JAVA - HashMap 检查特定键值（如果更大）

Question

Map<Integer, Integer> listInfo = new HashMap<Integer, Integer>();

I have been trying to search from Google, but I don't really know why I cannot find a correct solution. Anyways my HashMap will store two Integers. I would use normal "int[]" string-list or whatever it is called for, but it will return me exception something about "array outofbounds" or something related to that, because it's index will be greater than int. Not sure, but anyways let's stay in the topic.

My english is pretty bad, but if you don't understand what I'm talking about hope this helps. (Just the idea, not based in real code)

if(listInfo.containsKey(key)) {
    if(listInfo.getKeyValue(key).valueOfKey() > valueToBeAdded) {
        listInfo.put(key, valueToBeAdded);
    }
} else {
    listInfo.put(key, valueToBeAdded);
}

I have tried similar way than above, but with pretty much correct functions, but it will conflict, because it says that it cannot compare key value with int, because key value is object? Why it's object, because I've defined that it should be Integer? I've also tried for(Entry entry : ......) loop, but I have no clue how I can get specific key's value (I am not talking about key value, I'm talking about value that the specific key holds)

I want only update the value that the specific (existing) key holds if the value that key holds is greater and the value that will be added is less than the current one.

Answer 1

Find below a snippet which does what you are looking for (assuming that I right understood your intention).

Map<Integer, Integer> listInfo = new HashMap<>();
listInfo.put(1, 23);
listInfo.put(2, 45);
Integer valueToBeAdded = 42;
System.out.println("listInfo = " + listInfo);
if (listInfo.containsKey(1)) {
    if (listInfo.get(1) < valueToBeAdded) {
        listInfo.put(1, valueToBeAdded);
    }
} else {
    listInfo.put(1, valueToBeAdded);
}
System.out.println("listInfo = " + listInfo);

output

listInfo = {1=23, 2=45} // the initial listInfo entries (key, value)
listInfo = {1=42, 2=45} // after the value for key `1` has been updated

Answer 2

Looks like you want to only put values that are smaller.
For this you can use Map.compute :

final HashMap<Integer, Integer> map = new HashMap<Integer,Integer>();
map.put(123, 456);
System.out.println(map);
final int x = 234;
final BiFunction<? super Integer, ? super Integer, ? extends Integer> f =
    (k, v) -> v == null ? x : Math.min(v, x);
map.compute(123, f);
map.compute(999, f);
System.out.println(map);

Sadly, Java doesn't really support functional programming. It would be nice to have an easy way for partial application. Here's a version with a static method that partially applies some value and returns a BiFunction ( f is a higher-order function).

  public static void main(final String[] arrg) {
    final HashMap<Integer, Integer> map = new HashMap<Integer,Integer>();
    map.put(123, 456);
    System.out.println(map);
    map.compute(123, f(345));
    map.compute(123, f(99999));
    map.compute(999, f(888));
    System.out.println(map);
  }

  static BiFunction<? super Integer, ? super Integer, ? extends Integer> f(final int x) {
    return (k, v) -> v == null ? x : Math.min(v, x);
  }