https://www.softwaretestinghelp.com/core-java-interview-questions/

1. What is Java?

Java is a high-level programming language and is platform independent.
Java is a collection of objects. It was developed by Sun Microsystems. There are a lot of applications, websites and Games that are developed using Java.

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Platform independent = programming language or framework that will work on any OS (Java is, but C and C++, for instance, are not).

2. What are the features in Java?

Platform independent

OOPS concepts

• OO
• inheritance
• encapsulation
• polymorphism
• abstraction
  
  

JIT

• Just In Time compiler
• enables high performance in Java
• JIT converts the bytecode into machine language and then the JVM starts the execution
  
  

Multi-threaded

• thread = one flow of execution
• JVM = creates a thread called main thread
• user can create multiple threads by extending the thread class or by implementing a runnable interface

3. How does Java enable high performance?

Java uses Just In Time compiler to enable high perf. JIT is used to convert the instructions into bytecodes.

Warning: is JIT used to convert instruction > bytecodes or bytecodes to machine language (as mentioned in Q2)

4. What are the Java IDE's?

Eclipse & NetBeans

Constructor overloading is a technique in Java in which a class can have any number of constructors that differ in parameter list. The compiler differentiates these constructors by taking into account the number of parameters in the list and their type.

6. What is meant by local variable and instance variable?

Local variable

defined in the method and scope of the variables that have existed inside the method itself.

Instance variable

defined inside the class and outside the method and scope of the variables exist throughout the class.

7. What is a class?

All Java codes are defined in a class. A class has variables and methods.
Variables are attributes which define the state of a class.
Methods are the place where the exact business logic has to be done. It contains a set of statements (or) instructions to satisfy the particular requirement.

8. What is an object?

An instance of a class is called object. The object has a state and behavior. Whenever the JVM reads the "new()" keyword, then it will create an instance of that class.

10. What is inheritance?

Inheritance means one class can extend to another class. So that the code can be reused from one class to another class. Existing class is known as super class whereas the derived class is known as a sub class.
Inheritance is applicable for public and protected members only. Private members cannot be inherited.

11. What is encapsulation?

Why?

• protects the code from others
• code maintainability
  
  

How?

• make the variable private or protected
• use public accessors methods such as set<property> and get<property>

12. What is polymorphism?

A single object can refer to the super class or sub-class depending on the reference type which is called polymorphism. Polymorphism is applicable for overriding and not for overloading.

14. What is meant by overloading?

Rem: can happen for different classes or within the same class

Conditions

• same method name
• different argument type
• possibly different return types
  
  

15. What is meant by interface?

Multiple inheritance cannot be achieved in Java. To overcome this problem, the interface concept has been introduced.
An interface is a template which has only method declarations and no method implementation.
The class which implements the interface should provide implementation for all the methods declared in the interface.

16. What is meant by abstract class?

We can create an abstract class by using the "abstract" keyword before the class name. An abstract class can have bot abstract methods and non-abstract methods that are a concrete class.
The concrete sub-class which extends an abstract class should provide the implementation for abstract methods.