比较超过 3 个 HashMap 的 Key

Question

I want to compare the keys of four hashmaps. All of the keys in the Hashmaps are equal. But I would like to evaluate each hashmap to compare the keys of all four hashmaps. In the code below, I am using 4 nested loops and then using a logical AND. If true , I want to add the Values to an Array List. Is this possible?

I have seen some code on the forum, but found nothing where we can compare 4 hashmaps. So do I need to add the Values to the ArrayList and not the Keys?

public static Map<String, Boolean> areEqualKeyValues(Map<String, String> firstMap, Map<String, String> secondMap,Map<String, String> thirdMap, Map<String, String> fourthMap) {

        ArrayList<String> reporValues = new ArrayList<>();
        HashMap<String, String> filteredMap = new HashMap<>();

        for (Map.Entry<String, String> entry1 : firstMap.entrySet()) {
            for (Map.Entry<String, String> entry2 : secondMap.entrySet()) {
                for (Map.Entry<String, String> entry3 : thirdMap.entrySet()) {
                    for (Map.Entry<String, String> entry4 : fourthMap.entrySet()) {
                            if (entry1.getKey().equals(entry2.getKey())
                                    && entry2.equals(entry3.getKey())
                                    && entry1.equals(entry3.getKey())
                                    && entry3.equals(entry4.getKey())) {
                                reporValues.add(entry1.getValue() + entry2.getValue() + entry3.getValue() + entry4.getValue());
                            }
                        }
                    }
                }
                filteredMap.put(entry1.getKey(), reporValues.toString());
            }

Answer 1

Your code has some problems:

1. The return type is wrong.
2. You need to compare a key with a key whereas you have compared some keys with the entry itself.
3. You haven't returned anything from the function.

Given below is the corrected function with a demo code:

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Main {
    public static void main(final String[] args) {
        // Test
        Map<String, String> firstMap = Map.of("a", "a1", "b", "b-first_", "c", "c1", "d", "d1");
        Map<String, String> secondMap = Map.of("x", "z1", "b", "b-second_", "y", "y1", "z", "z1");
        Map<String, String> thirdMap = Map.of("b", "b-third_", "m", "m1", "n", "n1");
        Map<String, String> fourthMap = Map.of("a", "a1", "p", "p1", "b", "b-fourth_");
        Map<String, String> filteredMap = new HashMap<>();
        List<String> reporValues = new ArrayList<>();
        System.out.println(areEqualKeyValues(firstMap, secondMap, thirdMap, fourthMap, filteredMap, reporValues));
        System.out.println(filteredMap);
        System.out.println(reporValues);
    }

    public static boolean areEqualKeyValues(Map<String, String> firstMap, Map<String, String> secondMap,
            Map<String, String> thirdMap, Map<String, String> fourthMap, Map<String, String> filteredMap,
            List<String> reporValues) {

        for (Map.Entry<String, String> entry1 : firstMap.entrySet()) {
            for (Map.Entry<String, String> entry2 : secondMap.entrySet()) {
                for (Map.Entry<String, String> entry3 : thirdMap.entrySet()) {
                    for (Map.Entry<String, String> entry4 : fourthMap.entrySet()) {
                        if (entry1.getKey().equals(entry2.getKey()) && entry2.getKey().equals(entry3.getKey())
                                && entry3.getKey().equals(entry4.getKey()) && entry4.getKey().equals(entry1.getKey())) {
                            reporValues
                                    .add(entry1.getValue() + entry2.getValue() + entry3.getValue() + entry4.getValue());
                            filteredMap.put(entry1.getKey(), reporValues.toString());
                            return true;
                        }
                    }
                }
            }
        }
        return false;
    }
}

Output:

true
{b=[b-first_b-second_b-third_b-fourth_]}
[b-first_b-second_b-third_b-fourth_]

Answer 2

Try this.

public static Map<String, String> areEqualKeyValues(Map<String, String>... maps) {
    if (maps.length <= 0)
        throw new IllegalArgumentException("maps");
    Set<String> keys = new HashSet<>();
    for (Map<String, String> map : maps)
        keys.addAll(map.keySet());
    Map<String, String> filteredMap = new HashMap<>();
    for (String key : keys) {
        StringBuilder sb = new StringBuilder();
        for (Map<String, String> map : maps)
            sb.append(map.get(key));
        filteredMap.put(key, sb.toString());
    }
    return filteredMap;
}

Answer 3

You should not iterate over the entrySet but use the keySet instead.
Then you can just use the retainAll of the first keySet repeatedly to only retain the objects that are in all the sets.