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Java 8 Streams - Collectors.toMap Examples

Java 8 Streams Java Java API 

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java.lang.Objectjava.lang.Objectjava.util.stream.Collectorsjava.util.stream.CollectorsLogicBig

The static overloaded methods, Collectors.toMap() return a Collector which produces a new instance of Map, populated with keys per provided keyMapper function and values per provided valueMap function.

<T,K,U> Collector<T,?,Map<K,U>> toMap(

                                  Function<? super T,? extends K> keyMapper,

                                  Function<? super T,? extends U> valueMapper)


<T,K,U> Collector<T,?,Map<K,U>> toMap(

                                  Function<? super T,? extends K> keyMapper,

                                  Function<? super T,? extends U> valueMapper,

                                  BinaryOperator<U> mergeFunction)


<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)

Parameters

keyMapper: A mapping function to produce the map keys for each input stream element.

valueMapper: A mapping function to produce the map values for each input stream element.

mergeFunction: A binary operator which is to resolve collisions between values associated with the same key. The inputs to this function are the values which belong to the same key.

mapSupplier: A function which provides a new instance of the desired implementation of the Map.


Examples


package com.logicbig.example.collectors;

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

public class ToMapExample {
    public static void main (String[] args) {
        Stream<String> s = Stream.of("apple", "banana", "orange");
        Map<Character, String> m = s.collect(
                            Collectors.toMap(s1 -> s1.charAt(0),
                                                s1 -> s1));
        System.out.println(m);
    }
}

Output

{a=apple, b=banana, o=orange}
Original Post




The method Collectors.toMap(keyMapper, valueMapper) will throw IllegalStateException if there are duplicate keys as provided by keyMapper function. 

package com.logicbig.example.collectors;

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

public class ToMapExample1 {
    public static void main (String[] args) {
        Stream<String> s = Stream.of("apple", "banana", "apricot", "orange");
        Map<Character, String> m = s.collect(
                            Collectors.toMap(s1 -> s1.charAt(0),
                                                s1 -> s1));
        System.out.println(m);
    }
}

Output

Caused by: java.lang.IllegalStateException: Duplicate key apple
	at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
	at java.util.HashMap.merge(HashMap.java:1253)
	at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
	at com.logicbig.example.collectors.ToMapExample1.main(ToMapExample1.java:17)
	... 6 more
Original Post




package com.logicbig.example.collectors;

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

public class ToMapExample2 {
    public static void main (String[] args) {
        Stream<String> s = Stream.of("apple", "banana", "apricot", "orange",
                            "apple");
        Map<Character, String> m = s.collect(
                            Collectors.toMap(s1 -> s1.charAt(0),
                                                s1 -> s1,
                                                (s1, s2) -> s1 + "|" + s2));
        System.out.println(m);
    }
}

Output

{a=apple|apricot|apple, b=banana, o=orange}
Original Post




package com.logicbig.example.collectors;

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

public class ToMapExample3 {
    public static void main (String[] args) {
        Stream<String> s = Stream.of("apple", "banana", "apricot", "orange",
                            "apple");
        LinkedHashMap<Character, String> m = s.collect(
                            Collectors.toMap(s1 -> s1.charAt(0),
                                                s1 -> s1,
                                                (s1, s2) -> s1 + "|" + s2,
                                                LinkedHashMap::new));
        System.out.println(m);
    }
}

Output

{a=apple|apricot|apple, b=banana, o=orange}
Original Post

See Also