Java：NullPointerException二維數組對象甚至數組初始化並填充

Question

在方法forest（）中，我創建了一個二維數組對象，然後填充它。我確信該數組已填充，因爲我可以在方法forest（）中的Forest類中顯示數組的內容，也可以在setTree（）方法的Tree樹中顯示數組的內容，如下所示。不幸的是，我無法通過showContentOfTree（）方法或其他方法獲取此內容。錯誤apears：空指針異常

問題是爲什麼發生，我應該改變？

public class Forest extends JPanel { 
    private LoadImage loadImage; 
    private Tree[][] tree; 

    public forest(){ 
     setLayout(null); 
     loadImage = new LoadImage(); 
     loadImage.Image(); 
     Tree[][] tree = new Tree[16][16]; 
     for (int y = 0; y < 16; y++){ 
      for (int x = 0; x < 16; x++){ 
       tree[x][y] = new Tree(); 
       tree[x][y].setTree(loadImage.loadForest(x,y)); 
       System.out.println("Tree species " + tree[x][y].treeSpecies); 
       //here System displays loaded treeSpecies successfully 
      } 
     } 
     showContentOfTree(); 
    } 
    public void showContentOfTree(){ 
    for (int y = 0; y < 16; y++){ 
     for (int x = 0; x < 16; x++){ 
      System.out.println("Tree species" + tree[x][y].treeSpecies); 
          //here System DOES NOT displays loaded treeSpecies: 
          //Error apears: java.lang.NullPointerException 
          //at Forest.showContentOfTree(Forest.java:31) 
     } 
    } 
} 

public class Tree{ 
    String treeSpecies; 
    public void setTree(String treeSpecies){ 
     this.treeSpecies = treeSpecies; 
     System.out.println("Tree species " + treeSpecies); 
       //here System also displays loaded treeSpecies successfully 
    } 
} 

堆棧跟蹤：

Exception in thread "main" java.lang.NullPointerException at Forest.showContentOfTree(Forest.java:31) 
at Forest.<init>(Forest.java:26) 
at Okno.<init>(Okno.java:19) 

在Glowny.main（Glowny.java:10）

Answer 1

你在你的forest方法shadowing您tree變量。

public forest(){ 
     setLayout(null); 
     loadImage = new LoadImage(); 
     loadImage.Image(); 
     tree = new Tree[16][16]; //<-- remove Tree[][]