創建遞歸樹時的運行時錯誤（空點異常）

Question

我想將以下內容作爲Json輸入並將其轉換爲java中的樹形數據結構。

   { 
        "component": "A", 
        "status": 0, 
        "children": [ 
         { 
          "component": "AA", 
          "status": 0, 
          "children": [ 
           { 
            "component": "AAA", 
            "status": 0, 
            "children": [] 
           }, 
           { 
            "component": "AAB", 
            "status": 0, 
            "children": [] 
           } 
          ] 
         }, 
         { 
          "component": "AB", 
          "status": 0, 
          "children": [ 
           { 
            "component": "ABA", 
            "status": 0, 
            "children": [] 
           }, 
           { 
            "component": "ABB", 
            "status": 0, 
            "children": [] 
           } 
          ] 
         } 
       } 

我寫了下面的代碼，但是它顯示運行時錯誤，如果任何人可以找到錯誤。 for循環執行，直到那裏有沒有孩子通過遞歸節點時它應該回到它正顯示出空指針異常點

  import java.io.FileNotFoundException; 
      import java.io.FileReader; 
      import java.io.IOException; 
      import java.util.List; 
      import java.io.BufferedReader; 
      import org.json.*; 

      public class Sample { 

       public static void main(String[] args) { 

         BufferedReader in = new BufferedReader(new FileReader("json.txt")); 
         StringBuilder builder = new StringBuilder(); 
         String line; 
         while ((line = in.readLine()) != null) { 
          builder.append(line); 

         object = new JSONObject(builder.toString()); 
         imlementation im = new imlementation(); 
         im.createnode(object);              
       }     
      } 
      public class node { 
        public String component; 
        public int status; 
        public List<node> children; 
      } 

      import org.json.JSONArray; 
      import org.json.JSONException; 
      import org.json.JSONObject; 

      public class imlementation { 

       public node createnode(JSONObject ob) 
       { 
        node n = new node(); 
        try 
        { 
         JSONArray children = ob.getJSONArray("children"); 
         String component = ob.getString("component"); 
         int status = ob.getInt("status"); 

         n.component = component; 
         n.status = status; 
         n.children=null; 
         System.out.println("component " + component + "status " + status); 

         int i; 
         for(i=0;i<children.length();i++) 
         { 
          n.children.add(createnode(children.getJSONObject(i))); 
         } 
         return n; 

        } 
        catch (JSONException ex) 
        { 
         ex.printStackTrace(); 
        } 

       } 

       } 

堆棧跟蹤是

  java.lang.NullPointerException 
       at imlementation.createnode(imlementation.java:30) 
       at imlementation.createnode(imlementation.java:30) 
       at SampleSample.main(SampleSample.java:45) 
      java.lang.NullPointerException 
       at imlementation.createnode(imlementation.java:30) 
       at SampleSample.main(SampleSample.java:45) 

Answer 1

您已設置

n.children=null; 

，然後在for循環中你正在做

n.children.add(....) 

這可能會導致空指針異常。

你可能需要做加法之前做

n.children = new List<Node>() 

。

另一個潛在的原因是在某些遞歸調用期間，children變量可能爲null。 children.length會導致空指針異常。