What is the difference between primitives and wrapper classes?

Java has eight primitives: boolean, byte, short, char, int, long, float and double. Each has a corresponding wrapper such as Integer or Double. Autoboxing converts between them automatically, but the conversion is not free and the identity rules are surprising.

Integer a = 127;
Integer b = 127;
Integer c = 128;
Integer d = 128;

System.out.println(a == b);
System.out.println(c == d);

Where each is appropriate

• Use primitives for arithmetic, loop counters and hot paths
• Use wrappers when null is meaningful, such as an optional database column
• Use wrappers in generics because type parameters cannot be primitives
• Use equals rather than == when comparing wrapper values

Unboxing a null wrapper throws NullPointerException. That commonly appears when a missing database value is assigned to an int field. Prefer primitive types in method signatures when absence is not allowed, and validate wrapper inputs at the boundary.