Topic 3.5 — Methods: How to Write Them

Goal: write method headers and bodies with correct return types, parameters, and control flow, and understand how methods help with abstraction, reuse, and organization.

The big idea

A method is a named block of code that performs a task. Writing good methods means designing clear inputs (parameters), clear outputs (return values), and a body that does exactly what the header promises.

Method header anatomy

Part	Example	Meaning
access	`public`	who can use it
return type	`int` / `boolean` / `void`	what it gives back
name	`calcArea`	what it’s called
parameters	`(int w, int h)`	inputs to the method

Example: a method that returns a value

public int calcArea(int width, int height) {
  int area = width * height;
  return area;
}

Return type is int, so it must return an int.
return ends the method immediately.

Void methods

A void method does a task but does not return a value.

public void reset() {
  value = 0;
}

You can still use return; in a void method to exit early, but it returns nothing.

Return-type mistakes

If return type isn’t void, every path must return a value.
Don’t return the wrong type (Java will error).

// Bug: missing return when x <= 0
public int f(int x) {
  if (x > 0) return x;
  // ❌ no return here
}

Parameters are local variables

Parameters behave like variables inside the method.
Changes to primitive parameters (int, double, boolean) do not change the original variable outside.
You can use parameters to avoid repeated code.

public boolean isEven(int n) {
  return n % 2 == 0;
}

Writing methods: a simple workflow

Decide the method’s job (1 sentence).
Pick parameter types/names (inputs).
Pick return type (output) or void.
Write the body using variables + selection/iteration.
Check: do all paths return (if needed)?

Quick self-check

What are the 4–5 main parts of a method header?
What does void mean?
Why must every path return a value for a non-void method?
Are parameters local variables?
Write a method max(int a, int b) that returns the larger value.

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