英文:
Adding new object by pressing left mouse
问题
有一个包含Object实例（Sprites）的数组，有一个通过按下鼠标左键来向数组中添加新对象的方法：
private void addSprite() {
addMouseListener(new MouseAdapter() {
@Override
public void mousePressed(MouseEvent e) {
int x = e.getX();
int y = e.getY();
Sprite sprite = new Ball(x, y);
if (sprites[sprites.length - 1] == null){
sprites[sprites.length - 1] = sprite;
}else if (sprites[sprites.length - 1] != null){
Sprite[] temp = new Sprite[sprites.length * 2];
System.arraycopy(sprites, 0, temp, 0, sprites.length);
sprites = temp;
}
}
});
}
如果arr已满，我会创建一个新的临时数组，大小为sprites.length * 2，并将所有元素复制到一个临时数组中，然后更改链接sprites = temp，但是我遇到了NullPointerException。任务是仅使用数组而不是ArrayList来添加新对象。
我做错了什么？
英文:
Got an array of Object instances (Sprites), got method to add a new object to array by pressing left mouse button:
private void addSprite() {
addMouseListener(new MouseAdapter() {
@Override
public void mousePressed(MouseEvent e) {
int x = e.getX();
int y = e.getY();
Sprite sprite = new Ball(x, y);
if (sprites[sprites.length - 1] == null){
sprites[sprites.length - 1] = sprite;
}else if (sprites[sprites.length - 1] != null){
Sprite[] temp = new Sprite[sprites.length * 2];
System.arraycopy(sprites, 0, temp, 0, sprites.length);
sprites = temp;
}
}
});
}
If arr is full I create new temporary array, size of sprites.length * 2, and copy all elements in a temporary array, and change link sprites = temp, and I've got NullPointerException. Task is adding new objects using only arrays, not ArrayList.
What am I doing wrong?
答案1
得分: -1
像这样工作：
private void addSprite() {
addMouseListener(new MouseAdapter() {
@Override
public void mousePressed(MouseEvent e) {
int x = e.getX();
int y = e.getY();
Sprite sprite = new Ball(x, y);
if (sprites[count] == null){
sprites[count] = sprite;
count++;
}else if (sprites[count] != null){
Sprite[] temp = new Sprite[sprites.length + 1];
System.arraycopy(sprites, 0, temp, 0, sprites.length);
temp[sprites.length] = sprite;
sprites = temp;
}
}
});
}
如果创建数组大小：`sprites.length * 2`：
问题是 Java 会尝试对数组的 `null` 元素进行 `update` 和 `render`。
如果创建 `sprites.length + 1` 并在最后一个 `index` 处添加元素，数组中就不会有 `null` 元素，它将正常工作。
英文:
work like this:
private void addSprite() {
addMouseListener(new MouseAdapter() {
@Override
public void mousePressed(MouseEvent e) {
int x = e.getX();
int y = e.getY();
Sprite sprite = new Ball(x, y);
if (sprites[count] == null){
sprites[count] = sprite;
count++;
}else if (sprites[count] != null){
Sprite[] temp = new Sprite[sprites.length + 1];
System.arraycopy(sprites, 0, temp, 0, sprites.length);
temp[sprites.length] = sprite;
sprites = temp;
}
}
});
}
If create array size: sprites.length * 2:
problem is java try to update and render null elements of array
If create sprites.length + 1 and add on an last index element there is no null elements in array, it's work fine.
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