Why create a new Object - Java Tetris Tutorial

Question

Im just new to Java and I found this good tutorial for creating a Java Tetris Game.

I dont have a mentor or a tutor to help me with this - Ive been looking for one for ages :( so currently im self learning Java and PHP :)

Anyways heres the website I found: http://zetcode.com/tutorials/javagamestutorial/tetris/

One method of the program I dont get in the Shape.java class:

public Shape rotateLeft() 
{
 if (pieceShape == Tetrominoes.SquareShape)
     return this;

 Shape result = new Shape();
 result.pieceShape = pieceShape;

 for (int i = 0; i < 4; ++i) {
     result.setX(i, y(i));
     result.setY(i, -x(i));
 }
 return result;
}

Why do we need to create a new Object Shape result = new Shape(); if can already get the current piece from the pieceShape variable?

Answer 1

It seems the naming is a bit misleading in this tutorial. The class called Shape represents a single item that falls down. The Tetrominoes enum seems to be describing which kind of item it is (i.e. it's "shape"!).

So the code you posted creates a new item and specifies its shape.

The rotateRight() and rotateLeft() methods don't modify the shape itself to allow the tryMove() method to check if the move is legal and ignore it if it isn't (for example if you'd rotate an item into the wall). tryMove() simply keeps the old values (including the old Shape instance) when the move is not allowed. If rotateLeft()/rotateRight() modified the Shape then it would have to undo that operation, which would complicate the code.

Also, there are a few nitpicks with this code:

• I'd call the Tetrominoes class Tetromino, as enum types are usually named in the singular (since you often reference a single element: Tetromino.SquareShape.
• I'd add the information about the concrete coordinate of each Tetromino into that enum, effectively putting much of the logic from the setShape() method into it.

• The Board class mixes the logic and the presentation, it should be separated (makes it much easier to test).

  For example the Board class could implement all the logic without any of the graphics (i.e. don't reference anything from java.awt or javax.swing). Then you'd write a BoardPanel that draws the state of the Board and interacts with the user, calling the appropriate Board methods.

Answer 2

The method you've posted returns a shape that is rotated left. If you didn't create a new Shape, the original shape, which is a field of the class and used everywhere else, would have been rotated.

In the case of the square shape, which isn't changed when rotated left, you can still return the original one.

Answer 3

Without looking thoroughly at the tutorial I'd say this: Since Shape seems to represent an individual item and rotateLeft() might be a instance method of Shape it might also be ok to rotate the item in place, i.e. not to return a rotated copy but change the block' coordinates of the current shape. Creating a new item to fall down would then mean to create a new Shape with default orientation.

Answer 4

It appears that the author doesn't mutate the Shape in rotateLeft() because it might not be accepted as a valid move. In Board the inner TAdapter class calls tryMove() and only sets the current Shape (variable curShape) if it's acceptable. If it were mutated prior to that check it'd have to set it back when invalid. The method should perhaps be named rotatedLeftCopy() to indicate that it's not changing the state. Or the check should be performed beforehand and then rotateLeft() would be safe to change the Shape in place.