METHODS.JAVA

Methods

Write something once, use it anywhere.

Download week 4 Four files, one per problem.

A method is a machine

Something goes in, something comes back. Drag it.

n 5

goes in5

public static int square(int n) {
    return n * n;
}

comes back25

The trade

The same lines get copied twice, once for each fighter. A method lets them be written once and called twice. When the formula is wrong, fix it in one place.

The shape

public staticalways this

intgives back

squarename

(int n)takes in

Methods go inside the class, next to main, never inside it.

public class Squares {
    public static void main(String[] args) {
        System.out.println(square(7));
    }

    public static int square(int n) {
        return n * n;
    }
}

Order does not matter

Java finds square whether written above or below main. Place methods after main so the reader sees the program first.

Parameters

A parameter is a variable the method is handed when it is called.

public static void greet(String who) {
    System.out.println("Hello, " + who + "!");
}

Code	who becomes	It prints
`greet("Sam")`	`"Sam"`	Hello, Sam!
`greet("Adi")`	`"Adi"`	Hello, Adi!

The name who only exists inside the method. Nothing outside can see it.

Returning a value

Two kinds of method. The difference is what sits before the name.

	Does	Call it
`void`	Acts, hands nothing back	on its own line
`int`, `double`, `String`	Works something out and hands it back	where a value would go

greet("Sam");                    // void, a whole statement

int answer = square(7);          // returns, so it goes on the right of =
System.out.println(square(7));   // or straight into a println

The one that catches everyone

return ends the method immediately. Anything written after it never runs.

Problems

1. Say it three times

Hello.java

Write sayHello(). It takes nothing and returns nothing. Call it three times from main.

Example

Hello there!
Hello there!
Hello there!

Hint

No value goes in and none comes back, so it is public static void sayHello() with empty brackets.

2. Greet by name

Greet.java

Ask for a name, then pass it to greet.

Example

Your name: Sam
Hello, Sam!

Hint 1

The method already has its parameter. Only the call is missing: greet(name);

Hint 2

The variable in main and the parameter need not share a name. greet(name) passes the value, and inside the method it is called who.

3. Squares

Squares.java

Finish square so it hands back n * n. Print the square of 3, 7 and 12.

Example

9
49
144

Hint

Replace return 0; with return n * n;

Then try this

Add cube next to it, and print square(3) + cube(2). Two method calls in one expression work fine.

4. Battle Sim, without the copy and paste

BattleScore.java

In Battle Sim the combat formula is typed twice, once per fighter. Put it in a method and call it twice.

Example

Fighter 1 strength, armor, stamina? 80 60 55
Fighter 2 strength, armor, stamina? 70 90 40

Fighter 1 combat = 68.8
Fighter 2 combat = 67.5

Hint 1

The method takes three int values and hands back a double, so it starts public static double warriorScore(int strength, int armor, int stamina).

Hint 2

Keep the 4.0. All three parameters are whole numbers, so dividing by 4 would throw the decimal away. Same trap as week 2.

Hint 3

Now change the formula to divide by 5.0 and run it. Notice only one line needed editing for both fighters. That is the whole point of the week.

Rules

Methods go inside the class, beside main, never inside it.
Every method that promises a type must return that type on every path.
Keep a method short. If it spans more than one screen, it is doing too much.
Name it for what it does. square, not doIt.
Name, date and one sentence at the top of every file.
Stuck fifteen minutes on one thing? Stop and ask.

What this unlocks

Every assignment from here on can use methods, and the rule that banned them is gone. Going back and rewriting Battle Sim with them is worth an hour.

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