英文:
TreeSet throws nullpointer exception when using custom comparator
问题
以下是您提供的代码段的翻译部分：
我在尝试从TreeSet中删除特定元素（非null）时遇到了错误消息。我不确定为什么会发生这种情况，我尝试使用contains方法检查集合中是否已经存在相同的元素，这样可以正常工作。异常在调用reset方法时抛出。
java.lang.NullPointerException
在第55行，Leaderboard$1.compare
在第52行，Leaderboard$1.compare
在第374行，java.base/java.util.TreeMap.getEntryUsingComparator
在第343行，java.base/java.util.TreeMap.getEntry
在第601行，java.base/java.util.TreeMap.remove
在第276行，java.base/java.util.TreeSet.remove
在第86行，Leaderboard.reset
在第71行，__Driver__.__helperSelectMethod__
在第91行，__Driver__.__helper__
在第112行，__Driver__.main
> 这是我的代码：
class Leaderboard {
TreeSet<Integer> TS;
HashMap<Integer, Integer> HS = new HashMap<>();
public Leaderboard() {
TS = new TreeSet<>(new Comparator<Integer>(){
@Override
public int compare(Integer a, Integer b){
if(HS.get(a).equals(HS.get(b))){
return Integer.compare(a, b);
}
return HS.get(a).compareTo(HS.get(b));
}
});
}
public void addScore(int playerId, int score) {
HS.put(playerId, HS.getOrDefault(playerId, 0) + score);
TS.add(playerId);
TreeSet<Integer> test = TS;
HashMap<Integer, Integer> test2 = HS;
}
public int top(int K) {
Iterator<Integer> iterator = TS.iterator();
int res = 0;
while(K > 0 && iterator.hasNext()){
res += HS.get(iterator.next());
K--;
}
return res;
}
public void reset(int playerId) {
Integer id = new Integer(playerId); //尝试过，无效
System.out.println(HS.containsKey(id)); // true
System.out.println(TS.contains(id)); // true
HS.remove(id);
TS.remove(id);
}
}
/**
* 将实例化您的Leaderboard对象，并按以下方式调用：
* Leaderboard obj = new Leaderboard();
* obj.addScore(playerId, score);
* int param_2 = obj.top(K);
* obj.reset(playerId);
*/
英文:
I got an error msg when I trying to remove a specific element in treeset (not null). I'm not sure why this happened, and I tried to use contains to check if there has the same element in the set already which would work fine. The exception throws when calling reset method.
java.lang.NullPointerException
at line 55, Leaderboard$1.compare
at line 52, Leaderboard$1.compare
at line 374, java.base/java.util.TreeMap.getEntryUsingComparator
at line 343, java.base/java.util.TreeMap.getEntry
at line 601, java.base/java.util.TreeMap.remove
at line 276, java.base/java.util.TreeSet.remove
at line 86, Leaderboard.reset
at line 71, __Driver__.__helperSelectMethod__
at line 91, __Driver__.__helper__
at line 112, __Driver__.main
> Here is my code:
class Leaderboard {
TreeSet<Integer> TS;
HashMap<Integer, Integer> HS = new HashMap<>();
public Leaderboard() {
TS = new TreeSet<>(new Comparator<Integer>(){
@Override
public int compare(Integer a, Integer b){
if(HS.get(a).equals(HS.get(b))){
return Integer.compare(a, b);
}
return HS.get(a).compareTo(HS.get(b));
}
});
}
public void addScore(int playerId, int score) {
HS.put(playerId, HS.getOrDefault(playerId, 0) + score);
TS.add(playerId);
TreeSet<Integer> test = TS;
HashMap<Integer, Integer> test2 = HS;
}
public int top(int K) {
Iterator<Integer> iterator = TS.iterator();
int res = 0;
while(K > 0 && iterator.hasNext()){
res += HS.get(iterator.next());
K--;
}
return res;
}
public void reset(int playerId) {
Integer id = new Integer(playerId); //tried, not work
System.out.println(HS.containsKey(id)); // true
System.out.println(TS.contains(id)); // true
HS.remove(id);
TS.remove(id);
}
}
/**
* Your Leaderboard object will be instantiated and called as such:
* Leaderboard obj = new Leaderboard();
* obj.addScore(playerId,score);
* int param_2 = obj.top(K);
* obj.reset(playerId);
*/
答案1
得分: 0
如@VGR所提到的：
您确定在您的比较方法中HS.get(a)永远不会返回null吗？
这是因为TreeMap是基于HashMap进行比较的。如果使用HS.remove(a)删除了HS的值，无论何时treemap调用HS.get(a)，都会返回null作为返回值。当在比较中为HS.get(a).compareTo调用时，将会是null.compareTo，这就是抛出空指针异常的原因。
英文:
As @VGR mentioned:
Are you certain that HS.get(a) never returns null in your compare method?
This is because that the TreeMap is based on HashMap to compare. If delete the HS value using HS.remove(a), whenever the treemap call HS.get(a) there will be a null as return value. which when called in compare for HS.get(a).compareTo will be null.compareTo, and this is the reason that throws an NullPointerException.
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