Java 8: Create LinkedHashMap with Collectors.toMap

In this article, I will show you how to create a LinkdHashMap with Collectors.toMap() method.

Assume that you have a LinkedHashMap with the key as a string and the value as an Integer. Now you want to iterate this map and modify the value of each key and then create an another map to hold that value.

Refer the below code to know how to do that.



import java.util.LinkedHashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {

    public static void main(String[] args) {

        Map map1 = new LinkedHashMap();

        map1.put("key1", 1);
        map1.put("key2", 2);
        map1.put("key3", 3);

        System.out.println(map1);

        Map map2 = map1.keySet()
                .stream()
                .collect(Collectors.toMap(key -> key,
                        key -> (map1.get(key) + 10),
                        (e1, e2) -> e1,
                        LinkedHashMap::new));

        System.out.println(map2);
    }
}


By default, the Collectors.toMap function creates a HashMap but we can override it by specifying the mapSupplier argument. The corresponding function from Collectors class which should be used in this case.


    public static <T, K, U, M extends Map<K, U>>
    Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
                                Function<? super T, ? extends U> valueMapper,
                                BinaryOperator<U> mergeFunction,
                                Supplier<M> mapSupplier) 

So in my code,I am passing the key, updated value, merge function(consider the first key if there is a duplicate key), finally the LinkedHashMap to the Collectors.toMap() function.

The output will be like this,


 map1{key1=1, key2=2, key3=3}
 map2{key1=11, key2=12, key3=13}

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