Transform HashMap in Java 8 and above

This post will discuss how to transform HashMap key-value pairs from one type into another type in Java.

1. Transforming `HashMap<K₁,V>` to `HashMap<K₂,V>`

import java.util.HashMap;

import java.util.Map;

import java.util.stream.Collectors;

class Main

{

public static void main(String[] args)

{

Map<String, String> map = new HashMap<>();

map.put("1", "One");

map.put("2", "Two");

map.put("3", "Three");

Map<Integer, String> hashMap =

map.entrySet().stream().collect(Collectors.toMap(

entry -> Integer.parseInt(entry.getKey()),

entry -> entry.getValue()));

System.out.println(hashMap);

}

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Output:

{1=One, 2=Two, 3=Three}

2. Transforming `HashMap<K,V₁>` to `HashMap<K,V₂>`

import java.util.HashMap;

import java.util.Map;

import java.util.stream.Collectors;

class Main

{

public static void main(String[] args)

{

Map<String, String> map = new HashMap<>();

map.put("One", "1");

map.put("Two", "2");

map.put("Three", "3");

Map<String, Integer> hashMap =

map.entrySet().stream().collect(Collectors.toMap(

entry -> entry.getKey(),

entry -> Integer.parseInt(entry.getValue()))

);

System.out.println(hashMap);

}

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Output:

{One=1, Two=2, Three=3}

This is equivalent to:

import java.util.HashMap;

import java.util.Map;

class Main

{

public static void main(String[] args)

{

Map<String, String> map = new HashMap<>();

map.put("One", "1");

map.put("Two", "2");

map.put("Three", "3");

Map<String, Integer> hashMap = new HashMap<>();

for (Map.Entry<String, String> entry : map.entrySet()) {

if (hashMap.put(entry.getKey(), Integer.parseInt(entry.getValue())) != null) {

throw new IllegalStateException("Duplicate key");

}

System.out.println(hashMap);

}

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Output:

{One=1, Two=2, Three=3}

3. Transforming `HashMap<K₁,V₁>` to `HashMap<K₂,V₂>`

import java.util.HashMap;

import java.util.Map;

import java.util.stream.Collectors;

class Main

{

public static void main(String[] args)

{

Map<String, String> powerMap = new HashMap<>();

powerMap.put("1", "1");

powerMap.put("2", "4");

powerMap.put("3", "9");

Map<Integer, Integer> hashMap =

powerMap.entrySet().stream().collect(Collectors.toMap(

entry -> Integer.parseInt(entry.getKey()),

entry -> Integer.parseInt(entry.getValue())));

System.out.println(hashMap);

}

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Output:

{1=1, 2=4, 3=9}

Similarly, we can convert HashMap of other types. For instance, we can use String.valueOf() method in place of Integer.parseInt() to convert an Integer key/value to a string.

That’s all about transforming HashMap in Java.

Transform HashMap in Java 8 and above

1. Transforming HashMap<K1,V> to HashMap<K2,V>

2. Transforming HashMap<K,V1> to HashMap<K,V2>

3. Transforming HashMap<K1,V1> to HashMap<K2,V2>

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1. Transforming `HashMap<K₁,V>` to `HashMap<K₂,V>`

2. Transforming `HashMap<K,V₁>` to `HashMap<K,V₂>`

3. Transforming `HashMap<K₁,V₁>` to `HashMap<K₂,V₂>`