Java Interview Questions and Answers Set 1

What do you know about Java?

What is the difference between JDK, JRE and JVM?

Is Java platform-independent? If yes, then why?

Yes, Java is platform-independent because of the Java Virtual Machine (JVM). When Java code is compiled, it is converted into bytecode, which is not specific to any platform. This bytecode can be executed on any device that has a compatible JVM, making Java programs run consistently across different operating systems.

What are the four main OOP principles in Java?

The four main Object-Oriented Programming (OOP) principles in Java are:

1. Encapsulation – Bundling data and methods that operate on the data within a single unit (class) and restricting access using access modifiers.
2. Inheritance – Allowing one class to inherit fields and methods from another class, promoting code reusability.
3. Polymorphism – Allowing one interface to be used for different underlying data types, enabling method overloading and overriding.
4. Abstraction – Hiding complex implementation details and showing only the necessary features to the user.

List any five features of Java?

1. Platform Independence – Java programs can run on any device with a JVM, making them platform-independent.
2. Object-Oriented – Java follows the OOP principles, promoting modular and reusable code.
3. Multithreading – Java supports multithreading, allowing multiple tasks to run concurrently.
4. Automatic Garbage Collection – Java automatically manages memory by cleaning up unused objects, freeing up resources.
5. Security – Java provides a robust security model, including bytecode verification and a security manager, to prevent unauthorized access.

Why is Java Architectural Neutral?

Java is considered architectural neutral because its bytecode, generated by the Java compiler, can be executed on any platform with a compatible Java Virtual Machine (JVM). This ensures that Java programs can run on different hardware and operating systems without modification, making Java platform-independent and able to work across various architectures.

How Java enabled High Performance?

Why is Java considered dynamic?

What is Java Virtual Machine (JVM), and how is it related to Java's platform-independent feature?

The Java Virtual Machine (JVM) is an abstract computing machine that enables a computer to run Java programs. It is a part of the Java Runtime Environment (JRE) that executes Java bytecode. When a Java program is compiled, it is not converted into platform-specific machine code. Instead, it is compiled into an intermediate form called bytecode, which is platform-independent.

This bytecode can be distributed across any system or over the web. The JVM on each platform (Windows, Linux, macOS etc.) interprets or compiles this bytecode into native machine code specific to that platform at runtime.

Because every platform has its own JVM implementation that understands the same bytecode, Java achieves platform independence.

List two Java IDE's?

Two popular Java IDEs are Eclipse and IntelliJ IDEA.

What do you mean by Object in Java?

In Java, an object is an instance of a class. It is a real-world entity that contains both state (data/fields) and behavior (methods/functions). Objects are created using the new keyword and are the foundation of object-oriented programming.

For example, a Car object may have properties like color and speed, and behaviors like drive() or brake().

Define class in Java.

class Car {
    String color;
    void drive() {
        System.out.println("The car is driving.");
    }
}

What kind of variables can a class consist of in Java?

What is Singleton class?

class Singleton {
    private static Singleton instance;

    private Singleton() {}

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

When should we use a Singleton class?

Why should we use a Singleton Class?

What do you mean by Constructor in Java?

A constructor in Java is a special type of method used to initialize objects when they are created. It has the same name as the class and does not have a return type. Constructors are called automatically when an object of a class is instantiated using the new keyword. They can be default constructors (no parameters) or parameterized constructors (with parameters to initialize the object's state).

List the three steps for creating an Object for a class?

1. Declare: Declare a reference variable of the class type.
2. Instantiate: Use the new keyword to create an object.
3. Initialize: Initialize the object using a constructor or set values for its fields.

What is the default value of byte datatype in Java?

The default value of the byte datatype in Java is 0.

What is the default value of float and double datatype in Java?

The default value of float is 0.0f and the default value of double is 0.0 in Java.

When a byte datatype is used?

The byte datatype in Java is used when you need to store small integer values, specifically in the range of -128 to 127, and when memory efficiency is a concern, as it consumes only 1 byte of memory. It is often used in situations like file I/O, network communication or handling raw binary data.

What is a static variable?

A static variable in Java is a class-level variable declared with the static keyword, outside methods, constructors or blocks. It is shared by all instances of the class, meaning all objects refer to the same variable.

What do you mean by Access Modifier?

An access modifier in Java is a keyword used to set the visibility or accessibility of classes, variables, methods, and constructors. It defines who can access these members within and outside the class. Java provides four types of access modifiers:

1. public – The class, method or variable is accessible from any other class.
2. private – The class, method or variable is accessible only within the same class.
3. protected – The class, method or variable is accessible within the same package or by subclasses.
4. Default (no modifier) – If no access modifier is specified, the class, method or variable is accessible only within the same package.

What is protected access modifier?

The protected access modifier in Java allows variables, methods, and constructors to be accessed within the same package and by subclasses in other packages. This means protected members are visible to all classes in the same package and to subclasses regardless of their package, providing a balance between package-level and public access.

1. Access from a Class in the Same Package

// File: animals/Animal.java
package animals;

public class Animal {
    protected void makeSound() {
        System.out.println("Animal makes a sound");
    }
}
// File: animals/Zoo.java
package animals;

public class Zoo {
    public void showAnimalSound() {
        Animal animal = new Animal();
        animal.makeSound();  // Allowed: same package access
    }
}

// File: animals/Animal.java
package animals;

public class Animal {
    protected void makeSound() {
        System.out.println("Animal makes a sound");
    }
}
// File: pets/Dog.java
package pets;

import animals.Animal;

public class Dog extends Animal {
    public void bark() {
        makeSound();  // Accessing protected method from superclass
        System.out.println("Dog barks");
    }
}

What do you mean by synchronized non-access modifier in Java?

public class Counter {
    private int count = 0;
    public synchronized void increment() {
        count++;
    }
    public int getCount() {
        return count;
    }
}

According to Java Operator precedence, which operator is considered to be with highest precedence?

In Java, the postfix operators (), [] and . have the highest precedence.

Which data types can be used with variables in a switch statement?

In Java, the variable used in a switch statement (known as the switch expression) must be of a compatible type. Valid data types include the primitive types byte, short, char and int, as well as their corresponding wrapper classes Byte, Short, Character and Integer. Starting from Java 5, enum types are also supported in switch statements, providing a type-safe way to handle fixed sets of constants. From Java 7 onward, String values can also be used in switch expressions, allowing developers to write more readable and expressive code. Other data types like float, double, long or objects not listed above are not allowed in switch statements.

When parseInt() method can be used?

The parseInt() method in Java is used to convert a String representing a number into its corresponding integer value (int). It is a static method of the Integer class. This method can be used when you have a string that represents a valid integer and you need to perform numeric operations with it. If the string cannot be parsed as an integer (e.g. it contains non-numeric characters), the method throws a NumberFormatException.

String number = "123";
int result = Integer.parseInt(number);  // Converts the string to an integer
System.out.println(result);  // Outputs 123

Why is String class considered immutable?

The String class in Java is considered immutable because once a String object is created, its value cannot be changed. Any modification to a String creates a new object. This immutability makes Strings thread-safe, as they can be shared across multiple threads without risk of data inconsistency, which is crucial in multithreaded programming.

Why is StringBuffer called mutable?

The StringBuffer class in Java is called mutable because its contents can be modified after creation. Unlike the String class (which is immutable), StringBuffer allows for efficient manipulation of characters, such as appending, deleting, or modifying characters without creating new objects. It's ideal for scenarios requiring frequent modifications to string data.

What is the difference between StringBuffer and StringBuilder class?

Which package is used for pattern matching with regular expressions?

java.util.regex package is used for pattern matching with regular expressions.

What is finalize() method?

The finalize() method in Java is called by the garbage collector just before an object is removed from memory. It allows the object to perform cleanup operations, such as releasing resources.