What is the difference between primitive types and reference types in Java?

Java has exactly 8 primitive types: byte, short, int, long, float, double, char, boolean. A primitive variable stores its value directly (in a stack frame, or inline within an object/array) — there's no separate object, no header, no reference indirection.

Everything else — classes, interfaces, arrays, enums — is a reference type. A reference-type variable stores a pointer to an object allocated on the heap; multiple variables can reference the same object, and the variable itself may be null.

int x = 5;              // value lives directly in x
Integer boxed = 5;      // x is a reference to a heap-allocated Integer object

int[] arr = {1, 2, 3};  // arr is a reference to a heap array object

Key practical differences:

• Defaults: numeric primitives default to 0/0.0, boolean to false, char to ' '; references default to null.
• Equality: == compares primitive values directly, but compares references (identity) for objects — use .equals() for value equality on objects.
• Passing to methods: both are passed by value, but for a reference type that "value" is the reference itself, so mutating the referenced object is visible to the caller, while reassigning the parameter is not.