How to Create a Map of Maps in Java - Step by Step Examples

We can create a map of maps in Java by using the HashMap data structure. This example demonstrates creating a map where both the outer and inner maps use integers for keys and values.

For example,

import java.util.HashMap;
import java.util.Map;

public class MapOfMaps {
    public static void main(String[] args) {
        // Declare a map of maps
        Map<Integer, Map<Integer, Integer>> outerMap = new HashMap<>();

        // Populate the inner maps
        Map<Integer, Integer> innerMap1 = new HashMap<>();
        innerMap1.put(1, 10);
        innerMap1.put(2, 20);
        innerMap1.put(3, 30);

        Map<Integer, Integer> innerMap2 = new HashMap<>();
        innerMap2.put(4, 40);
        innerMap2.put(5, 50);
        innerMap2.put(6, 60);

        // Insert the inner maps into the outer map
        outerMap.put(1, innerMap1);
        outerMap.put(2, innerMap2);

        // Print the resulting map of maps
        System.out.println("Map of maps with integer keys and values:");
        for (Map.Entry<Integer, Map<Integer, Integer>> outerEntry : outerMap.entrySet()) {
            System.out.println("Outer key: " + outerEntry.getKey());
            for (Map.Entry<Integer, Integer> innerEntry : outerEntry.getValue().entrySet()) {
                System.out.println("  Inner key: " + innerEntry.getKey() + ", value: " + innerEntry.getValue());
            }
        }
    }
}

Map of maps with integer keys and values:
Outer key: 1
  Inner key: 1, value: 10
  Inner key: 2, value: 20
  Inner key: 3, value: 30
Outer key: 2
  Inner key: 4, value: 40
  Inner key: 5, value: 50
  Inner key: 6, value: 60

We can create a map of maps in Java by using the HashMap data structure. This example demonstrates creating a map where the outer map uses strings for keys and the inner maps use integers for keys and strings for values.

For example,

import java.util.HashMap;
import java.util.Map;

public class MapOfMaps {
    public static void main(String[] args) {
        // Declare a map of maps
        Map<String, Map<Integer, String>> outerMap = new HashMap<>();

        // Populate the inner maps
        Map<Integer, String> innerMap1 = new HashMap<>();
        innerMap1.put(1, "one");
        innerMap1.put(2, "two");
        innerMap1.put(3, "three");

        Map<Integer, String> innerMap2 = new HashMap<>();
        innerMap2.put(4, "four");
        innerMap2.put(5, "five");
        innerMap2.put(6, "six");

        // Insert the inner maps into the outer map
        outerMap.put("first", innerMap1);
        outerMap.put("second", innerMap2);

        // Print the resulting map of maps
        System.out.println("Map of maps with mixed key and value types:");
        for (Map.Entry<String, Map<Integer, String>> outerEntry : outerMap.entrySet()) {
            System.out.println("Outer key: " + outerEntry.getKey());
            for (Map.Entry<Integer, String> innerEntry : outerEntry.getValue().entrySet()) {
                System.out.println("  Inner key: " + innerEntry.getKey() + ", value: " + innerEntry.getValue());
            }
        }
    }
}

Map of maps with mixed key and value types:
Outer key: first
  Inner key: 1, value: one
  Inner key: 2, value: two
  Inner key: 3, value: three
Outer key: second
  Inner key: 4, value: four
  Inner key: 5, value: five
  Inner key: 6, value: six

We can create a map of maps in Java by using the HashMap data structure. This example demonstrates creating a map where both the outer and inner maps use strings for keys and doubles for values.

For example,

import java.util.HashMap;
import java.util.Map;

public class MapOfMaps {
    public static void main(String[] args) {
        // Declare a map of maps
        Map<String, Map<String, Double>> outerMap = new HashMap<>();

        // Populate the inner maps
        Map<String, Double> innerMap1 = new HashMap<>();
        innerMap1.put("A", 1.1);
        innerMap1.put("B", 2.2);
        innerMap1.put("C", 3.3);

        Map<String, Double> innerMap2 = new HashMap<>();
        innerMap2.put("D", 4.4);
        innerMap2.put("E", 5.5);
        innerMap2.put("F", 6.6);

        // Insert the inner maps into the outer map
        outerMap.put("group1", innerMap1);
        outerMap.put("group2", innerMap2);

        // Print the resulting map of maps
        System.out.println("Map of maps with string keys and double values:");
        for (Map.Entry<String, Map<String, Double>> outerEntry : outerMap.entrySet()) {
            System.out.println("Outer key: " + outerEntry.getKey());
            for (Map.Entry<String, Double> innerEntry : outerEntry.getValue().entrySet()) {
                System.out.println("  Inner key: " + innerEntry.getKey() + ", value: " + innerEntry.getValue());
            }
        }
    }
}

Map of maps with string keys and double values:
Outer key: group1
  Inner key: A, value: 1.1
  Inner key: B, value: 2.2
  Inner key: C, value: 3.3
Outer key: group2
  Inner key: D, value: 4.4
  Inner key: E, value: 5.5
  Inner key: F, value: 6.6