在哈希表中如何根据键更新值？

更加严谨的解决方案是避免NullPointerException：

map.replace(key, map.get(key) + 1);

由于声望不够，我无法评论一些答案，因此我将发布我应用的解决方案。

for(String key : someArray)
{
   if(hashMap.containsKey(key)//will check if a particular key exist or not 
   {
      hashMap.put(hashMap.get(key),value+1);// increment the value by 1 to an already existing key
   }
   else
   {
      hashMap.put(key,value);// make a new entry into the hashmap
   }
}

Integer i = map.get(key);
if(i == null)
   i = (aValue)
map.put(key, i + 1);

或者

Integer i = map.get(key);
map.put(key, i == null ? newValue : i + 1);

for (int i =0; i<5; i++){
    HashMap<String, Integer> map = new HashMap<String, Integer>();
    map.put("beer", 100);

    int beer = map.get("beer")+i;
    System.out.println("beer " + beer);
    System.out ....

}

public class ExampleToUpdateMapValue {

    public static void main(String[] args) {
        Map<String,String> bookAuthors = new TreeMap<>();
        bookAuthors.put("Genesis","Moses");
        bookAuthors.put("Joshua","Joshua");
        bookAuthors.put("Judges","Samuel");

        System.out.println("---------------------Before----------------------");
        bookAuthors.entrySet().stream().forEach(System.out::println);
        // To update the existing value using Java 8
        bookAuthors.computeIfPresent("Judges", (k,v) -> v = "Samuel/Nathan/Gad");

        System.out.println("---------------------After----------------------");
        bookAuthors.entrySet().stream().forEach(System.out::println);
    }
}

尝试：

HashMap hm=new HashMap<String ,Double >();

注意：

String->give the new value; //THIS IS THE KEY
else
Double->pass new value; //THIS IS THE VALUE

你可以更改哈希表中的键或值，但不能同时更改两者。