The questions are organized into five main themes: Basics and Syntax, Object-Oriented Programming (OOP), Exception Handling and Assertions, Core APIs, and Concurrency. By addressing these areas, this article aims to equip you with the confidence and knowledge needed to excel in Java interviews, whether you’re tackling core Java interview questions or more specific Java 8 interview questions.
1. Basics and Syntax
Question 1: What is Java?
Answer: Java is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is a general-purpose programming language intended to let application developers write once, run anywhere (WORA), meaning that compiled Java code can run on all platforms that support Java without the need for recompilation.
Question 2: Explain the JDK, JRE, and JVM.
Answer:
- JVM (Java Virtual Machine): The JVM is an engine that provides a runtime environment to drive the Java Code or applications. It converts Java bytecode into machine language.
- JRE (Java Runtime Environment): JRE is a part of software that is designed to run other software. It contains the set of libraries + other files that JVM uses at runtime.
- JDK (Java Development Kit): The JDK is a software development environment used for developing Java applications and applets. It physically exists. It contains JRE + development tools.
Question 3: What are variables in Java?
Answer: Variables are containers for storing data values. In Java, each variable must be declared with a data type that designates the type and quantity of value it can hold. Java is a statically typed language, meaning variables must be defined before they are used.
Question 4: What is typecasting in Java?
Answer: Typecasting is the process of converting a variable from one type to another. In Java, there are two types of casting:
- Widening Casting (Implicit): automatic type conversion from a smaller to a larger type
- Narrowing Casting (Explicit): needs explicit conversion to convert a larger type to a smaller type
Question 5: How do you declare an array in Java?
Answer: An array is a container object that holds a fixed number of values of a single type. The declaration of an array in Java is as follows:
int[] myIntArray = new int[10]; // declares an array of integers
String[] myStringArray = new String[50]; // declares an array of strings
Question 6: Explain the main method in Java.
Answer: The main method is the entry point for any Java program. It must be public, static, return no value (void), and accept a String array as a parameter. It signature is:
public static void main(String[] args) {
// code to be executed
}
Question 7: What are literals in Java?
Answer: Literals refer to the fixed values assigned to variables in Java. For example, 100, -90, 3.14F, ‘A’, and “Hello” are all literals.
Question 8: What is a constructor in Java?
Answer: A constructor in Java is a block of code similar to a method that’s called when an instance of an object is created. Unlike methods, constructors have no explicit return type and have the same name as the class itself.
Question 9: Explain method overloading in Java.
Answer: Method overloading is a feature that allows a class to have more than one method having the same name, if their parameter lists are different. It is related to compile-time (or static) polymorphism.
Question 10: What is a package in Java?
Answer: A package in Java is a namespace that organizes a set of related classes and interfaces. Conceptually you can think of packages as being similar to different folders on your computer.
2. Object-Oriented Programming (OOP)
Question 11: What is Object-Oriented Programming?
Answer: Object-oriented programming (OOP) is a programming paradigm based on the concept of “objects”, which can contain data in the form of fields (often known as attributes or properties) and code in the form of procedures (often known as methods).
Question 12: What are the main principles of OOP?
Answer:
- Encapsulation: The binding (or wrapping) of data and methods that operate on the data into a single unit called a ‘class’. It also means hiding data (i.e., private variables) from direct access.
- Abstraction: Hiding the complex implementation details of an operation while exposing a simple interface.
- Inheritance: Allows a new class to inherit properties and methods of an existing class.
- Polymorphism: The ability of different classes to provide a unique interface by exposing a method that can behave differently.
Question 13: What is inheritance?
Answer: Inheritance in Java is a mechanism where one object acquires all the properties and behaviors of a parent object. It is an important part of OOPs (Object-Oriented programming systems).
Question 14: What is an interface?
Answer: An interface in Java is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. Interfaces cannot contain instance fields. The methods in interfaces are abstract by default.
Question 15: Explain the difference between abstract classes and interfaces.
Answer:
- Abstract Class: Can have both abstract and non-abstract methods. Abstract classes are used to provide a base for subclasses to extend and implement the abstract methods.
- Interface: Typically contains abstract methods only. Starting from Java 8, it can also contain default and static methods. Interfaces are used to implement abstraction.
Question 16: What is polymorphism?
Answer: Polymorphism in Java is the ability of an object to take on many forms. Most commonly, it is when a parent class reference is used to refer to a child class object.
Question 17: Explain method overriding.
Answer: Method overriding, in object-oriented programming, is a language feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its superclasses or parent classes. The method that is overridden is called an overridden method.
Question 18: What is the “super” keyword?
Answer: The super keyword in Java is a reference variable that is used to refer to parent class objects. The keyword can be used to call superclass methods and to access the superclass constructor.
Question 19: What are getters and setters in Java?
Answer: Getters and setters are methods that get and set the value of a private variable. For example:
public class Data {
private int num;
public int getNum() {
return num;
}
public void setNum(int num) {
this.num = num;
}
}
Question 20: What is static in Java?
Answer: The keyword static means that the particular member belongs to a type itself, rather than to an instance of that type. This means that only one instance of that static member is created which is shared across all instances of the class.
