Types, variables, and values

Java is statically typed: the compiler checks what values a variable may hold and which operations are valid.

Primitive and reference types

The eight primitives are byte, short, int, long, float, double, char, and boolean. A reference variable holds a reference to an object or null. Primitives are values and cannot be null.

int attempts = 3;
BigDecimal amount = new BigDecimal("19.95");
String customer = "Amina";

Use BigDecimal for exact decimal money rules. Construct it from a string or another exact representation; new BigDecimal(0.1) captures the binary floating-point approximation.

Declaration, initialization, and scope

Fields receive default values. Local variables must be definitely assigned before use. Keep scope as narrow as possible. A variable that exists only inside one loop is easier to reason about than shared mutable state.

var asks the compiler to infer a local variable's static type from its initializer. It is not dynamic typing and cannot be used without an initializer. Use it when the type remains obvious.

Conversions

Widening primitive conversions such as int to long are generally safe. Narrowing conversions require a cast and can lose information. Numeric promotion means arithmetic on byte or short normally produces int.

Autoboxing converts between primitives and wrappers such as int and Integer. Unboxing a null wrapper throws NullPointerException. Wrapper identity with == is not a value comparison.

Strings

String is immutable. Concatenation in a loop can create many intermediate objects; use StringBuilder for repeated assembly. Compare string values with equals, not ==.

Feynman check

A primitive variable is a labeled box containing a value. A reference variable is a labeled card pointing to an object. Two cards can point to the same object.