Core Java Interview Question

1. What are the main features of Java?

1. Object-Oriented: Everything in Java is an object.
2. Platform-Independent: Java code is compiled into bytecode, which can run on any platform with a JVM.
3. Robust: Java has strong memory management, exception handling, and garbage collection.
4. Secure: Java provides a secure runtime environment with features like bytecode verification and no explicit pointer usage.
5. Multithreaded: Java supports concurrent execution of threads.
6. High Performance: Java uses Just-In-Time (JIT) compilers for better performance.

2. What is the difference between JDK, JRE, and JVM?

1. JDK (Java Development Kit): A development environment that includes tools like the compiler (javac), debugger, and JRE.
2. JRE (Java Runtime Environment): Provides the libraries and JVM required to run Java applications.
3. JVM (Java Virtual Machine): Executes Java bytecode and provides platform independence.

3. What is the difference between == and equals() in Java?

• == compares references (memory addresses) for objects and values for primitives.
• equals() compares the content of objects. By default, it behaves like ==, but it can be overridden in classes like String.

Example:

​​Strings1=newString("Hello");
Strings2=newString("Hello");
System.out.println(s1 == s2); // false (different references)
System.out.println(s1.equals(s2)); // true (same content)

4. What is the difference between String, StringBuilder, and StringBuffer?

• String: Immutable; every modification creates a new object.
• StringBuilder: Mutable and not synchronized; faster for single-threaded operations.
• StringBuffer: Mutable and synchronized; thread-safe but slower than StringBuilder.

5. What are the access modifiers in Java?

1. Public: Accessible from anywhere.
2. Protected: Accessible within the same package and subclasses.
3. Default (no modifier): Accessible within the same package.
4. Private: Accessible only within the same class.

6. What is the difference between final, finally, and finalize()?

• final: A keyword used to declare constants, prevent method overriding, or inheritance.
• finally: A block in exception handling that always executes, regardless of whether an exception is thrown.
• finalize(): A method called by the garbage collector before an object is destroyed (deprecated in modern Java).

7. What is the difference between Array and ArrayList?

• Array: Fixed size, can store primitives and objects.
• ArrayList: Dynamic size, can only store objects, and provides utility methods like add(), remove(), etc.

8. What is the difference between HashMap and Hashtable?

• HashMap: Not synchronized, allows one null key and multiple null values.
• Hashtable: Synchronized, does not allow null keys or values.

9. What is the difference between Abstract Class and Interface?

• Abstract Class:
  
  • Can have abstract and concrete methods.
  • Supports constructors.
  • Can have instance variables.
• Interface:
  • All methods are abstract (until Java 8, which introduced default and static methods).
  • Cannot have constructors.
  • Only constants (static final variables) are allowed.
  
  

10. What is the difference between throw and throws?

• throw: Used to explicitly throw an exception.
• throws: Declares exceptions that a method might throw.

Example:

voidmethod() throws IOException {
thrownewIOException("Exception thrown");
}

11. What is the difference between checked and unchecked exceptions?

• Checked Exceptions: Must be handled or declared using throws (e.g., IOException, SQLException).
• Unchecked Exceptions: Runtime exceptions that do not need to be declared or handled (e.g., NullPointerException, ArithmeticException).

12. What is the difference between static and instance variables?

• Static Variables: Shared across all instances of a class; belong to the class.
• Instance Variables: Unique to each object; belong to the instance.

13. What is the difference between overloading and overriding?

• Overloading: Same method name, different parameter lists (compile-time polymorphism).
• Overriding: Subclass provides a specific implementation of a method in the parent class (runtime polymorphism).

14. What is the purpose of the transient keyword?

The transient keyword is used to mark a variable that should not be serialized.

15. What is the purpose of the synchronized keyword?

​The synchronized keyword ensures that only one thread can access a block of code or method at a time, preventing race conditions.

16. What is the difference between wait(), notify(), and notifyAll()?

1. wait(): Causes the current thread to wait until another thread calls notify() or notifyAll() on the same object.
2. notify(): Wakes up one thread waiting on the object's monitor.
3. notifyAll(): Wakes up all threads waiting on the object's monitor.

17. What is the difference between Comparable and Comparator?

1. Comparable: Used to define natural ordering of objects. Implemented using compareTo().
2. Comparator: Used to define custom ordering. Implemented using compare().

18. What is the purpose of the volatile keyword?

The volatile keyword ensures that changes to a variable are immediately visible to all threads. It prevents caching of the variable by threads.

19. What is the difference between HashSet and TreeSet?

• HashSet: Unordered, allows null, and uses a hash table for storage.
• TreeSet: Ordered (sorted), does not allow null, and uses a red-black tree for storage.

20. What is the purpose of the default keyword in interfaces (Java 8)?

The default keyword allows interfaces to have concrete methods with a default implementation. This helps in adding new methods to interfaces without breaking existing implementations.