How to insert triple items in a map in Java?

Question

So, the main idea is to create for example:

Map<Map<String, String>, Integer> map = new LinkedHashMap<>();

And I need to put in this map for example items:

{{France=Paris}=12, {Spain=Madrid}=2, ... }

How to make this? How to implement the "put" function? map.put("France","Paris",2) does not work.

Answer 1

You need to first create a Map (e.g. Map.of("France", "Paris")) and put it inside the bigger Map.

Do it as follows:

import java.util.LinkedHashMap;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        Map<Map<String, String>, Integer> map = new LinkedHashMap<>();
        map.put(Map.of("France", "Paris"), 2);
        System.out.println(map);
    }
}

Output:

{{France=Paris}=2}

Alternatively,

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

public class Main {
    public static void main(String[] args) {
        Map<Map<String, String>, Integer> map = new LinkedHashMap<>();
        Map<String, String> data = new HashMap<>();
        data.put("France", "Paris");
        map.put(data, 2);
        System.out.println(map);
    }
}

Caution: As already mentioned by Mureinik, having mutable objects as the keys (the alternative solution) in a map is error-prone. You can check this answer for an explanation.

Answer 2

Though Arvind Kumar Avinash's answer perfectly solves the question, please note that you should reconsider your data structure.

Having a Map as a key for another Map is not advisable. You should rather craft another class that serves the purpose of being a key of a Map. If you are interested in having capitals as keys, consider implementing an immutable class such as this one, in which equals() and hashcode() are properly overridden:

public class Capital {

    private final String country;
    private final String name;

    public Capital(String country, String name) {
        this.country = country;
        this.name = name;
    }

    public boolean equals(final Object o) {
        if (o == this) return true;
        if (!(o instanceof Capital)) return false;
        final Capital other = (Capital) o;
        if (!other.canEqual((Object) this)) return false;
        final Object this$country = this.country;
        final Object other$country = other.country;
        if (this$country == null ? other$country != null : !this$country.equals(other$country)) return false;
        final Object this$name = this.name;
        final Object other$name = other.name;
        if (this$name == null ? other$name != null : !this$name.equals(other$name)) return false;
        return true;
    }

    protected boolean canEqual(final Object other) {
        return other instanceof Capital;
    }

    public int hashCode() {
        final int PRIME = 59;
        int result = 1;
        final Object $country = this.country;
        result = result * PRIME + ($country == null ? 43 : $country.hashCode());
        final Object $name = this.name;
        result = result * PRIME + ($name == null ? 43 : $name.hashCode());
        return result;
    }

    public String toString() {
        return "Capital(country=" + this.country + ", name=" + this.name + ")";
    }
}

If you want to get rid of the boilerplate code, consider using the Lombok Project. The above-listed class is the delomboked version of the following code:

@ToString
@EqualsAndHashCode
@RequiredArgsConstructor
public class Capital {

    private final String country;
    private final String name;
    
}

---

Update after Holger's comment:

Other implementations for equals() and hashCode() than the above-mentioned, delomboked version are possible, differing in some criterias such as code style and performance. Same as Holger, I think the delomboked implementation should not be hand-crafted. It is simply the implementation that results through the usage of the Lombok @EqualsAndHashCode annotation.

There are requirements to be fulfilled in order to properly override hashCode() and equals():

• hashCode()
• equals()

Though performance and good code-sytle are important, they are not amongst these requirements. With the usage of Lombok, in my opinion, code-style is less important, since the code is generated. One usually does not see it during development.

The canEqual() method is present since Lombok also generates it.

Answer 3

As you are using one pair of string as the key, you can use Map.Entry as key of map rather use Map<String, String>

Map<Map.Entry<String, String>, Integer> map = new LinkedHashMap<>();
map.put(Map.entry("France", "Paris"), 12);
map.put(Map.entry("Spain", "Madrid"), 2);
System.out.println(map.get(Map.entry("France", "Paris")));

Output : 12