Primitive types vs reference types

Java has 8 primitives: byte, short, int, long, float, double, char, boolean. A primitive variable holds the value itself. Every other type (classes, interfaces, arrays, enums, records) is a reference type: the variable holds a reference to an object on the heap.

int a = 5; int b = a; b++;             // a is still 5: the value was copied
int[] x = {5}; int[] y = x; y[0]++;    // x[0] is now 6: both refer to one array

Java is always pass-by-value: for references, the reference is copied, so a method can change the object but cannot make the caller's variable point somewhere else. Wrapper classes (Integer, Double, …) bridge the two worlds via autoboxing.