Implement a Map with multiple keys (MultiKeyMap) in Java

This post will implement MultiKeyMap in plain Java and cover its possible implementations in Apache Commons Collection and Guava library. Basically, we need a map implementation that uses multiple keys to map the value in Java.

1. Using Plain Java

The idea is to construct a custom class Key, which consists of all keys, and use an instance of the Key class in our map as a key to map the value. The Key class should override equals() and hashCode() methods to test equality on hash-based map. This is demonstrated below for two keys. Note that we can easily extend the solution to any number of keys.

import java.util.HashMap;

import java.util.Map;

class Key<K1, K2>

{

public K1 key1;

public K2 key2;

public Key(K1 key1, K2 key2)

{

this.key1 = key1;

this.key2 = key2;

}

@Override

public boolean equals(Object o)

{

if (this == o) {

return true;

}

if (o == null || getClass() != o.getClass()) {

return false;

}

Key key = (Key) o;

if (key1 != null ? !key1.equals(key.key1) : key.key1 != null) {

return false;

}

if (key2 != null ? !key2.equals(key.key2) : key.key2 != null) {

return false;

}

return true;

}

@Override

public int hashCode()

{

int result = key1 != null ? key1.hashCode() : 0;

result = 31 * result + (key2 != null ? key2.hashCode() : 0);

return result;

}

@Override

public String toString() {

return "[" + key1 + ", " + key2 + "]";

}

class Main

{

public static void main(String[] args)

{

// Create a `HashMap` with `Key` as key

Map<Key, String> multiKeyMap = new HashMap<>();

// [key1, key2] -> value1

Key k12 = new Key("key1", "key2");

multiKeyMap.put(k12, "value1");

// [key3, key4] -> value2

Key k34 = new Key("key3", "key4");

multiKeyMap.put(k34, "value2");

// print multikey map

System.out.println(multiKeyMap);

// print value corresponding to key1 and key2

System.out.println(multiKeyMap.get(k12));

}

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Output:

{[key1, key2]=value1, [key3, key4]=value2}
value1

2. Using Apache Commons Collections

We can also use Apache Commons Collection, which provides an efficient map implementation MultiKeyMap that maps multiple keys to a value. MultiKeyMap provides get, containsKey, put, and remove for individual keys.

import org.apache.commons.collections4.map.LinkedMap;

import org.apache.commons.collections4.map.MultiKeyMap;

class Main

{

public static void main(String[] args)

{

// creates an ordered map

MultiKeyMap multiKeyMap = MultiKeyMap.multiKeyMap(new LinkedMap());

// [key1, key2] -> value1

multiKeyMap.put("key1", "key2", "value1");

// [key3, key4] -> value2

multiKeyMap.put("key3", "key4", "value2");

// print multikey map

System.out.println(multiKeyMap);

// print value corresponding to key1 and key2

System.out.println(multiKeyMap.get("key1", "key2"));

}

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Output:

{MultiKey[key1, key2]=value1, MultiKey[key3, key4]=value2}
value1

3. Using Google’s Guava

There are several ways to accomplish this with Google’s Guava library. An elegant solution is to use Guava’s Table interface, which associates an ordered pair of keys, called a row key and a column key, with a single value.

import com.google.common.collect.HashBasedTable;

import com.google.common.collect.Table;

class Main

{

public static void main(String[] args)

{

// create a map

Table<String, String, String> table = HashBasedTable.create();

// [key1, key2] -> value1

table.put("key1", "key2", "value1");

// [key3, key4] -> value2

table.put("key3", "key4", "value2");

// print multikey map

System.out.println(table);

// print value corresponding to key1 and key2

System.out.println(table.get("key1", "key2"));

}

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Output:

{key1={key2=value1}, key3={key4=value2}}
value1

We can also use ImmutableList to build your keys in order, as shown below:

import com.google.common.collect.ImmutableList;

import com.google.common.collect.Maps;

import java.util.Map;

class Main

{

public static void main(String[] args)

{

// Create a map

Map<ImmutableList<String>, String> listKeyMap = Maps.newHashMap();

// [key1, key2] -> value1

listKeyMap.put(ImmutableList.of("key1", "key2"), "value1");

// [key3, key4] -> value2

listKeyMap.put(ImmutableList.of("key3", "key4"), "value2");

// print multikey map

System.out.println(listKeyMap);

// print value corresponding to key1 and key2

System.out.println(listKeyMap.get(ImmutableList.of("key1", "key2")));

}

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Output:

{[key1, key2]=value1, [key3, key4]=value2}
value1

That’s all about MultiKeyMap implementation in Java.

Implement a Map with multiple keys (MultiKeyMap) in Java

1. Using Plain Java

2. Using Apache Commons Collections

3. Using Google’s Guava

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