Java中ArrayList的空指针异常 (Lib GDX)

huangapple
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英文:

Java Nullpointer Exception in ArrayList (Lib GDX)

问题

I have a problem with a "Nullpointer Exception" in LibGDX.
When calling the method "public static ArrayList getList(Stage stage, String className)," I get the Nullpointer Exception.

Would be very grateful for help...

Here the function:

  1. public static ArrayList<BaseActor> getList(Stage stage, String className)
  2. {
  3.     ArrayList<BaseActor> list = new ArrayList<BaseActor>();
  5.     Class theClass;
  6.     theClass=null;
  8.     try
  9.     {
  10.         theClass = Class.forName(className);
  11.     }
  12.     catch (Exception error)
  13.     {
  14.         error.printStackTrace();
  15.     }
  17.     for (Actor a : stage.getActors())
  18.     {
  19.         if ( theClass.isInstance( a ) )
  20.             list.add( (BaseActor)a );
  21.     }
  23.     return list;
  24. }

Here is the call to the function:

  1. public void update(float dt)
  2. {
  4.     try{
  5.         for (BaseActor rockActor : BaseActor.getList(mainStage, "Rock"))
  6.             turtle.preventOverlap(rockActor);
  7.     }
  8.     catch (Exception error)
  9.     {
  10.         error.printStackTrace();
  11.     }
  12. }

Java中ArrayList的空指针异常 (Lib GDX)

英文:

I have a problem with a "Nullpointer Exception" in LIb GDX.
When calling the method " public static ArrayList<BaseActor> getList(Stage stage, String className)"
I get the Nullpointer Exception.

Would be very grateful for help...

Java中ArrayList的空指针异常 (Lib GDX)

Here the function:

  1.  public  static ArrayList&lt;BaseActor&gt; getList(Stage stage, String className)
  2. {
  3.     ArrayList&lt;BaseActor&gt; list = new ArrayList&lt;BaseActor&gt;();
  5.     Class theClass;
  6.     theClass=null;
  8.     try
  9.     {
  10.         theClass = Class.forName(className);  }
  11.     catch (Exception error)
  12.     {  error.printStackTrace();  }
  14.     for (Actor a : stage.getActors())
  15.     {
  16.         if ( theClass.isInstance( a ) )
  17.             list.add( (BaseActor)a );
  18.     }
  20.     return list;
  21. }

Here is the call to the function:

  1.  public void update(float dt)
  2. {
  4.     try{
  5.         for (BaseActor rockActor : BaseActor.getList(mainStage, &quot;Rock&quot;))
  6.             turtle.preventOverlap(rockActor);
  7.     }
  8.     catch (Exception error)
  9.     {  error.printStackTrace();  } ......

...

答案1

得分: 1

这是因为try/catch的结果可能导致theClass仍然为null。 处理这种情况的一种方法是在try中包含您的列表逻辑，像这样：

  1. try
  2. {
  3.     theClass = Class.forName(className);
  5.     for (Actor a : stage.getActors())
  6.     {
  7.         if (theClass.isInstance(a))
  8.             list.add((BaseActor)a);
  9.     }
  10. }
  11. catch (Exception error)
  12. { error.printStackTrace(); }
  14. return list;

而且Chaosfire是对的；您可能需要做的不仅仅是打印堆栈跟踪。

英文:

This is happening because the result of the try/catch could result in theClass still being null. One way to handle this would be to include your list logic inside of the try, like this:

  1. try
  2. {
  3.     theClass = Class.forName(className);
  5.     for (Actor a : stage.getActors())
  6.     {
  7.         if ( theClass.isInstance( a ) )
  8.             list.add( (BaseActor)a );
  9.     }
  10. }
  11. catch (Exception error)
  12. {  error.printStackTrace();  }
  14. return list;

And Chaosfire is right; you may need to do more than just print the stack trace.

huangapple
  • 本文由 发表于 2023年4月10日 20:55:05
  • 转载请务必保留本文链接：https://go.coder-hub.com/75977321.html
  • android
  • class
  • java
  • libgdx
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