Calling Methods: Parameters and Return Values

Arguments are copied in; results come back only through the return value

A method call has three moving parts: the object (or class) you call it on, the arguments you pass, and the value it returns. A method whose header says void returns nothing, so writing double r = box.print(); is a compile-time error, not a run-time surprise.

public static int doubled(int n) {
    n = n * 2;
    return n;
}

Java passes every argument by value. For a primitive that means the method receives a copy of the number, so changing n inside doubled leaves the caller's variable untouched. For an object the copied thing is the reference: the method cannot make your variable point somewhere else, but it can call mutator methods on the object you both share, and those changes are visible to the caller.

The other half of the rule is that a returned value is thrown away unless you store it or use it. doubled(score); on a line by itself compiles and runs, computes the answer, and discards it. You must write score = doubled(score);. Static methods are called on the class (Math.abs(-4)), instance methods on an object (word.length()); mixing the two up is a compile-time error.