可变引用的示例

在这种情况下，类 Point 是可变的，一些用户可以修改该类的对象的状态。

class Point {
    private int x, y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public int getX() {
        return x;
    }
    
    public void setX(int x) {
        this.x = x;
    }
    
    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }
}

//...

public final class ImmutableCircle {
    private final Point center;
    private final double radius;

    public ImmutableCircle(Point center, double radius) {
        // we create new object here because it shouldn't be changed
        this.center = new Point(center.getX(), center.getY());
        this.radius = radius;
    }