是否可以在使用方法的过程中初始化Dimension的新实例，如果是这样，为什么？

Question

I'm reading through some java tutorial code on 2d game design and came across this statement .

setPreferredSize(new Dimension(B_WIDTH, B_HEIGHT))

I don't understand how a new instance of Dimension can be initialized inside the use of the method setPrefferedSize(). I'm not even sure if I'm using all the proper vocabulary but if someone understands what I'm asking help would be appreciated =). Below is the full code

public class Board extends JPanel {

private final int B_WIDTH = 350;
private final int B_HEIGHT = 350;
private final int INITIAL_X = -40;
private final int INITIAL_Y = -40;    
private final int INITIAL_DELAY = 100;
private final int PERIOD_INTERVAL = 25;

private Image star;
private Timer timer;
private int x, y;

public Board() {


    initBoard();        
}

private void loadImage() {

    ImageIcon ii = new ImageIcon("star.png");
    star = ii.getImage();        
}

private void initBoard() {

    setBackground(Color.BLACK);
    setPreferredSize(new Dimension(B_WIDTH, B_HEIGHT));
    setDoubleBuffered(true);

    loadImage();

    x = INITIAL_X;
    y = INITIAL_Y;

    timer = new Timer();
    timer.scheduleAtFixedRate(new ScheduleTask(), 
            INITIAL_DELAY, PERIOD_INTERVAL);        
}


@Override
public void paintComponent(Graphics g) {
    super.paintComponent(g);

    drawStar(g);
}

private void drawStar(Graphics g) {

    g.drawImage(star, x, y, this);
    Toolkit.getDefaultToolkit().sync();
}


private class ScheduleTask extends TimerTask {

    @Override
    public void run() {
        x += 1;
        y += 1;

        if (y > B_HEIGHT) {
            y = INITIAL_Y;
            x = INITIAL_X;
        }

        repaint();
    }
}

}

Answer 1

There is nothing unusual. You could have written

Dimension d=new Dimension(B_WIDTH, B_HEIGHT); setPreferredSize(d);

It's the same thing. But since you won't use it anywhere else, it's the fastest way of writing it.

Answer 2

When you call a method, as in :

setPreferredSize(new Dimension(B_WIDTH, B_HEIGHT))

first the arguments are evaluated. In this case there is a single argument - new Dimension(B_WIDTH, B_HEIGHT) - an its evaluation creates a Dimension instance.

Then that instance is then passed to the setPreferredSize method.

Answer 3

The result of constructing an object is the object itself.

Dimension prefSize = new Dimension(B_WIDTH, B_HEIGHT);

An object can be constructed without assigning it to a variable, and the result is still a new object.

new Dimension(B_WIDTH, B_HEIGHT);

Though the above by itself is not a valid statement, because we're required to do something with the object. Passing it to a method is a valid something.

The author could have written it like

Dimension prefSize = new Dimension(B_WIDTH, B_HEIGHT);
setPreferredSize(prefSize);

but there was no point in the intermediate assignment to a variable because the Dimension object was being created solely for the purpose of setting that one component's preferred size.