在 forest() 方法中，我创建了一个二维对象数组，然后填充它。我确定数组已填充，因为我可以在方法 forest() 的类 Forest 中以及方法 setTree() 的类 Tree 中显示数组的内容，如下所示。不幸的是，我无法通过方法 showContentOfTree() 或其他地方获取此内容。出现错误:NullPointerException
问题是为什么会这样，我应该改变什么？
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
}
}
堆栈跟踪:
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)
最佳答案
你是shadowing forest 方法中的 tree 变量。
public forest(){
setLayout(null);
loadImage = new LoadImage();
loadImage.Image();
tree = new Tree[16][16]; //<-- remove Tree[][]
关于Java:NullPointerException 二维对象数组甚至数组已初始化并填充，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/21043139/