游戏循环为什么我一直在使用Java。Lang.NullPointerException

Question

我一直在用Java。此代码上的Lang.NullPointerException：

private void render(){
    BufferStrategy bs = this.getBufferStrategy();
    if(bs == null){
        this.createBufferStrategy(3);
    }

    Graphics g = bs.getDrawGraphics();

    g.dispose();
    bs.show();
}

有人能说出我做错了什么吗

Answer 1

你应该试试这个：

private void render(){
    BufferStrategy bs = this.getBufferStrategy();
    if(bs == null){
        this.createBufferStrategy(3);
        bs = this.getBufferStrategy(); // reassign bs
    }

    Graphics g = bs.getDrawGraphics();

    g.dispose();
    bs.show();
}

Answer 2

即使在调用this.createBufferStrategy(3);时，bs变量仍保持未赋值状态

创建后，您需要将其读回：

if(bs == null){
    this.createBufferStrategy(3);
    bs = this.getBufferStrategy();
}

最好添加一个检查，以确保在调用createBufferStrategy后返回一个非空值：

this.createBufferStrategy(3);
bs = this.getBufferStrategy();
if (bs == null) throw new IllegalStateException("Buffered structure is not created.");

Answer 3

如果新的BufferStrategy是null到to变量bs，您会忘记分配它。换成

if (bs == null) {
    bs = this.createBufferStrategy(3); // in case it returns BufferStrategy.
    bs = this.getBufferStrategy(); // otherwise
}