Variables and types

Java variables have an explicit type declared before the name: int count = 0;. Unlike JavaScript or PHP, you cannot silently change a variable from int to String without a compile error.

Primitive types (common)

• int, long — integers
• double, float — floating point (prefer double)
• boolean — true or false only
• char — single Unicode character in single quotes

Reference types

Objects and arrays are references: String name = "Ada";, int[] scores = {90, 85};. Primitives hold values directly; references point to heap objects.

final and var

final int MAX = 100; cannot be reassigned. Java 10+ var infers type locally: var list = new ArrayList<String>();

Important interview questions and answers

1. Q: Primitive vs wrapper?
   A: int is a primitive; Integer is an object wrapper used in generics and nullable contexts.
2. Q: Is String a primitive?
   A: No—String is an immutable reference type.
3. Q: What is autoboxing?
   A: Automatic conversion between primitives and wrappers, e.g. Integer n = 5;

Self-check

1. Name two primitive numeric types.
2. What type does "hello" have?

Pitfall: Use equals for String content—== compares references and breaks comparisons of separate String objects.