It doesn't work!

Everybody's programs go wrong, all the time. That is normal. Here are the mistakes that happen most often, and how to fix them. Find the one that sounds like your problem.

My sprite does not appear

You forgot this.add(...). Creating a sprite with new is not enough. It also has to be put on the stage:

Bunny bunny = new Bunny();
this.add(bunny);   // without this line, the bunny exists but is not on the stage

The stage is never created. Something has to run new MyStage(). In the browser and in Studio that is the void main() at the top. In BlueJ, right-click MyStage and choose new MyStage().

It is off the stage or very small. The middle of the stage is x: 0, y: 0. A stage is usually 480 wide and 360 high, so x goes from -240 to 240 and y from -180 to 180. Check setPosition and setSize.

"Costume not found" or a picture is missing

Look at the output below the editor, or the console in BlueJ. Scratch for Java tells you which name it could not find and suggests the closest names it knows.

Names are case-sensitive: bunny1_stand works, Bunny1_Stand does not.
Look the name up on the Sprites page. Click a picture there to copy the line that uses it.
Your own picture must be in the project folder, and the path must match exactly, including .png or .jpg.

Pressing keys does nothing

Click on the stage first. The stage only gets the keys when it has the focus, just like a text field needs a click before you can type into it.

The method is never called. whenKeyPressed must be written exactly like this, or Java treats it as a new method of yours that nobody calls:

public void whenKeyPressed(KeyCode key) {
  if (key == KeyCode.SPACE) {
    this.say("Jump!");
  }
}

Put @Override on the line above. Then Java reports an error if the name or the parameter is not quite right. The same goes for run, whenClicked and whenIReceive.

The program freezes

There is a loop in run() that never ends, or one that waits:

public void run() {
  while (true) {        // never finishes, so the next frame is never drawn
    this.move(1);
  }
}

run() is already called again and again, about 60 times a second. Take the loop out and let run() do the repeating. To wait, use a timer: Differences to Scratch shows how.

Something happens far too often

Everything in run() happens in every frame. A coin that touches the basket for half a second counts 30 times:

public void run() {
  if (this.isTouchingSprite(Basket.class)) {
    this.score = this.score + 1;     // happens in every frame while touching
  }
}

Make sure the reason goes away: move the coin back to the top, hide it, or remember in a boolean that it has already been counted.

A text comparison is never true

Texts are compared with equals, not with ==:

if (message.equals("go")) {   // right
}
if (message == "go") {        // compiles, but is usually false
}

A division gives the wrong number

7 / 2 is 3 in Java, because two whole numbers give a whole number. Write 7.0 / 2 or use double variables if you want 3.5.

What the error messages mean

Java checks your program before it runs. The error message names the line, and the mistake is usually there or on the line just before it.

Message	What it usually means
`cannot find symbol`	A name is misspelled, or has the wrong capital letters (`setx` instead of `setX`). Or the `import` line at the top is missing.
`';' expected`	A semicolon is missing at the end of the line before.
`reached end of file while parsing`	A `}` is missing. Every `{` needs one.
`class, interface, enum, or record expected`	There is one `}` too many, or code outside a class.
`incompatible types: String cannot be converted to int`	You used text where a number is needed. `Integer.parseInt(text)` turns text into a number.
`missing return statement`	A method that promises a value (`int`, `boolean`, …) has a way to end without `return`.
`method does not override or implement a method from a supertype`	Your `@Override` caught a typo in the method's name or parameter, see keys.
`class Coin is public, should be declared in a file named Coin.java`	The file name must match the class name exactly.
`non-static method … cannot be referenced from a static context`	You called a method on the class instead of on an object: `Coin.move(5)` instead of `coin.move(5)`.

Still stuck?

Compare with the finished project at the end of the tutorial. The download link is there.
Read your code out loud, line by line, and say what each line does.
When you ask someone for help, show them three things: your code, the exact error message, and what you expected to happen.