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.

an alien catching falling coins while the score counts up

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();
    }
  }
}

isTouchingSprite(Basket.class) asks "am I touching any basket?" rather than naming one particular sprite. That is why the coin does not need to be told which basket exists.

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().

All three classes together, running here — click the stage and use the arrow keys:

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.

Project: Catch the Coins 100%

Things to try

Faster and faster

Make the coins fall faster the more you have caught.

Hint

Give Coin a field speed and use it in changeY. Every time the coin is caught, make speed a little bigger.

Solution

Each coin speeds up every time it is caught, so the more you catch, the faster they fall.

import org.openpatch.scratch.*;
 
public class Coin extends Sprite {
  private double speed = 3;
 
  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(-this.speed);
 
    if (this.isTouchingSprite(Basket.class)) {
      ((CatchStage) this.getStage()).addPoint();
      this.speed = this.speed + 0.5;
      this.dropFromTop();
    }
 
    if (this.getY() < -200) {
      this.dropFromTop();
    }
  }
}

Watch out for bombs

Add a bomb that costs you a point. bomb is one of the built-in sprites.

Hint

A bomb behaves almost like a coin, so start with a copy of the Coin class and call it Bomb. The stage needs a second method, removePoint(), next to addPoint().

Solution

import org.openpatch.scratch.*;
 
public class Bomb extends Sprite {
  public Bomb() {
    this.addCostume("bomb");
    this.setSize(40);
    this.dropFromTop();
  }
 
  private void dropFromTop() {
    this.setX(Random.randomInt(-270, 270));
    this.setY(Random.randomInt(300, 600));
  }
 
  public void run() {
    this.changeY(-4);
 
    if (this.isTouchingSprite(Basket.class)) {
      ((CatchStage) this.getStage()).removePoint();
      this.dropFromTop();
    }
 
    if (this.getY() < -200) {
      this.dropFromTop();
    }
  }
}

The stage adds a bomb and gets the new method:

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());
    }
    this.add(new Bomb());
  }
 
  public void addPoint() {
    this.score = this.score + 1;
    this.playSound("handleCoins");
    this.showScore();
  }
 
  public void removePoint() {
    this.score = this.score - 1;
    this.showScore();
  }
 
  private void showScore() {
    this.scoreText.showText("Coins: " + this.score);
  }
}

Thirty seconds

Stop the game after 30 seconds.

Hint

The stage has a run() too, and a timer: this.getTimer().afterMillis(30000) is true once 30 seconds have passed. Window.getInstance().pause() stops the game and leaves the last picture on the screen.

Solution

Add this method to CatchStage:

public void run() {
  if (this.getTimer().afterMillis(30000)) {
    this.scoreText.showText("Time's up! Coins: " + this.score);
    Window.getInstance().pause();
  }
}