Question 1

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

Accepted Answer

== compares object references; .equals() compares content/value. == checks if two references point to the same object in memory. .equals() checks if the objects are logically equal. Always use .equals() to compare Strings.

Question 2

What is a NullPointerException in Java?

Accepted Answer

An exception thrown when you try to use a null object reference. NullPointerException occurs when you call a method or access a field on a null reference. It's one of the most common runtime exceptions in Java.

Question 3

What is the key difference between ArrayList and LinkedList in Java?

Accepted Answer

ArrayList uses a dynamic array (fast random access); LinkedList uses a doubly linked list (fast insert/delete at ends). ArrayList provides O(1) random access but O(n) insertion/deletion in the middle. LinkedList provides O(1) insert/delete at ends but O(n) random access.

Question 4

What does the 'final' keyword do when applied to a variable in Java?

Accepted Answer

Makes the variable a constant — its value cannot be reassigned after initialization. 'final' on a variable prevents reassignment. On a method, it prevents overriding. On a class, it prevents inheritance. All three uses enforce immutability at different levels.

Question 5

What is the difference between checked and unchecked exceptions in Java?

Accepted Answer

Checked exceptions must be declared or caught at compile time; unchecked exceptions extend RuntimeException. Checked exceptions (e.g., IOException) are enforced by the compiler — you must catch or declare them. Unchecked exceptions (e.g., NullPointerException) extend RuntimeException and need not be declared.

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