Java Interview Questions (2023)

Java is a high-level programming language originally developed by Sun Microsystems and released in 1995. Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX.

Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX/Linux like HP-Unix, Sun Solaris, Redhat Linux, Ubuntu, CentOS, etc.

Some features include Object Oriented, Platform Independent, Robust, Interpreted, Multi-threaded

It’s compiler generates an architecture-neutral object file format, which makes the compiled code to be executable on many processors, with the presence of Java runtime system.

Java uses Just-In-Time compiler to enable high performance. Just-In-Time compiler is a program that turns Java bytecode, which is a program that contains instructions that must be interpreted into instructions that can be sent directly to the processor.

It is designed to adapt to an evolving environment. Java programs can carry extensive amount of run-time information that can be used to verify and resolve accesses to objects on run-time.

When Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by virtual Machine (JVM) on whichever platform it is being run.

Netbeans, Eclipse, etc.

Some Java keywords are import, super, finally, etc.

Object is a runtime entity and it’s state is stored in fields and behavior is shown via methods. Methods operate on an object's internal state and serve as the primary mechanism for object-to-object communication.

A class is a blue print from which individual objects are created. A class can contain fields and methods to describe the behavior of an object.

A class consist of Local variable, instance variables and class variables.

Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and it will be destroyed when the method has completed.

Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded.

These are variables declared with in a class, outside any method, with the static keyword.

Singleton class control object creation, limiting the number to one but allowing the flexibility to create more objects if the situation changes.

Constructor gets invoked when a new object is created. Every class has a constructor. If we do not explicitly write a constructor for a class the java compiler builds a default constructor for that class.

An Object is first declared, then instantiated and then it is initialized.

Default value of byte datatype is 0.

Default value of float and double datatype in different as compared to C/C++. For float its 0.0f and for double it’s 0.0d

This data type is used to save space in large arrays, mainly in place of integers, since a byte is four times smaller than an int.

Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.

Java provides access modifiers to set access levels for classes, variables, methods and constructors. A member has package or default accessibility when no accessibility modifier is specified.

Variables, methods and constructors which are declared protected in a superclass can be accessed only by the subclasses in other package or any class within the package of the protected members' class.

Java provides these modifiers for providing functionalities other than Access Modifiers, synchronized used to indicate that a method can be accessed by only one thread at a time.

Postfix operators i.e () [] . is at the highest precedence.

Variables used in a switch statement can only be a string, enum, byte, short, int, or char.

This method is used to get the primitive data type of a certain String.

The String class is immutable, so that once it is created a String object cannot be changed. Since String is immutable it can safely be shared between many threads ,which is considered very important for multithreaded programming.

The String class is considered as immutable, so that once it is created a String object cannot be changed. If there is a necessity to make alot of modifications to Strings of characters then StringBuffer should be used.

Use StringBuilder whenever possible because it is faster than StringBuffer. But, if thread safety is necessary then use StringBuffer objects.

java.util.regex package is used for this purpose.

java.util.regex consists of three classes − Pattern class, Matcher class and PatternSyntaxException class.

It is possible to define a method that will be called just before an object's final destruction by the garbage collector. This method is called finalize( ), and it can be used to ensure that an object terminates cleanly.

An exception is a problem that arises during the execution of a program. Exceptions are caught by handlers positioned along the thread's method invocation stack.

It is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.

It is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compliation.

The Exception class has two main subclasses : IOException class and RuntimeException Class.

If a method does not handle a checked exception, the method must declare it using the throwskeyword. The throws keyword appears at the end of a method's signature.

An exception can be thrown, either a newly instantiated one or an exception that you just caught, by using throw keyword.

The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.

While creating your own exception −

• All exceptions must be a child of Throwable.

• If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.

• You want to write a runtime exception, you need to extend the RuntimeException class.

It is the process where one object acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.

If the method overrides one of its superclass's methods, overridden method can be invoked through the use of the keyword super. It can be also used to refer to a hidden field.

Polymorphism is the ability of an object to take on many forms. The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.

It refers to the ability to make a class abstract in OOP. It helps to reduce the complexity and also improves the maintainability of the system.

These classes cannot be instantiated and are either partially implemented or not at all implemented. This class contains one or more abstract methods which are simply method declarations without a body.

If you want a class to contain a particular method but you want the actual implementation of that method to be determined by child classes, you can declare the method in the parent class as abstract.

It is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. Therefore encapsulation is also referred to as data hiding.

The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this Encapsulation gives maintainability, flexibility and extensibility to our code.

An interface is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

It includes −

• Interface cannot be instantiated

• An interface does not contain any constructors.

• All of the methods in an interface are abstract.

A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and name space management.

Packages are used in Java in-order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations, etc., easier.

A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution.

Thread can be created by: implementing Runnable interface, extending the Thread class.

An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal.

An applet extends java.applet.Applet class.

It uses garbage collection to free the memory. By cleaning those objects that is no longer reference by any of the program.

An immutable object can’t be changed once it is created.

It is used with variables or methods and used to call constructer of same class.

It is a collection of element which cannot contain duplicate elements. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.

It is a Set implemented when we want elements in a sorted order.

It is used to sort collections and arrays of objects using the collections.sort() and java.utils. The objects of the class implementing the Comparable interface can be ordered.

It includes:

• Throw is used to trigger an exception where as throws is used in declaration of exception.

• Without throws, Checked exception cannot be handled where as checked exception can be propagated with throws.

The following shows the explanation individually −

• public − it is the access specifier.

• static − it allows main() to be called without instantiating a particular instance of a class.

• void − it affirns the compiler that no value is returned by main().

• main() − this method is called at the beginning of a Java program.

• String args[ ] − args parameter is an instance array of class String

Java Runtime Environment is an implementation of the Java Virtual Machine which executes Java programs. It provides the minimum requirements for executing a Java application;

JAR files is Java Archive fles and it aggregates many files into one. It holds Java classes in a library. JAR files are built on ZIP file format and have .jar file extension.

This is Web Archive File and used to store XML, java classes, and JavaServer pages. which is used to distribute a collection of JavaServer Pages, Java Servlets, Java classes, XML files, static Web pages etc.

It improves the runtime performance of computer programs based on bytecode.

Object based programming languages follow all the features of OOPs except Inheritance. JavaScript is an example of object based programming languages.

The java compiler creates a default constructor only if there is no constructor in the class.

No, this is not possible.

It is used to initialize the static data member, It is excuted before main method at the time of classloading.

Holding the reference of the other class within some other class is known as composition.

If a class has multiple functions by same name but different parameters, it is known as Method Overloading.

If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding.

Method overloading increases the readability of the program. Method overriding provides the specific implementation of the method that is already provided by its super class parameter must be different in case of overloading, parameter must be same in case of overriding.

Final classes are created so the methods implemented by that class cannot be overridden. It can’t be inherited.

A NullPointerException is thrown when calling the instance method of a null object, accessing or modifying the field of a null object etc.

A thread can enter the waiting state by invoking its sleep() method, by blocking on IO, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.

When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.

The Vector class provides the capability to implement a growable array of objects. Vector proves to be very useful if you don't know the size of the array in advance, or you just need one that can change sizes over the lifetime of a program.

Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.

These are classes that allow primitive types to be accessed as objects. Example: Integer, Character, Double, Boolean etc.

The Frame class extends Window to define a main application window that can have a menu bar.

javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components.

The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.

It is used to create objects that provide access to the files and directories of a local file system.

The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

The Class class is used to obtain information about an object's design and java.lang.Class class instance represent classes, interfaces in a running Java application.

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

It is part of the analysis of a program and describes a situation that a program might encounter and what behavior the program should exhibit in that circumstance.

Sub class inherits all the public and protected methods and the implementation. It also inherits all the default modifier methods and their implementation.

If there is a button instance called b1, you may add menu short cut by calling b1.setMnemonic('F'), so the user may be able to use Alt+F to click the button.

Yes, just add a main() method to the applet.

AWT components are heavy-weight, whereas Swing components are lightweight. Heavy weight components depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button.

Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

Yes, since inheritance inherits everything from the super class and interface, it may make the subclass too clustering and sometimes error-prone when dynamic overriding or dynamic overloading in some situation.

When copying elements between different arrays, if the source or destination arguments are not arrays or their types are not compatible, an ArrayStoreException will be thrown.

Yes, use this() syntax.

The code sleep(2000); puts thread aside for exactly two seconds. The code wait(2000), causes a wait of up to two second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

The ArithmeticException is thrown when integer is divided by zero or taking the remainder of a number by zero. It is never thrown in floating-point operations.

A transient variable is a variable that may not be serialized during Serialization and which is initialized by its default value during de-serialization,

Synchronization is the capability to control the access of multiple threads to shared resources. synchronized keyword in java provides locking which ensures mutual exclusive access of shared resource and prevent data race.

The Collections API is a set of classes and interfaces that support operations on collections of objects.

Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.

Panel is the immediate superclass. A panel provides space in which an application can attach any other component, including other panels.

The = operator is right associative.

A break statement results in the termination of the statement to which it applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.

A private variable may only be accessed within the class in which it is declared.

The purpose of the System class is to provide access to system resources.

The eight primitive types are byte, char, short, int, long, float, double, and boolean.

When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.

Component class is the immediate super class.

The Java runtime system generates RuntimeException and Error exceptions.

The garbage collector invokes an object's finalize() method when it detects that the object has become unreachable.

A dead thread cannot be restarted.

Integer / and % can result in the throwing of an ArithmeticException.

The default value of the boolean type is false.

Yes

A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class.

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation.

Program throws "NoSuchMethodError" error at runtime.

Null, unless it is defined explicitly.

No, a top level class can not be private or protected. It can have either "public" or no modifier.

We can pass them around as method parameters where a method expects an object. It also provides utility methods.

An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. Exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist.

It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block or a finally block.

A thread is in the ready state as initial state after it has been created and started.

The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

Synchronized methods are methods that are used to control access to an object. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass.

Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding means that the code associated with a given procedure call is not known until the time of the call at run-time.

No, constructor cannot be inherited.

ArrayList can grow dynamically and provides more powerful insertion and search mechanisms than arrays.

Deletion in linked list is fast because it involves only updating the next pointer in the node before the deleted node and updating the previous pointer in the node after the deleted node.

If you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially, then LinkedList should be used. If you need to support random access, without inserting or removing elements from any place other than the end, then ArrayList should be used.

It is a collection returned by the values() method of the Map Interface, It contains all the objects present as values in the map.

The dot operator(.) is used to access the instance variables and methods of class objects.It is also used to access classes and sub-packages from a package.

Private constructor is used if you do not want other classes to instantiate the object and to prevent subclassing.T

Type casting means treating a variable of one type as though it is another type.

A thread is a execution in a program. The life cycle of a thread include −

• Newborn state
• Runnable state
• Running state
• Blocked state
• Dead state

The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.

setBounds() method is used for this purpose.

The range of the short type is -(2^15) to 2^15 - 1.

MenuItem class

No, Java does not allow Default Arguments.

Octal Numbers are denoted by leading zero in java, example: 06

Hexadecimal Numbers are denoted by leading 0x or 0X in java, example − 0XF

Yes, an example can be break one;

Import statement is allowed at the beginning of the program file after package statement.

It is used to pause or temporarily stop the execution of the thread.

It is used to find out whether a thread is still running or not.

It is a public static method used to obtain a reference to the current thread.

The main thread is created automatically and it begins to execute immediately when a program starts. It ia thread from which all other child threads originate.

Life cycle involves the following steps −

• Initialization

• Starting

• Stopping

• Destroying

• Painting

It initializes the applet and is the first method to be called.

getImage(URL object, filename) is used for this purpose.

It is used to specify the name of the applet class.

It is a simple drawing surface which are used for painting images or to perform other graphical operations.

It refers to writing programs that execute across multiple devices (computers), in which the devices are all connected to each other using a network.

Sockets provide the communication mechanism between two computers using TCP. A client program creates a socket on its end of the communication and attempts to connect that socket to a server.

Sockets are flexible and sufficient. Efficient socket based programming can be easily implemented for general communications. It cause low network traffic.

Socket based communications allows only to send packets of raw data between applications. Both the client-side and server-side have to provide mechanisms to make the data useful in any way.

java.net.ServerSocket class is used by server applications to obtain a port and listen for client requests

java.net.Socket class represents the socket that both the client and server use to communicate with each other.

Generics provide compile-time type safety that allows programmers to catch invalid types at compile time. Java Generic methods and generic classes enable programmers to specify, with a single method declaration, a set of related methods or, with a single class declaration, a set of related types.

CLASSPATH and PATH are the two variables.

No, there is no need to import this package. It is by default loaded internally by the JVM.

If a class is declared within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Nested top-level class is an Inner class.

Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism.

No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.

Daemon thread is a low priority thread, which runs intermittently in the back ground doing the garbage collection operation for the java runtime system.

setDaemon method is used to create a daemon thread.

All tasks must implement the run() method

The GregorianCalendar provides support for traditional Western calendars

The SimpleTimeZone class provides support for a Gregorian calendar .

The size is the number of elements actually stored in the vector, while capacity is the maximum number of elements it can store at a given instance of time.

Yes a Vector can contain heterogenous objects. Because a Vector stores everything in terms of Object.

An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It allows sequential access to all the elements stored in the collection.

Path and Classpath are operating system level environment variales. Path is defines where the system can find the executables(.exe) files and classpath is used to specify the location of .class files.

No, it's not possible to accessed outside it's package.

A static method should not refer to instance variables without creating an instance and cannot use "this" operator to refer the instance.

Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.

Polymorphism

An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock.

It is the casting from a general to a more specific type, i.e. casting down the hierarchy.

Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left.

A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

When a class is defined within a scope of another class, then it becomes inner class. If the access modifier of the inner class is static, then it becomes nested class.

Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides.

A child object constructor always first needs to construct its parent. In Java it is done via an implicit call to the no-args constructor as the first statement.

Yes, a double value can be cast to a byte.

When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.

If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type.