Question

基本上，我创建了一个地图来存储唯一键和项目列表。起初地图是null所以在我的类dosomething我检查地图是否为null但它返回一个异常。请参阅下面的代码。有谁知道我能做些什么来解决这个问题？

public class MyClass{
 private static Map<String, List<MyList>> MyMap = null;
 private static void doSomething(){
  String myKey = "hello";
  if(MyMap.get(myKey) == null ){ // Here is where i got the exception "java.lang.NullPointerException"
    //do something
  }
 }
}
public class MyList{
// do my List
}

Answer 1

private static Map<String, List<MyList>> MyMap = null; // null here and not initialized

MyMap.get(myKey)

你的MyMap为null所以它正在抛出NPE

Answer 2

您的MyMap为空。请执行以下操作..

 public class MyClass{
     private static Map<String, List<MyList>> MyMap = new HashMap<String, List<MyList>>(); // creating instance
     private static void doSomething(){
      String myKey = "hello";
      if(MyMap.get(myKey) == null ){ // Here is where i got the exception "java.lang.NullPointerException"
        //do something
      }
     }
    }
    public class MyList{
    // do my List
    }

Answer 3

您的MyMap正在抛出NullPointerException。这是因为您已将其明确设置为null：

private static Map<String, List<MyList>> MyMap = null;

相反，您应首先初始化MyMap：

private static Map<String, List<MyList>> MyMap = new HashMap<String, List<MyList>>();

Answer 4

你必须初始化你的HasMap。

Map<String, List<String>> myMap = new HashMap<String, List<String>>();
    if(myMap.get("bla") == null){
    //do somethig
}

检查null时出现NullPointerException

4 个答案: