Java Stream Collectors.toUnmodifiableMap
October 04, 2020
Collectors.toUnmodifiableMap example introduced in Java 10.
1. The
Collectors.toUnmodifiableMap returns a Collector that accumulates the input elements into an unmodifiable Map. Their keys and values are obtained by applying the specified mapping functions to the input elements.
2. To handle duplicate keys, we need to pass
BinaryOperator otherwise it will throw error.
1. toUnmodifiableMap Example
Find the method declaration of Collectors.toUnmodifiableMap from Java doc.
public static <T,K,U> Collector<T,?,Map<K,U>> toUnmodifiableMap(
Function<? super T, ? extends K> keyMapper,
Function<? super T, ? extends U> valueMapper)
IllegalStateException is thrown when the collection operation is performed.
Example1.java
package com.concretepage;
import java.util.Arrays;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;
public class Example1 {
public static void main(String[] args) {
Map<Integer, Integer> map1 = Stream.of(10, 15, 20)
.collect(Collectors.toUnmodifiableMap(i -> i, i -> i * 2));
System.out.println(map1);
Map<Integer, Integer> map2 = IntStream.range(20, 25).boxed()
.collect(Collectors.toUnmodifiableMap(i -> i + 2, i -> i * 5));
System.out.println(map2);
Map<Integer, String> map3 = Arrays.asList(1, 2, 3).stream().map(i -> i * 2)
.collect(Collectors.toUnmodifiableMap(i -> i * 10, i -> "A" + i));
System.out.println(map3);
}
}
{20=40, 10=20, 15=30}
{26=120, 25=115, 24=110, 23=105, 22=100}
{20=A2, 60=A6, 40=A4}
Map<Integer, Integer> map1 = Stream.of(10, 10, 15, 20) .collect(Collectors.toUnmodifiableMap(i -> i, i -> i * 2));
java.lang.IllegalStateException: Duplicate key 10 (attempted merging values 20 and 20)
2. toUnmodifiableMap Example with BinaryOperator
Find the method declaration of Collectors.toUnmodifiableMap from Java doc with BinaryOperator.
public static <T,K,U> Collector<T,?,Map<K,U>> toUnmodifiableMap(
Function<? super T,? extends K> keyMapper,
Function<? super T,? extends U> valueMapper,
BinaryOperator<U> mergeFunction)
Find the example.
Example2.java
package com.concretepage;
import java.util.Arrays;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class Example2 {
public static void main(String[] args) {
Map<Integer, Integer> map1 = Stream.of(10, 10, 15, 20)
.collect(Collectors.toUnmodifiableMap(i -> i, i -> i * 2, (x, y) -> x + y));
System.out.println(map1);
Map<Integer, String> map2 = Arrays.asList(1, 1, 2, 2, 3).stream().map(i -> i * 2)
.collect(Collectors.toUnmodifiableMap(i -> i * 10, i -> "A" + i, (x, y) -> x + "-" + y));
System.out.println(map2);
}
}
{20=40, 15=30, 10=40}
{60=A6, 20=A2-A2, 40=A4-A4}
Example3.java
package com.concretepage;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class Example3 {
public static void main(String[] args) {
Student s1 = new Student(22, "Mahesh");
Student s2 = new Student(22, "Suresh");
Student s3 = new Student(23, "Krishna");
Map<Integer, String> map = Stream.of(s1, s2, s3)
.collect(Collectors.toUnmodifiableMap(Student::getAge, Student::getName, (x, y) -> x + "-" + y));
System.out.println(map);
}
}
class Student {
private Integer age;
private String name;
public Student(Integer age, String name){
this.age=age;
this.name=name;
}
//Sets and Gets
}
{22=Mahesh-Suresh, 23=Krishna}
