield() is a static method, it does not release any lock.

Question: Can you explain the thread lifecycle in Java?

Answer: The thread lifecycle has the following states and follows the following order:

  • New – In the very first state of the thread lifecycle, the thread instance is created and the start() method is yet to be invoked. The thread is considered alive now.
  • Runnable – After invoking of the start() method but before invoking the run() method, a thread is in the runnable state. A thread can also return to the runnable state from waiting or sleeping state.
  • Running – The thread enters the running state after the run() method is invoked. This is when the thread begins execution.
  • Non-Runnable – Although the thread is alive, it is not able to run. Typically, it returns to the runnable state after some time.
  • Terminated – The thread enters the terminated state once the run() method complete its execution. It is not alive now.

Question: Take a look at the two code snippets below:

i.

class Adder {
Static int add(int a, int b)
{
return a+b;
}
Static double add( double a, double b)
{
return a+b;
}
public static void main(String args[])
{
System.out.println(Adder.add(11,11));
System.out.println(Adder.add(12.3,12.6));
}}

ii.

class Car {
void run(){
System.out.println(“car is running”);
}
Class Audi extends Car{
void run()
{
System.out.prinltn(“Audi is running safely with 100km”);
}
public static void main( String args[])
{
Car b=new Audi();
b.run();
}
}

What is the important difference between the two?

Answer: Code snippet i. is an example of method overloading while the code snippet ii. demonstrates method overriding.

Question: What do you understand by Synchronization in Java? What is its most significant disadvantage?

Answer: If several threads try to access a single block of code then there is an increased chance of producing inaccurate results. To prevent this, synchronization is used.

Using the synchronization keyword makes a thread to need a key in order to access the synchronized code. Simply, synchronization allows only one thread to access a block of code at a time.

Each Java object has a lock and every lock has only one key. A thread is able to access a synchronized method only if it can get the key to the lock of the object. Following example demonstrates synchronization:

public class ExampleThread implements Runnable {
public static void main (String[] args){
Thread t = new Thread();
t.start();
}
public void run(){
synchronized(object){
{
}
}

Note: It is recommended to avoid implementing synchronization for all methods. This is because when only one thread is able to access the synchronized code, the next thread needs to wait. Consequently, it results in slower performance of the program.

Question: Is it possible to write multiple catch blocks under a single try block?

Answer: Yes, it is possible to write several catch blocks under a single try block. However, the approach needs to be from specific to general. Following example demonstrates the same:

public class Example {
public static void main(String args[]) {
try {
int a[]= new int[10];
a[10]= 10/0;
}
catch(ArithmeticException e)
{
System.out.println("Arithmetic exception in first catch block");
}
catch(ArrayIndexOutOfBoundsException e)
{
System.out.println("Array index out of bounds in second catch block");
}
catch(Exception e)
{
System.out.println("Any exception in third catch block");
}
}

Question: Can you tell the difference between execute(), executeQuery(), and executeUpdate()?

Answer:

  • execute() – Used for executing an SQL query. It returns TRUE if the result is a ResultSet, like running Select queries, and FALSE if the result is not a ResultSet, such as running an Insert or an Update query.
  • executeQuery() – Used for executing Select queries. It returns the ResultSet, which is not null even if there are no records matching the query. The executeQuery() method must be used when executing select queries so that it throws the java.sql.SQLException with ‘executeQuery method cannot be used for update’ message when someone tries to execute an Insert or Update statement.
  • executeUpdate() – Used for executing Delete/Insert/Update statement or DDL statements that returns nothing. The output varies depending on whether the statements are Data Manipulation Language (DML) statements or Data Definition Language (DDL) statements. The output is an integer and equals to the total row count for the former case, and 0 for the latter case.

Note: The execute() method needs to be used only in a scenario when there is no certainty about the type of statement. In all other cases, either use executeQuery() or executeUpdate() method.

Question: Provide an example of Hibernate architecture:

Answer:Hibernate architecture

Question: Could you demonstrate how to delete a cookie in JSP with a code example?

Answer: Following code demonstrates deleting a cookie in JSP:

Cookie mycook = new Cookie("name1","value1");
response.addCookie(mycook1);
Cookie killmycook = new Cookie("mycook1","value1");
killmycook . set MaxAge ( 0 );
killmycook . set Path ("/");
killmycook . addCookie ( killmycook 1 );

Question: Write suitable code examples to demonstrate the use of final, finally, and finalize.

Answer:

Final

The final keyword is used for restricting a class, method, and variable. A final class can’t be inherited, a final method is disabled from overriding, and a final variable becomes a constant i.e. its value can’t be changed.

class FinalVarExample {
public static void main( String args[])
{
final int a=10;
a=50; // Will result in an error as the value can’t be changed now
}

Finally

Any code inside the finally block will be executed, irrespective of whether an exception is handled or not.

class FinallyExample {
public static void main(String args[]){
try {
int x=100;
}
catch(Exception e) {
System.out.println(e);
}
finally {
System.out.println("finally block is executing");}
}
}
}

Finalize

The finalize method performs the clean up just before the object is garbage collected.

class FinalizeExample {
public void finalize() {
System.out.println("Finalize is called");
}
public static void main(String args[])
{
FinalizeExample f1=new FinalizeExample();
FinalizeExample f2=new FinalizeExample();
f1= NULL;
f2=NULL;
System.gc();
}
}

Question: How does the throw keyword differ from the throws keyword?

Answer: While the throws keyword allows declaring an exception, the throw keyword is used to explicitly throw an exception. Checked exceptions can’t be propagated with throw only but throws allow doing so without the need of anything else.

The throws keyword is followed by a class, whereas the throw keyword is followed by an instance. The throw keyword is used within the method but the throws keyword is used with the method signature. Furthermore, it is not possible to throw multiple exceptions, but it is possible to declare multiple exceptions.

Question: How will you distinguish processes from threads?

Answer: There are several fundamental differences between a process and a thread, stated as follows:

  • Definition – A process is an executing instance of a program whereas, a thread is a subset of a process.
  • Changes – A change made to the parent process doesn’t affect child processes. However, a change in the main thread can yield changes in the behavior of other threads of the same process.
  • Communication – While processes require inter-process communication for communicating with sibling processes, threads are able to directly communicate with other threads belonging to the same process.
  • Control – Processes are controlled by the operating system and are able to control only child processes. On the contrary, threads are controlled by the programmer and are capable of exercising control over threads of the same process to which they belong.
  • Dependence – Processes are independent entities while threads are dependent entities
  • Memory – Threads run in shared memory spaces but processes run in separate memory spaces.

Qestion: Could you draw a comparison between Array and ArrayList?

Answer: An array necessitates for giving the size during the time of declaration while an array list doesn’t necessarily require size as it changes size dynamically.

In order to put an object into an array, there is the need to specify the index. However, no such requirement is in place for an array list. While an array list is parameterized, an array is not parameterized.

Question: Please explain what do you mean by an Abstract class and an Abstract method?

Answer: An abstract class in Java is a class that can’t be instantiated. Such a class is typically used for providing a base for subclasses in order to extend as well as implementing the abstract methods and overriding or using the implemented methods defined in the abstract class.

In order to create an abstract class, it needs to be followed by the abstract keyword. Any abstract class can have both abstract as well as non-abstract methods.

A method in Java that only has the declaration and not implementation is known as an abstract method. Also, an abstract method name is followed by the abstract keyword. Any concrete subclass that extends the abstract class must provide an implementation for abstract methods.

Question: What do you know about Interface in Java?

Answer: A Java interface is a template that has only method declarations and not method implementations. It is a workaround for achieving multiple inheritances in Java.

Some worth remembering important points regarding Java interfaces are:

  • A class that implements the interface must provide an implementation for all methods declared in the interface
  • All methods in an interface are internally public abstract void
  • All variables in an interface are internally public static final
  • Classes do not extend but implement interfaces

Question: What is the join() method? Give an example.

Answer: We use the join() method for joining one thread with the end of the currently running thread. It is a non-static method and has an overloaded version. Consider the example below:

public static void main (String[] args) {
Thread t = new Thread();
t.start();
t.join();
}

The main thread starts execution in the aforementioned example. As soon as the execution reaches the code t.start() then the thread t starts its own stack for execution. The JVM switches between the main thread and the thread there.

Once the execution reaches the t.join() then the thread t alone is executed and allowed to complete its task. Afterward, the main thread resumes execution.

Question: How do you make a thread stop in Java?

Answer: There are three methods in Java to stop the execution of a thread:

  • Blocking – This method is used to put the thread in a blocked state. The execution resumes as soon as the condition of the blocking is met. For instance, the ServerSocket.accept() is a blocking method that listens for incoming socket connection and resumes the blocked thread only when a connection is made.
  • Sleeping – This method is used for delaying the execution of the thread for some time. A thread upon which the sleep() method is used is said to enter the sleep state. It enters the runnable state as soon as it wakes up i.e. the sleep state is finished. The time for which the thread needs to enter the sleep state is mentioned inside the braces of the sleep() method. It is a static method.
  • Waiting – Although it can be called on any Java object, the wait() method can only be called from a synchronized block.

Question: What purpose do the Volatile variable serve in Java?

Answer: The value stored in a volatile variable is not read from the thread’s cache memory but from the main memory. Volatile variables are primarily used during synchronization.

Question: Please compare Serialization with Deserialization in Java.

Answer: Serialization is the process by which Java objects are converted into the byte stream. Deserialization is the exact opposite process of serialization where Java objects are retrieved from the byte stream.

A Java object is serialized by writing it to an ObjectOutputStream and deserialized by reading it from an ObjectInputStream.

Question: What do you understand by OutOfMemoryError in Java?

Answer: Typically, the OutOfMemoryError exception is thrown when the JVM is not able to allocate an object due to running out of memory. In such a situation, no memory could be reclaimed by the garbage collector.

There can be several reasons that result in the OutOfMemoryError exception, out of which most notable ones are:

  •         Holding objects for too long
  •         Trying to process too much data at the same time
  •         Using a third-party library that caches strings
  •         Using an application server that doesn’t perform a memory cleanup post the deployment
  •         When a native allocation can’t be satisfied

That completes the list of top Java interview questions. What do you think about the list we compiled? Let us know by dropping your comments in the dedicated window below. Also, check out these best Java tutorials to further refine your Java skill set.

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20 Comments, RSS

  1. Avatar

    Alethea Koonce April 10, 2019 @ 11:44 am

    Difference between String, String Builder, and String Buffer.

    • Avatar

      Ryan Matthews April 19, 2019 @ 2:53 pm

      String is immutable i.e. if you try to change the original value then another object gets created. String Builder and String Buffer, on the other hand, are mutable, which is that their values can be altered. Also, String Buffer is thread-safe while String Builder isn’t.

  2. Avatar

    Dusty Koenig April 10, 2019 @ 11:45 am

    What are wrapper classes?

    • Avatar

      Kim Drake April 19, 2019 @ 2:54 pm

      An object of the wrapper class contains, or as technically known wraps, its respective primitive data type. Byte, Double, Float, Integer, Long, and Short are various wrapper classes and they are in actuality subclasses belonging to the abstract class Number

  3. Avatar

    Richelle Ritter April 10, 2019 @ 11:48 am

    Difference between Default and Protected access specifiers.

    • Avatar

      Rachel McCartney April 19, 2019 @ 2:55 pm

      Protected data members and methods are accessible by the classes belonging to the same package in addition to the subclasses belonging to any other package. Default data members and methods, on the other hand, are only accessible by the classes belonging to the same package.

  4. Avatar

    Arron Nunley April 10, 2019 @ 11:49 am

    Difference between equals() and ==?

    • Avatar

      Ram Naresh April 19, 2019 @ 2:56 pm

      Although both equals() and == are used to compare objects for equality in Java, there is one big distinguishing factor between the two. While == checks whether the two objects point to the same memory location, equals() checks whether the two objects have the same value or not.

  5. Avatar

    Rona Moon April 10, 2019 @ 11:50 am

    What is the difference between HashSet and TreeSet?

    • Avatar

      Kyle Mannequin April 19, 2019 @ 2:57 pm

      The biggest difference between HashSet and TreeSet is in terms of performance. HashSet is much faster than TreeSet. However, HashSet doesn’t guarantee any order. On the contrary, TreeSet maintains objects in some sorted order defined by the Comparator or Comparable method

  6. Avatar

    Celesta Lavoie April 10, 2019 @ 11:50 am

    Can you override a private or static method in Java?

    • Avatar

      Mahmood Khan April 19, 2019 @ 2:57 pm

      No, it isn’t possible to override either a private method or a static method in Java. Because the scope of the private access specifier is limited within a class, it can’t be overridden. Overriding the static method won’t yield any error, but the output will turn out to have deviated.

  7. Avatar

    Walker Almond April 10, 2019 @ 11:54 am

    What do you mean by aggregation?

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      Rocky Rickman April 19, 2019 @ 2:58 pm

      Also described as a “has-a” and “whole/part” relationship, aggregation is a relationship between two Java classes. The aggregate class has a reference to some other class. A class with all its data members public is an aggregate class. Aggregate classes have neither constructor definitions nor virtual functions.

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    Nidia Graff April 10, 2019 @ 11:54 am

    How does cookies work in Servlets?

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      Jimmy Gerald April 19, 2019 @ 2:59 pm

      A Servlet container sets a cookie in the header of the HTTP response with the cookie name along with the unique session ID as its value. The cookie is stored in the user browser, which sends it back to the server for all the following requests until the cookie is valid.

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    Chung Jack April 10, 2019 @ 11:55 am

    Explain about Map and their types.

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      Jackie Smith April 19, 2019 @ 2:59 pm

      A Map is a Java object that maps keys to values. Each key in a Map object can have only one value and a Map object can’t have duplicate keys. It is of three types:
      i. HashMap – Implemented as a hash table with no order
      ii. LinkedHashMap – Implemented as a hash table with insertion order
      iii. TreeMap – Implementation based on red-black tree structure and ordered by the key

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    Karena Noonan April 10, 2019 @ 11:56 am

    Difference between notify() method and notifyAll() method in Java.

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      Sam Willy April 19, 2019 @ 3:00 pm

      When multiple threads are waiting on any lock in Java, then while the notify() method sends a notification to only one of them, the notifyAll() method sends notifications to all of the waiting threads