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Top Core Java Interview Questions & Answers

These questions cover fundamental Java concepts as discussed in the Code Decode tutorial.

1. Why is Java not a purely Object-Oriented language?​

Java is not considered 100% object-oriented because it supports primitive data types like int, char, float, double, etc. In a purely object-oriented language, everything should be an object.

2. What makes Java platform independent?​

Java's independence comes from its byte code. When you compile a Java program, the compiler converts the source code into byte code rather than machine-specific code. This byte code can run on any operating system (Windows, Linux, Mac) provided the system has a Java Virtual Machine (JVM) to interpret it.

3. Why is Java both interpreted and compiled?​

Java uses a two-step execution process:

  1. Compiler: The javac compiler converts source code (.java) into byte code (.class).
  2. Interpreter/JIT: The JVM interprets the byte code. To improve performance, it uses a Just-In-Time (JIT) compiler to compile frequently executed byte code into native machine code.

4. Why are Strings immutable in Java?​

Strings are immutable (cannot be changed once created) for several key reasons:

  • String Pool: It allows multiple references to point to the same string literal, which saves significant memory.
  • Security: Since Strings are used for parameters like file paths, network connections, and database URLs, immutability ensures these values cannot be altered maliciously.
  • Caching: The hashcode of a String is cached at the time of creation, making it very efficient for use as a key in a HashMap.

5. What is a Marker Interface?​

A Marker Interface is an interface that does not contain any methods or fields. Examples include Serializable, Cloneable, and Remote. They serve as a "tag" to inform the JVM or a compiler that the implementing class has a specific behavior or capability.

6. Can we override a static method?​

No. If you define a static method with the same signature in a subclass, it is known as method hiding, not method overriding. Static methods are bound to the class at compile-time (static binding), whereas overriding relies on the object type at runtime (dynamic binding).

7. What is the difference between final, finally, and finalize?​

  • final: A keyword used to define constants (variables), prevent method overriding, or prevent class inheritance.
  • finally: A block used in exception handling (try-catch-finally) that is guaranteed to execute whether an exception is thrown or not, typically used for closing resources.
  • finalize(): A protected method in the Object class that the Garbage Collector calls just before an object is destroyed to perform cleanup operations.

8. How do you create an Immutable Class?​

To create a custom immutable class, you should:

  1. Declare the class as final so it cannot be extended.
  2. Make all fields private and final.
  3. Do not provide any "setter" methods.
  4. Initialize all fields through a constructor.
  5. If the class contains mutable objects, perform deep copies in the constructor and getter methods to prevent external modification.

9. What is a Singleton Class and how is it created?​

A Singleton class ensures that only one instance of the class is created within a single JVM.

  • Private Constructor: Ensures the class cannot be instantiated from outside.
  • Private Static Variable: Holds the single instance of the class.
  • Public Static Method: (often named getInstance()) Provides the global access point to the instance, often implementing lazy initialization.