Unit 3.5: Methods: How to Write Them

Methods define the behaviors of an object—the actions it can perform.

Methods allow us to organize code into reusable blocks.

Void Methods

A void method performs an action but does not give any data back to the code that called it.

• The method header must include the word void.

Task: Implementing a basic void method.

public void sayHello() {
    System.out.println("Hello!"); // Just performs an action
}

Non-Void Methods

A non-void method is like a vending machine: you call it, and it "spits out" a single piece of data (a value).

• Instead of void, the header specifies the return type (like int, double, or String).

Task: Implementing a non-void method that returns a value.

public int getLuckyNumber() {
    return 7; // Gives back an integer
}

Return by Value

When a non-void method finishes, it evaluates the expression next to the return keyword and sends that value back. This is called return by value.

Task: Returning the result of a calculation.

public double calculateTax(double price) {
    return price * 0.08; // Evaluates this math and returns the result
}

The return Keyword

The return keyword immediately stops the method and sends control back to whoever called it.

• Unreachable Code: You cannot put any code after a return statement in the same block. It will cause a compiler error.
• Early Exit: A return can be used inside an if statement or a loop to stop the method early. If it's inside a loop, it stops the loop and exits the method immediately.

Task: Using return for early method termination.

public void checkAge(int age) {
    if (age < 0) {
        return; // Stops the method immediately if age is invalid
    }
    System.out.println("Age is valid.");
}

Accessor Methods (Getters)

An accessor method allows other classes to "see" or get a copy of an object's private data.

• Accessors are always non-void methods.

Task: Implementing an accessor (getter) method.

public class Player {
    private int score;
    
    // Accessor (Getter)
    public int getScore() {
        return score; 
    }
}

Mutator Methods (Setters)

A mutator method (also called a modifier) is used to change the value of an object's data.

• Mutators are often void methods.

Task: Implementing a mutator (setter) method.

public class Player {
    private int score;

    // Mutator (Setter)
    public void setScore(int newScore) {
        score = newScore;
    }
}

Method Parameters

Parameters are the inputs a method needs to do its job. They are defined in the parentheses of the method header.

Task: Defining a method with multiple parameters.

// 'name' and 'times' are parameters
public void repeatName(String name, int times) {
    for (int i = 0; i < times; i++) {
        System.out.println(name);
    }
}

Pass by Value (Primitives)

When you pass a primitive (like int, double, or boolean) to a method, Java sends a copy of that value.

• If the method changes its local copy of the parameter, it does not change the original variable outside the method.

Task: Demonstrating that primitives are passed by value.

public void increment(int x) {
    x = x + 1; // Only changes the local copy
}

// --- In main ---
int myNum = 10;
increment(myNum);
System.out.println(myNum); // Still prints 10!

Examples

Complete Class: TemperatureSensor

This example combines all the concepts of method design.

Task: Reviewing a complete class with accessors, mutators, and behaviors.

public class TemperatureSensor {
    private double currentTemp;

    public TemperatureSensor(double startTemp) {
        currentTemp = startTemp;
    }

    // Accessor
    public double getTemp() {
        return currentTemp;
    }

    // Mutator with validation
    public void setTemp(double newTemp) {
        if (newTemp > -273.15) { // Cannot be below absolute zero
            currentTemp = newTemp;
        }
    }

    // Void method with parameter
    public void logStatus(String location) {
        System.out.println("Sensor at " + location + " reads: " + currentTemp);
    }
}