Core Java interview questions and Answers

In Java, there are 10 OOPS concepts as follows:

• Abstraction
• Encapsulation
• Inheritance
• Polymorphism
• Association
• Composition
• Aggregation
• Delegation
• Coupling
• Cohesion

Here are some of the most important Java 10 features:

• Local-Variable Type Inference
• Consolidate the JDK Forest into a Single Repository
• Garbage-Collector Interface
• Parallel Full GC for G1
• Application Class-Data Sharing
• Thread-Local Handshakes
• Heap Allocation on Alternative Memory Devices
• Experimental Java-Based JIT Compiler
• Root Certificates
• Time-Based Release Versioning

A part of Java Run Environment (JRE), JVM is a virtual machine that provides runtime environment to drive Java programs or applications. It assists in converting bytecode into machine language and handles system memory.

Java does not support multiple inheritances because it may lead to overriding of methods when extended classes have the same method names.

Also, supporting multiple inheritances through classes may give rise to ambiguity errors. However, multiple inheritance is possible through the interface.

Java is not a pure OOP language as it uses primitive datatypes such as int float char double. A pure OOP language should use nothing but objects, and everything should be a class. But in Java, not everything is a class, such as byte, int, float, char, etc. This is the reason Java is a not a pure OOP language.

From version JDK6 onwards, the main method is mandatory to start code execution. If your program does not contain "main" method, you will get a run-time error. Please note that in the absence of "main" method your entire program will successfully compile but it will throw an error at runtime.

Access modifiers help to restrict the scope of a class, variable, constructor, method or data.

Four types of access modifiers are available:

• Default
• Private
• Protected
• Public