爲什麼我不斷收到顯示java.lang.NullPointerException

Question

我不斷收到這個代碼顯示java.lang.NullPointerException：

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

    Graphics g = bs.getDrawGraphics(); 

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

誰能告訴我在做什麼錯？

Answer 1

你應該試試這個：

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

即使你打電話this.createBufferStrategy(3);您bs變量保持未分配。

你需要創建後讀回：

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

它是增加一個檢查，以確保的createBufferStrategy通話後，你得到一個非空一個好主意：

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

Answer 3

您忘記將新的BufferStrategy指定爲null以指定給變量bs。將其更改爲

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

Answer 4

owww我很愚蠢的我忘了迴應該是這樣

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

    Graphics g = bs.getDrawGraphics(); 

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