无法在迭代器（java）中引用以前使用的元素

Question

当我使用迭代器遍历条目集的元素时，我无法引用之前已被迭代的元素。 这是更具体的问题：

public void adjSetter(HashMap<ArrayList<Integer>,miniSquare> mp) {
    Iterator it = mp.entrySet().iterator();
    while (it.hasNext()) {
        System.out.println("Hi~!");

        Map.Entry pair = (Map.Entry)it.next();
        ArrayList<Integer> lis=(ArrayList<Integer>)pair.getKey();
        System.out.println(lis.get(0) + "  " + lis.get(1) + "Yerr");
        miniSquare minSqua=(miniSquare)pair.getValue();
        if((minSqua.yCord!=600)){
            lis.set(1,minSqua.yCord+50);
            minSqua.topSquare=mp.get(lis);
             System.out.println("This Square is " + minSqua.xCord + "  " +minSqua.yCord + " and its top square is " + lis.get(0) + " " + lis.get(1) + " And it's actually " + minSqua.topSquare.xCord + " " + minSqua.topSquare.yCord  ); }
        if(minSqua.yCord!=250){
            lis.set(1,minSqua.yCord-50);
            minSqua.bottomSquare=mp.get(lis);
            System.out.println("This Square is " + minSqua.xCord + "  " + minSqua.yCord + " and its bottom square is " + lis.get(0) + " " + lis.get(1) + " And it's actually " + minSqua.bottomSquare.xCord + " " + minSqua.bottomSquare.yCord );
            }
        if(minSqua.xCord!=550){
            lis.set(1,minSqua.yCord);
            lis.set(0,minSqua.xCord+50);
            minSqua.rightSquare=mp.get(lis);
            System.out.println("This Square is " + minSqua.xCord + "  " + minSqua.yCord + " and its right square is " + lis.get(0) + " " + lis.get(1) + " And it's actually " + minSqua.rightSquare.xCord + " " + minSqua.rightSquare.yCord );
        lis.set(0,450); lis.set(1,450); //The error line
            System.out.println(mp.get(lis).xCord+ "Yerd");}
        if(minSqua.xCord!=200){
            lis.set(1,minSqua.yCord);
            lis.set(0,minSqua.xCord-50);
            minSqua.leftSquare=mp.get(lis);
            System.out.println("This Square is " + minSqua.xCord + "  " + minSqua.yCord + " and its left square is " + lis.get(0) + " " + lis.get(1) + " And it's actually " + minSqua.leftSquare.xCord + " " + minSqua.leftSquare.yCord );// This line throws a null pointer exception if the other problem line is deleted 
        }

    }

}

通过键（450,450）遍历元素后，该行

lis.set(0,450); lis.set(1,450);

产生一个空指针异常。 任何人都可以帮助您了解如何解决此问题，甚至是真正的问题所在吗？ 很抱歉，如果没有足够的信息，或者代码难以阅读。

Answer 1

我不确定您得到NullPointerException的确切原因，但是您滥用HashMap 。 即使您没有遍历HashMap的条目，更改Map的键（在您的情况下为ArrayList<Integer> ）也会破坏HashMap ，因为它会更改已经在其中的键的hashCode() 。 Map （这意味着密钥可能存储在与其新的hashCode()不匹配的存储桶中）。

这可能会破坏迭代器代码，并导致NullPointerException 。 您不应该更改HashMap的键。 如果您不遍历条目，则可以从Map删除该条目，对键进行突变，然后将其放回Map ，但是在遍历Map不允许这样做。

PS，而不是使用原始的Iterator和Map.Entry ，后者需要稍后进行转换，请使用：

Iterator<Map.Entry<ArrayList<Integer>,miniSquare>> it = mp.entrySet().iterator();

和

Map.Entry<ArrayList<Integer>,miniSquare> pair = it.next();

这不会解决您的异常，但这是一个好习惯。