java 在哈希图上使用 removeif
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Using removeif on a hashmap
提问by UsefulUserName
I am trying to remove entries from a Hashmap, if i have already used them. Sadly, I'm not familier with Java 8 lambda expressions, so I'm not sure how to remove the entries correctly. Could somebody help me or explain what I have to do?
如果我已经使用过它们，我正在尝试从 Hashmap 中删除它们。遗憾的是，我不熟悉 Java 8 lambda 表达式，所以我不确定如何正确删除条目。有人可以帮助我或解释我必须做什么吗？
Here is the way I've tried doing it:
这是我尝试过的方法：
ArrayList<Integer> range10 = new ArrayList<Integer>();
ArrayList<Integer> range15 = new ArrayList<Integer>();
ArrayList<Integer> rangeMax = new ArrayList<Integer>();
for (int age = 16; age <= 100; age++){
for (Entry<Integer, Partner> entry : dbMap.entrySet()){
int key = entry.getKey();
Partner person = entry.getValue();
if (person.getAge() == alter && person.getAgeRange() == 10){
range10.add(key);
entry.setValue(null);
}
else if (person.getAge() == alter && person.getAgeRange() == 15){
range15.add(key);
entry.setValue(null);
}
else if (person.getAge() == age){
rangeMax.add(key);
entry.setValue(null);
}
dbMap.entrySet().removeIf(entries->entries.getValue().equals(null));
}
And I get a java.lang.NullPointerExceptionfor it. I don't think this is a duplicate to asking what a NullPointerexception is, since I'm primarily asking how to use the removeif-function.
我得到了java.lang.NullPointerException它。我不认为这与询问 NullPointerexception 是什么重复，因为我主要询问如何使用 removeif 函数。
回答by Adnan Isajbegovic
You get that because you call .equals() on getValue() object, which is null, so it will not work. That happens here:
你得到它是因为你在 getValue() 对象上调用了 .equals() ，它是null，所以它不会工作。这发生在这里：
dbMap.entrySet().removeIf(entries->entries.getValue().equals(null));
What you have to do is this:
你需要做的是：
dbMap.entrySet().removeIf(entries->entries.getValue() == null);
