Catch the Coins

Coins fall from the sky and you catch them. This is the first project with a score, which means the first project that has to remember something.

You need nothing but the three classes below. Every picture and sound comes with Scratch for Java.

Step 1: The catcher

Make a class called Basket. It is the same arrow-key movement as in your first program, just sitting near the bottom of the stage.

import org.openpatch.scratch.*;
 
public class Basket extends Sprite {
  public Basket() {
    this.addCostume("alienGreen_stand");
    this.setSize(35);
    this.setY(-150);
    this.setRotationStyle(RotationStyle.LEFT_RIGHT);
  }
 
  public void run() {
    if (this.isKeyPressed(KeyCode.RIGHT)) {
      this.setDirection(90);
      this.move(5);
    }
    if (this.isKeyPressed(KeyCode.LEFT)) {
      this.setDirection(-90);
      this.move(5);
    }
    this.ifOnEdgeBounce();
  }
}

Step 2: A falling coin

Make a class called Coin. Two new ideas appear here.

The first is a method of your own. dropFromTop() is not a Scratch for Java method, it is one you invent because you need the same three lines in two different places. In Scratch this is a custom block.

The second is Random.randomInt(a, b), which gives a whole number between a and b. It is the pick random block.

import org.openpatch.scratch.*;
 
public class Coin extends Sprite {
  public Coin() {
    this.addCostume("coinGold");
    this.setSize(40);
    this.dropFromTop();
  }
 
  private void dropFromTop() {
    this.setX(Random.randomInt(-270, 270));
    this.setY(Random.randomInt(200, 400));
  }
 
  public void run() {
    this.changeY(-3);
 
    if (this.isTouchingSprite(Basket.class)) {
      ((CatchStage) this.getStage()).addPoint();
      this.dropFromTop();
    }
 
    if (this.getY() < -200) {
      this.dropFromTop();
    }
  }
}

The coins start at a random height above the stage, so they do not all fall in a neat row.

Step 3: The stage that counts

The score lives on the stage, because it belongs to the game rather than to any one coin.

import org.openpatch.scratch.*;
 
public class CatchStage extends Stage {
  private Text scoreText = new Text();
  private int score = 0;
 
  public CatchStage() {
    super(600, 400);
    this.addBackdrop("background");
    this.addSound("handleCoins");
 
    this.scoreText.setPosition(0, 170);
    this.scoreText.setTextSize(22);
    this.add(this.scoreText);
    this.showScore();
 
    this.add(new Basket());
    for (int i = 0; i < 4; i++) {
      this.add(new Coin());
    }
  }
 
  public void addPoint() {
    this.score = this.score + 1;
    this.playSound("handleCoins");
    this.showScore();
  }
 
  private void showScore() {
    this.scoreText.showText("Coins: " + this.score);
  }
 
  public static void main(String[] args) {
    new CatchStage();
  }
}

Run it by right-clicking CatchStage and choosing new CatchStage().

Three things worth noticing:

• private int score is a variable that belongs to the stage. In Scratch you would have made a variable "for all sprites". This is the same idea: one number the whole game shares.
• The for loop creates four coins. In Scratch you would have copied the sprite four times by hand.
• addPoint() is called by the coin. A coin cannot change the stage's score directly, so the stage offers a method that does it. That is what ((CatchStage) this.getStage()).addPoint() means: "ask my stage to add a point".

The finished project

If something will not run, you can compare against the finished version. It needs no assets, only the library itself.

Things to try

• Make the coins fall faster the more you have caught.
• Add a second kind of sprite that costs you a point — bomb is in the built-in sprites.
• Stop the game after 30 seconds using getTimer().