20 Basic Core Java Interview Questions & Answers- TCS, Accenture, Cognizant, Infosys, Wipro, HCL

Summary

In this informative video, Nishant from CareerRight delves into essential Java interview questions tailored for freshers. He covers Java's origins, its versatility across platforms, and its robust community support. The video explains Java's execution process, from source code compilation to bytecode interpretation by the JVM. Nishant also discusses access specifiers, the significance of the main method, and the nuances of static methods, constructors, and inheritance. He touches on method overloading, the final keyword's role in preventing overrides, and the organization of classes within packages. The video is a comprehensive guide for beginners looking to solidify their Java foundations.

Takeaways

• 😀 Java is a high-level, general-purpose programming language created by James Gosling in 1995 and is now owned by Oracle.
• 📝 Java supports multiple paradigms such as object-oriented, functional, and imperative, and it has a vast community support.
• 📱 Java is used for developing mobile apps, desktop applications, web applications, web servers, games, database connections, and more.
• 💻 A Java program is executed by being compiled into bytecode, which is then translated into machine code by the Java Virtual Machine (JVM).
• 🌐 Java is platform-independent because it compiles source code into platform-independent bytecode that can run on any platform with a JVM.
• 🔑 Java has four access specifiers: public, private, protected, and default, which define the accessibility of classes, interfaces, and variables.
• 🛠️ A method in Java is a block of code that performs a specific task and can take input data in the form of parameters.
• 🚀 The main method in Java is the entry point of a Java program, and it must be declared with the 'public static void' signature.
• 🔗 String[] args is a parameter in the main method that stores command-line arguments supplied when executing a Java program.
• 📚 A static method can be called without creating an object of the class, as it belongs to the class itself and not to any object.
• ⚙️ Method overloading allows multiple methods with the same name but different parameters within the same class.
• 🏭 Constructors in Java are special methods used to initialize objects and must have the same name as the class with no return type.
• 📘 Inheritance in Java allows a class (subclass) to acquire attributes and methods of another class (superclass), promoting code reusability.
• 🔒 The 'final' keyword in Java is used to prevent overriding and modifying variables or methods.
• 📦 Packages in Java are used to group related classes and avoid name conflicts, consisting of built-in packages and user-defined packages.
• 📖 An abstract class in Java is a restricted class that cannot be instantiated and can have both regular and abstract methods.

Q & A

What is Java?

Java is a high-level, general-purpose programming language created in 1995 by James Gosling and is now owned by Oracle. It is popular for being secure, fast, and powerful, supporting multiple paradigms such as object-oriented, functional, and imperative. Java is used to develop mobile applications, desktop applications, web applications, web servers, games, and database connections, and it runs on over 3 billion devices worldwide.

How is a Java program executed?

A Java program is written in a Java file as source code, then compiled into bytecode by the Java compiler. This bytecode is not directly executable by the machine; it is translated into machine code by the Java Virtual Machine (JVM), which is platform-specific, allowing the program to run on any platform that has a JVM.

How is Java platform-independent?

Java's platform independence is achieved through the use of the Java compiler, which converts source code into bytecode. This bytecode is platform-independent and can be executed on any platform using the appropriate JVM. For example, bytecode compiled on Windows can be translated and executed on Linux or macOS through their respective JVMs.

What are the various access specifiers in Java?

In Java, access specifiers define the scope or accessibility of classes, interfaces, variables, etc. There are four access specifiers: public, private, protected, and default. Public allows access from anywhere, private is accessible only within the class, protected is accessible within the same package, subclass, and within the class, and default (no specifier) is accessible within the same package.

What is a method in Java?

A method in Java is a block of code that performs a specific task and can also be called a function. Methods can take input data in the form of parameters and must be declared within a class. They are defined by a name followed by parentheses. Methods can be predefined or user-defined.

What is the significance of the main method in a Java program?

The main method is the entry point of a Java program, where execution starts. It is one of the most important methods in Java. The Java compiler or JVM looks for the main method when executing a program.

What are string arguments in the Java main method?

String arguments in the Java main method are used to handle command-line arguments supplied when executing a Java program. They are stored as an array of String objects and are necessary for the main function to be treated as the entry point of the program.

What is a static method in Java?

A static method in Java is a method that can be accessed directly by specifying its name without creating an object of the class. It belongs to the class, not to the object, and does not require any object state to be accessed.

What is method overloading in Java?

Method overloading in Java allows multiple methods to have the same name but different parameters, achieved by changing the number or data type of arguments. Overloading the main method is also possible, and the JVM will recognize the entry point by checking for the string argument parameters.

What is a constructor in Java?

A constructor in Java is a special method used to initialize an object. It must have the same name as the class and cannot have a return type. The default constructor is called every time an object is created using the 'new' keyword.

How many types of constructors are there in Java?

There are three types of constructors in Java: no-argument constructor, parameterized constructor, and default constructor. No-argument constructors have no parameters, parameterized constructors contain arguments, and default constructors are created by the compiler when no other constructor is defined.

What is the difference between a Java constructor and a Java method?

A Java constructor is a special method used to initialize an object and must have the same name as the class, with no return type. It is invoked implicitly. In contrast, a method is a block of code that performs a task, can have a return type, and may or may not have the same name as the class. Methods are invoked explicitly.

What is inheritance in Java?

Inheritance in Java is a process where a class acquires attributes and methods of another class, promoting code reusability. A subclass inherits from a superclass, gaining access to its attributes and methods.

How is inheritance implemented in a Java program?

In Java, inheritance is implemented using the 'extends' keyword, allowing a class to inherit attributes and methods from another class.

What is a final variable in Java?

A final variable in Java is a variable that cannot be changed once it has been assigned a value. It is used to prevent overriding and modification. If a method is declared as final, it cannot be overridden by any subclasses.

What is a package in Java?

A package in Java is used to group related classes and is a collection of related classes. It helps avoid name conflicts and allows for better maintainable code.

What are the different types of packages in Java?

Packages in Java are divided into built-in packages, which are pre-written and stored in the Java API, and user-defined packages, which are created by the user.

What is an abstract class in Java?

An abstract class in Java is a restricted class from which objects cannot be created directly. It can have both regular methods and abstract methods without bodies. To access its members, it must be inherited by a subclass.