The Method Header and How a Method Works

Where you are: Week 0 review > The method header

Try This First

Decide whether this header is legal before reading on:

public int static larger(int a, int b)

Before You Start

Check each box you can do from memory. A box you cannot check yet is not a problem; it points you to a quick refresher, not a grade.

• I can name at least two primitive types that a method could return, such as int or boolean.
• I can state what a declared type fixes about a variable and why that type cannot change.
• I can write a valid variable declaration, including the type and an initial value.

See Variables and the Primitive Types if any box above gave you pause.

What You Need To Walk In With

Walk into the next class able to state these:

• A method is a named operation: fixed typed inputs in, one typed output out, or void for a side effect that returns nothing.
• The header order is fixed: public static ReturnType name(Type param). Rearranging the words is a compile error.
• A typed (non-void) method must return a value on every path that can reach the end.
• Calling a method substitutes its returned value back into the expression at the call site.

You should be able to: write a correct header from a description, and say whether a method should be void or typed.

How It Works

The six slots of a header

public static ReturnType name(Type param, Type param) {

The order is fixed. public int static foo(int n) does not compile.

A typed method returns a value

public static int larger(int a, int b) {
    if (a >= b) {
        return a;
    }
    return b;
}

Calling larger(10, 7) runs the body and substitutes 10 back into the call site. A typed method must return a value on every path that reaches the end; a println in a branch does not satisfy that rule.

A void method performs a side effect

public static void printBanner(String message) {
    System.out.println("== " + message + " ==");
}

A void method returns nothing. It may use a bare return; to exit early, but it must not return a value.

Worked Example: Predict, Then Check

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

System.out.println(doubled(6));

Predict the output before reading on.

A Common Mistake

A typed method that returns a value in one branch but only prints in another does not compile: the message is “missing return statement.” Every path that can reach the end of a typed method must end in a return (or throw). Add a return to the branch that lacks one. (Source: BJP (Reges and Stepp), Ch 3; JLS 25, §8.4.7.)

Go Deeper (optional)

For the curious: static is on every CS1 method because there is no object yet to call the method on. Once classes arrive, most methods drop static and run on a specific object, so the same name can behave differently depending on which object receives the call. The header slots stay in the same order; only the static slot changes.

Check Yourself

Close the notes and answer each one from memory, then reveal it. Pulling an idea back from memory is one of the strongest ways to make it stick.