Question

好吧，我改写了我的课程，我赶紧这么代码不干净但是现在它已经复制了，并且运行mainMethod问题仍然存在。

   import java.util.*;


   public class myClass {

            public Random myRandom;
            public HashMap<String, ArrayList<String>> myMap;
            public ArrayList<String> ar;
            public ArrayList<String> nexts;
        //     ArrayList<String> follows;
            public myClass(){
                myRandom = new Random();       
            }


            public void setRandom(int seed){
                myRandom = new Random(seed);

            }
            public HashMap<String, ArrayList<String>> buildHashMap(){
              HashMap<String, ArrayList<String>> myMap = new HashMap<String, ArrayList<String>>();
              ArrayList<String> ar = new ArrayList<String>();
              ar.add("2");
              ar.add("2");
             String test = "test";
             String anothertest = "anothertest";
              myMap.put(test, ar);
              myMap.put(anothertest, ar);

              return myMap;  
            }
            public ArrayList<String> arrayListGetter(String st){
                System.out.println("++++++++++++++++++++++++++++++++++");
                  System.out.println(myMap.size());
                ArrayList ar = myMap.get(st);

            return ar;  
        }

            public void mainMethod(){
                HashMap<String, ArrayList<String>> myMap = new HashMap<String, ArrayList<String>>();
                myMap = buildHashMap();
                System.out.println("\n\nNumber of keys found: " + myMap.size()); 
                for (String st : myMap.keySet()){
                System.out.println(st + ": ");
                ArrayList<String> al = myMap.get(st);

                    System.out.println(al.size());

                     }

                StringBuilder sb = new StringBuilder();
                String test = "test";
                System.out.println(myMap.get(test));

                System.out.println(sb);  
                System.out.println(myMap.get(test).size());
                System.out.println(myMap.get(test).size());
        //         ArrayList<String> follows = getFollows(key);
                System.out.println("something");
               int  index = myRandom.nextInt(myMap.get(test).size());
                System.out.println(index);
        //         index = myRandom.nextInt(follows.size());
                String next = myMap.get("test").get(index);
                sb.append(next);
                System.out.println(sb);  
                System.out.println(myMap.get("test").getClass());
                ArrayList<String> follows = new ArrayList<String>();<------new empty ArrayList
                System.out.println(follows.size() + " **********");
                ArrayList<String> nexts = new ArrayList<String>();
                nexts = arrayListGetter ("test");




            }
            }

我错过了一些明显的东西吗？提前感谢您的时间和耐心。

Answer 1

您通过在方法中重新声明myMap变量并将该字段保留为null来隐藏myMap变量。

import java.util.*;

public class MyClass2 {
    public HashMap<String, ArrayList<String>> myMap;  // this guy is null

    // ArrayList<String> follows;
    public HashMap<String, ArrayList<String>> buildHashMap() {
        HashMap<String, ArrayList<String>> myMap = new HashMap<String, ArrayList<String>>();
        return myMap;
    }

    public ArrayList<String> arrayListGetter(String st) {
        System.out.println("++++++++++++++++++++++++++++++++++");
        System.out.println(myMap.size());
        ArrayList ar = myMap.get(st);
        return ar;
    }

    public void mainMethod() {
        // this myMap is a local variable, and assigning it anything
        // will leave the class field null
        HashMap<String, ArrayList<String>> myMap = new HashMap<String, ArrayList<String>>();
        myMap = buildHashMap();  // the field in the class is still null!!!
        System.out.println("\n\nNumber of keys found: " + myMap.size());
        for (String st : myMap.keySet()) {
            System.out.println(st + ": ");
            ArrayList<String> al = myMap.get(st);

            System.out.println(al.size());

        }

        // this throws a NPE
        ArrayList<String> someMap =  arrayListGetter("test");
    }
}

这意味着您正在初始化一个已为某些方法声明为本地的HashMap。是的，你给它的名字是myMap，与类中的字段相同，但是通过在方法中声明它是局部的，变量只在该方法中可见，并且类中的字段保持为空

解决方案：不要这样做，不要给本地字段提供与字段相同的名称，如果要分配给类中的字段，则不要在本地范围内重新声明变量。

Java另一个HashMap <object，arraylist <string =“”>＆gt;空指针异常

1 个答案: