Differences to Scratch

Probably the most noticeable differences to Scratch are the following:

• There is not wait-block on a Sprite-level, you need to use Timers.

If you want to achieve something like this inside a Sprite:

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

You can use a timer.

public class Cat extends Sprite {
    // executes 60-times a second, if the sprite is added to a stage.
    public void run() {
        if (this.getTimer().everyMillis(1000)) {
            this.nextCostume();
        }
    }
}

• There is a sprite and sound library, like in Scratch: 838 pictures and 266 sounds ship with Scratch for Java, and you use them by name with addCostume("bunny1_stand"). Browse them on the Sprites and Sounds pages. What Scratch has and this does not is a choose from a categorised catalogue with previews inside the editor - here it is a documentation page you search.
• There are no built-in editors, so for drawing or recording your own files you need external tools like GIMP, Inkscape, Audacity and so on.
• If you want to share your project with others, you have to used external sharing platforms like Nextcloud, iCloud, Dropbox or better a code sharing platform like GitHub.
• You can not use a forever loop. This will halt you program.

If you want to achieve something like this inside a Sprite:

when green flag clicked
forever
move (10) steps

You can use the run-method of the Sprite-class.

public class Cat extends Sprite {
    // executes 60-times a second, if the sprite is added to a stage.
    public void run() {
        this.move(10);
    }
}

• If you want to have a variable for all sprites, you have to use a static attribute. Static attributes are available across all objects.

public class Cat extends Sprite {
    public static int hitCounter = 1;
}
 
public class MyProgram {
    public MyProgram() {
        Cat.hitCounter += 1;
    }
}