Your first program

If you have used Scratch before, you already know most of what is on this page. A sprite, a stage, costumes, move, say, when key pressed — Scratch for Java keeps all of those names. What changes is that you write them instead of dragging them.

This page gets a character moving on screen. It takes about ten minutes, and you do not have to download a single picture.

What you need

1. A development environment. BlueJ is the friendliest one to start with, and this page assumes it.
2. The Scratch for Java library, added to BlueJ once.

Both are on the Setup page. Come back here when BlueJ is running.

Step 1: A sprite on the stage

In BlueJ, create a new class called MyStage and replace everything in it with these lines:

import org.openpatch.scratch.*;
 
public class MyStage extends Stage {
  public MyStage() {
    Sprite bunny = new Sprite();
    bunny.addCostume("bunny1_stand");
    this.add(bunny);
  }
}

Compile, then right-click the MyStage box and choose new MyStage(). A window opens with a rabbit in it:

That is a whole program. Seven lines, and no image files anywhere.

Line by line:

• Line 1 brings in Scratch for Java. One import is enough for everything on this page.
• Line 3 says MyStage is a Stage. In Scratch every project has one stage; here you make your own, and it can already do everything a stage does.
• Line 5 creates a sprite, the same idea as clicking "add sprite".
• Line 6 gives it a costume, by name.
• Line 7 puts it on the stage. In Scratch a new sprite appears by itself; in Java you say so.

Step 2: Make it move

In Scratch each sprite has its own scripts. In Java each sprite gets its own class.

Create a second class called Bunny:

import org.openpatch.scratch.*;
 
public class Bunny extends Sprite {
  public Bunny() {
    this.addCostume("bunny1_stand");
    this.setSize(50);
    this.setRotationStyle(RotationStyle.LEFT_RIGHT);
  }
 
  public void run() {
    if (this.isKeyPressed(KeyCode.RIGHT)) {
      this.setDirection(90);
      this.move(4);
    }
    if (this.isKeyPressed(KeyCode.LEFT)) {
      this.setDirection(-90);
      this.move(4);
    }
    this.ifOnEdgeBounce();
  }
}

The important part is run(). It is called about 60 times a second for as long as the sprite is on a stage, so it is the Java version of this:

when green flag clicked
forever
if <key (right arrow v) pressed?> then
point in direction (90)
move (4) steps

There is no forever in Java, because run() is already the inside of the forever loop.

Now change MyStage to use the new sprite, and give the stage a backdrop:

import org.openpatch.scratch.*;
 
public class MyStage extends Stage {
  public MyStage() {
    this.addBackdrop("background");
    this.add(new Bunny());
  }
 
  public static void main(String[] args) {
    new MyStage();
  }
}

Run it again and hold the left and right arrow keys:

What just happened

The finished project

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

Where to go next

• Bouncing Hedgehog — your first real game, step by step.
• Sprites and Sounds — everything built in, with a search box.
• Differences to Scratch — the few places where Java makes you think differently.