Primitive Types & Wrappers

Java has eight primitives: byte, short, int, long, float, double, char, and boolean. Each is a fixed-size raw value, not an object.

Primitive locals live on the stack, primitive instance fields live on the heap inside their object, and primitive statics live in Metaspace with the class.

Arithmetic promotes operands to at least int and up to the widest type. Widening is automatic. Narrowing needs an explicit cast that can truncate or overflow. Compound += hides the cast, which is why b += 1 compiles but b = b + 1 does not.

Autoboxing wraps a primitive into a wrapper object via valueOf(). Unboxing extracts the primitive via intValue(). The Integer cache makes == work for -128 to 127 but fail outside that range.

Integer.valueOf() caches objects for -128 to 127. Inside the cache, == returns true because both references point to the same shared object. Outside, == returns false because each call creates a new object. Use .equals() to compare values.

Unboxing throws NullPointerException whenever a null wrapper reference is forced into a primitive, because the hidden intValue() or booleanValue() call dereferences null.

Each primitive has a wrapper class (int→Integer, char→Character, etc.) that boxes the value into an object for use in collections, generics, and nullable contexts. parseInt returns a primitive, valueOf returns a cached wrapper.

parseInt throws NumberFormatException for invalid input. Use the radix overload to parse hex (base 16) or binary (base 2). Use toHexString/toBinaryString to convert back.

char is a 16-bit unsigned type that holds one UTF-16 code unit. Most characters fit in one char, but emoji and rare scripts need two chars (a surrogate pair).

Widening goes from a smaller type to a larger type and is automatic. Narrowing goes from a larger type to a smaller type, requires a cast, and can lose data through truncation or overflow.

In a mixed expression, Java promotes both operands to a common type (at least int) before computing. This is called binary numeric promotion.