Java interview questions and answers for 2025

One of the commonly asked Java coding interview questions is about the features of the language. Some important Java features are as follows:

Ease of learning: Java is considered to be an easy-to-learn and understand language.

Security: Java is a secure language. The applications developed using this language are virus and tamper-proof.

OOP: OOP means Object-Oriented Programming Language. While Java is not considered to be purely object-oriented, the inclusion of features such as data abstraction, inheritance, and data-encapsulation, makes Java an object-oriented programming language. Thus, as such, everything in Java is considered an object.

Independent Platform: This is an important Java feature. Java does not depend on any specific platform and as long as the system has JRE installed, Java bytecode will be interpreted.

Another popular Java interview question is about memory allocation. Five important types of memory allocations in Java are - Heap memory, Class memory, Native method stock memory, Stack memory, and Program-counter memory.

Java is considered to be platform-independent because it does not depend on any hardware or software. When the Java programmer compiles Java code, the compiler converts the code into bytecode that can be run on different platforms. The only constraint is that the platform must have JRE (runtime environment) or Java Virtual Machine (JVM) installed on it. Answering this Java interview question can help the interviewer understand your depth of experience.

Data-encapsulation in Java is the process of hiding the variables (data) and the code applied to the data in a single unit. It is a significant feature of object-oriented programming. This helps Java developers to isolate different objects from each other and to create modules for each. By this, all objects would have their own behaviors, attributes, and functions. It prevents data or object mixing, hides data, enhances security.

Java, as a programming language, supports primitive data types. Wrapper classes help convert these primitive data types into objects or reference types and vice versa. Thus, the wrapper classes are so named because they ‘wrap’ these data types into identifiable objects of the specific data classes the primitive data types belong to. This Java interview question can help the interviewer assess whether you are comfortable using both old and new data types.

In Java, constructors can be understood as a block of codes applied to an object to initialize it. It can be understood as a special method for initializing an object. Constructors are called when a class instance is created such as creating an object using the new() keyword. When a class does not contain a constructor, a default constructor, provided by the Java compiler, is automatically called.

There are two types of Java constructors - default and parameterized. When no parameters are defined, the constructor is called a default constructor. When specific parameters are defined for a constructor, it is called a parameterized constructor. While you may be using the Java programming language for some time, this Java interview question shows the interviewer that you know the technicalities too.

Java supports primitive data types such as boolean, int, byte, long, float, etc. An object-oriented language must only support objects. However, the primitive data types are not objects, hence Java is not considered to be purely object-oriented.

No, unlike C++, Java doesn’t use pointers. Java’s main focus is simplicity. The use of pointers complicates the process, especially for new java programmers. The use of pointers can also lead to possible errors. Moreover, pointers grant direct access to memory and thus, compromise security. By excluding pointers from Java, a certain amount of abstraction is achieved. Java has automatic Garbage Collectors, the use of pointers can slow down the garbage collection process. This advanced Java interview question deals with the intricacies of the language and answering it can help set you apart from other candidates.

No. While it is possible to hide the parent static method with a static method of the same signature class, this is not called overriding in the strict sense. This is because, during the run-time, no polymorphism can take place.

ClassLoader is an integral element of the JRE (Java Runtime Environment). The function of the ClassLoader is to dynamically load Java classes directly to the JVM (Java Virtual Machine) during program execution, without notifying the Java run time system. The file to be executed is first loaded by ClassLoaders. Bootstrap, Extension, and Application are the ClassLoaders in Java. This is an advanced Java interview question and answering it can establish your expertise.

A session is any ongoing random conversation between a client and the server. The communication between the two is composed of requests and responses from both sides. There are several ways of managing a session. Applying cookies, using session management API, authenticating the user, hidden fields in HTML, and rewriting URL are some ways of session management. However, the most common way of managing a session is using a session ID when the communication between a client and the server takes place. This Java interview question shows your application orientation to the client.

With the help of JIT or a Just-in-time compiler, Java programmers can enhance the Java applications’ performance. This is achieved in a runtime environment by compiling bytecode to the code of the native machine. As a result, the compilation time for running the code reduces. In essence, the JIT compiler translates the source code to a code that can be executed by the machine.

Java programs contain machine-neutral bytecodes that can run on any machine with JVM. When a Java program is run, it accesses the class files, interprets the bytecode, and performs the relevant computation. However, during the processing, extra memory and processor are used which compromise the speed. With the help of the JIT compiler, the speed and performance of the program can be improved.

Java is a very secure programming language as it is hard to bug or tamper with. It is also very easy to learn and comprehend when developing applications. Most importantly, Java is a self-dependent language that runs on its own JRE platform.

Java is considered platform-independent as it does not depend on any software or hardware. However, it is flexible enough to be run on different platforms with the condition that the platform has a JRE environment installed on it.

The term inheritance occurs when one class in a code extends to another. This means that codes used in one class can be reused in another class.

The purpose of encapsulation is to maintain code and protect the code from others.

The solution to this problem is to use interfaces instead of inheritance. Interfaces allow a class to define multiple behaviors without the need to extend multiple classes.

A block of code used to initialize an object is called a constructor. The constructor must share the same name as the class and it has no return type. Default and parameterized constructors are the two types of constructors.

A new object is created when using new() whereas using literal, the object already existing might return with the same name.

Synchronization helps to execute threads one after another. It also prevents memory constituency errors due to shared access methods.

Java does not use pointers. This is because they are not safe and can increase the complexity of the program. Pointers are the exact opposite of Java’s simplicity.

A real entity that has a state and behavior is called an object. It is created using the new keyword. State, identity, and behavior are some of the characteristics of an object created.

OOP is an abbreviation for Object-Oriented Programming. It is a model where programs are organized around objects rather than functions and logic. Its main focus is on objects more than using logic and is mainly preferred for large and complex codes.

Double Brace Initialization is a technique that combines an anonymous inner class with an initialization block in order to initialize collections and improve code readability.

An association can be defined as a relationship between two or more that have no ownership over another.

A singleton class is a class that is capable of possessing only one object at a time.

The Java string pool is a repository of strings stored in the Java virtual machine (JVM). The pool stores only one instance of each literal string.

JRE has class libraries and other JVM files however, it does not have any tools for Java development such as a debugger and compiler. While JVM has a Just In Time(JIT) tool to convert all the Java codes into compatible machine language.

It's a process of creating multiple method signatures using one method. There are two ways of achieving this i.e changing the return type of the method and varying the number of arguments.

The stages of a thread’s lifecycle are as follows:

Newborn state- This is where a new thread is created. In this newborn state, the code has not been run or executed as yet.

Runnable state- A thread that is ready to run is put into the runnable state. The code may be running or ready to run at any given time but at this stage, the thread scheduler provides the time for the thread to run.

Running state- This stage is when the thread receives a CPU moving from the runnable to the running state.

Blocked state- When the thread is inactive for a period of time temporarily, the thread is said to be in a blocked state.

Dead state- Also known as the terminated state. This is when the thread has either finished its job and exists or when some unexpected event occurs such as a segmentation fault.

Enumeration also known as enum, is an interface in Java. It allows the sequential access of the elements stored in the collection.

Java packages can be defined as a collection of classes and interfaces which are bundled together while they relate to one another. It also helps developers to group a code for reuse and after being packaged it can be imported in other classes for repurposing.

JPA stands for Java Persistence API. It enables developers to create the persistence layer for desktop/ web applications.

Java is designed to adapt to any environment that is evolving. Its program has a large amount of information that is used to solve or access objects in real-time.

Steps needed to connect to a database in Java are as follows:

• Registering the driver class
• Creating connection
• Creating statement
• Executing queries
• Closing connects

To convert an object, an interface with the name serializable is implemented by the class.
All the objects of a class implementing a serializable interface get serialized and their state is saved in the byte stream.

An array groups data of the same primitive type and is static, while vectors can hold data of different types. Vectors are dynamic though.

Garbage collection in Java is done automatically by the Java Virtual Machine (JVM) at runtime. The JVM uses a garbage collection algorithm to identify and reclaim memory from objects that are no longer in use.

To create a copy of an object in Java, you can use the clone() method of the Object class. This method creates a shallow copy of the object, meaning that only the object's references are copied, not its values.

The main difference between stack and queue is that stack is based on LIFO (Last in first out) while a queue is based on FIFO( First in first out) principle. Both are used as placeholders for the collection of data.

Checked exceptions can be caught during the time of program compilation. The checked exceptions must be handled by using the try catch block in the code to successfully compile the code.

Java has its own garbage collection mechanism which destroys automatically objects when they are no longer referenced. Hence no destructors need to be defined.

Yes, it is possible to find the size of an object on the heap in Java. The object size can be calculated by calling the Object.getSize(Object)) method.

When a new class inside a particular block is defined, it's called a local class. Such a class is not usable outside the block where it is defined.

Java supplies a collection of APIs which provide useful methods to be applied to a set of objects. Examples of classes provided include TreeSet, HashMap, and ArrayList.

The final keyword in Java is used to indicate that a variable, method, or class cannot be changed or overridden. It is used to create constants that cannot be changed, and to prevent a method from being overridden by a subclass.

Float only takes 4 bytes in memory while double takes twice the bytes as that of a float in memory. Double is also a double-precision decimal number while float is a single-precision floating-point decimal number.

Threads can be created in two ways, by either extending the thread or by implementing the runnable interface.

This error occurs when JVM runs out of memory. OutOfMemoryError is the subclass of Java.lang

If an exception is not caught, it is thrown from the stack and falls down the call stack to the previous procedure. If then it is not caught there, it will fall back again to the previous function. The process repeats until caught or the call stack reaches the bottom.

JCA is an abbreviation of Java Cryptography Architecture and is used by developers to combine it with security applications. It, therefore, gives a platform and provides architecture and application programming interfaces that enable encryption and decryption.

It is an application framework in Java and inversion of control containers of Java. It creates enterprise applications in Java.

The applet lifecycles include the following:

• Initialization
• Start
• Stop
• Destroy
• Paint

Late binding is also known as dynamic binding or run-time binding. Late Binding happens when the method’s code segment is unknown until it is called during the runtime.

The function is faster in Java as the user needs to make a minor update to the pointer value so that the node can point to the next successor in the list.

It is a thread with the least priority. It is designed to run in the background during Garbage Collection in Java

Operator (>>) does the job of shifting sign bits while operator ( >>> ) out the zero-filled bits.

Eclipse and NetBeans are the two popular Java IDEs.

Polymorphism in Java is when we have many forms/classes related to each other by inheritance. A single object can refer to the sub and super class depending on the reference type which is called polymorphism’

These include:

• Collection
• List
• Set
• Map
• Sorted Set
• Sorted Map
• Queue

lt means the values that are stored in a collection are based on the values that are added to the collection.

A finally block in Java is a block of code that is always executed after a try/catch block, regardless of whether or not an exception has been thrown.

These are the different types of collectors:

• Serial Garbage Collector
• Parallel Garbage Collector
• CMS Garbage Collector
• G1 Garbage Collector

System.out is used to display messages and results while System.err is used to display error messages.

FailFast iterators do not allow modifications or changes or the Java collections. This implies that they fail when the latest element is added to a collection or an element gets removed from the collection.

The term aggregation refers to the relationship between two classes. This “has” and “whole” is the most specialized version of an association relationship as it contains the reference to another class.

C++, which is based on the C programming language, is compatible with most other high-level languages whereas Java is not compatible with C++.

A Copy Constructor is a constructor that initializes an object through another of the same class.

It is a paradigm that is based on the concepts of objects. It contains data that is in the form of fields, and regulations, that are in the form of procedures. Its main feature is that the object’s procedure can access and often modify the data fields.

Firstly, you need to make sure that the class has only one object and secondly, your classes must have global access to that object

The term is a particular word designed as a reference keyword. It is used to refer to the current class properties like method, variable, instance, and constructors

Generic methods and classes help programmers to specify a single method declaration and related types with an available class declaration.

Spring MVC has two ways to handle exceptions:

XML Configuration: The user can use the SimpleMappingExceptionResolver in Spring’s application file and map the exception.

Exception Handler: In this kind of exception handling, the user gets the @ExceptionHandler annotation type used to annotate a method to handle exceptions.

One or more catch blocks can follow a try block, though each catch must have a unique exception handler. Multiple tasks can now be performed using the Java multi-catch block.

The super keyword often referred to as parent class can be used when there are the same fields in both parent and child classes to access data members of the parent class.

The interface has only abstract methods in it whereas an abstract class can have both abstract and non-abstract methods.

In the case of deep copy, both primitive data types and object references are copied whereas in the shallow copy only primitive data are copied.

Throws can be used to declare multiple exceptions whereas throw can throw one exception at a time.

One approach to not allow serialization of attributes of a class is by using writeObject () and readObject() methods in the subclass and throwing a not serializable exception.

An infinite loop can be declared by breaking the logic in the instruction box.

The reasons for making the string final would be to destroy its immutability while also assisting in stopping others from trying to extend it. Strings will then be cached in the string pool, making them immutable.

Final is used to limit the scope of classes, variables, or methods. Finally is used to execute the code within a block without catching any exceptions. Finalize is used to invoke the code that performs garbage collection when an object is no longer in use.

A priority queue in Java is a specific kind of queue that holds elements that have an associated priority.

Checked Exceptions are a type of Exception that the compiler checks for during the compilation process.

Start() method creates a new thread, while the code inside run() executes in the new thread.

It is not recommended to implement all methods. This is because if one thread accesses the synchronized code then the next thread will have to wait. This will end result in slow performance.

Transient variables are not part of the serialization process. The values of the transient variables are set to a default value during deserialization. It is therefore not used with static variables.

Volatile variables are always read from the main memory and not from the thread’s cache memory. This is used during synchronization and is applicable to variables only.

When an object is serialized, the object is stamped with a version ID number for the class object. The ID is called SerialVersionUID. It is used during deserialization to verify the sender and receiver that are compatible with serialization

Serialization is the process which is used to convert objects into byte stream whereas Deserialization is the opposite process where we can get the objects back from the byte stream.

The sleep method is used to sleep the currently executing thread for a given amount of time. Sleep is used to delay the execution period because once the thread wakes up it can move to the runnable state.

These are:

• Blocked
• Waiting
• Sleeping

Notify() is used to send a signal to wake up a single thread in the waiting pool while notifyAll() sends the signal to wake up all the threads in the waiting pool.

The keywords for exception handling are:

• try
• catch
• finally

TreeSet maintains the elements in the sorted order. On the other hand, the elements in HashSet are inserted in random order.

Public and private access specifiers are methods and instance variables known as members. Public members are visible in the same package as well as outside whereas private members are visible in the same classes only and not for other classes in the same packages.

Yield() method moves the thread back to the runnable state only not to sleep(), wait(), or block but allowing other threads for execution.

This method is used to make the thread wait in the waiting pool. Once executed during the thread execution then instantly the thread gives up the lock on the object and goes into the waiting pool.

These are:

• Basic authentication
• Form-based authentication
• Digest authentication
• Client certificate authentication

Java works only as a “pass by value” as “pass by reference” needs the assistance of pointers. Java does not have pointers.

For a numerical value, it is O, for the boolean value, it is false and NULL value for objects.