Differences to Scratch

Most of Scratch carries over to Scratch for Java unchanged: the stage, sprites, costumes, events and messages. This page is about the few places where Java asks you to think differently. If you remember only one thing, remember this:

No forever: use run()

when green flag clicked
forever
move (10) steps

Everything you would put in a forever loop goes into run(). The library calls it for you, once per frame, for as long as the program runs:

public class Cat extends Sprite {
  public Cat() {
    this.addCostume("cat", "cat_idle_1");
  }
 
  public void run() {
    this.move(10);
  }
}

Do not write a while (true) loop inside run(). It never finishes, so the next frame is never drawn and the program freezes.

No wait in a sprite: use a timer

when green flag clicked
forever
next costume
wait (1) seconds

A sprite cannot stop and wait, because run() has to finish every frame. Instead you ask a timer whether enough time has passed:

public class Blinker extends Sprite {
  public Blinker() {
    this.addCostume("closed", "alienGreen_stand");
    this.addCostume("open", "alienGreen_front");
  }
 
  public void run() {
    if (this.getTimer().everyMillis(1000)) {
      this.nextCostume();
    }
  }
}

everyMillis(1000) is true once every second and false in all the frames in between. Times in Scratch for Java are usually in milliseconds: 1000 is one second. Make it Walk builds on this.

Things that do not hold up the script

In Scratch some blocks pause the script until they are done. In Java they start something and return straight away, and run() carries on:

If something should happen one after another, keep a variable that remembers which step you are at, and move on to the next step when the current one is done.

Variables belong to an object

In Scratch you choose between for this sprite only and for all sprites. In Java a variable is written at the top of a class, and every object of that class gets its own.

• For this sprite only is a variable in the sprite's class. Each coin of the class Coin has its own speed.
• For all sprites is a variable in the stage's class. The sprites reach it through their stage, like the score in Catch the Coins.

public class Coin extends Sprite {
  private int speed = 3;
 
  public Coin() {
    this.addCostume("coin", "coinGold");
  }
 
  public void run() {
    this.changeY(-this.speed);
    if (this.isTouchingEdge()) {
      ((CoinStage) this.getStage()).addPoint();
      this.setY(180);
    }
  }
}
 
public class CoinStage extends Stage {
  private int points = 0;
 
  public CoinStage() {
    this.add(new Coin());
  }
 
  public void addPoint() {
    this.points = this.points + 1;
  }
}

Variables are not shown on the stage by themselves, because a Java variable has no checkbox. showVariable puts one in the top left corner, like in Scratch:

public class Player extends Sprite {
  private int lives = 3;
 
  public Player() {
    this.addCostume("player", "bunny1_stand");
    this.showVariable("lives", () -> this.lives);
  }
}

Clones

Clones work like in Scratch. clone() is create clone of myself, whenStartsAsClone() is when I start as a clone, and deleteThisClone() removes it again. A clone starts with a copy of the original's variables. Its constructor does not run again.

when green flag clicked
forever
create clone of [myself v]
wait (1) seconds
when I start as a clone
go to [random position v]

public class Star extends Sprite {
  public Star() {
    this.addCostume("star", "star1");
  }
 
  public void whenStartsAsClone() {
    this.goToRandomPosition();
  }
 
  public void run() {
    if (!this.isClone() && this.getTimer().everyMillis(1000)) {
      this.clone();
    }
  }
}

Often you do not need clones at all: in Java you can simply create several objects of the same class with new, as the coins in Catch the Coins do.

Pictures and sounds

922 pictures and 266 sounds are built in. Browse them on the Sprites and Sounds pages and use them by name, for example addCostume("bunny1_stand"). You can also use your own files, see Costumes, Backdrops and Sounds.

Bringing a Scratch project along

Studio can import a .sb3 file from Scratch. It turns costumes, sounds and many scripts into Java and makes a list of the places that need your attention, such as waits and scripts running at the same time. Continue a Scratch project explains how to work through that list.