List<T> を IDをキーにしたMap に変換

すぐ忘れるのでメモ。

idが重複するとjava.lang.IllegalStateException: Duplicate keyが発生するので注意。

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.function.UnaryOperator;
import java.util.stream.Collectors;


public class ListToMapSample {
    public static void main(String[] args) {
        List<Foo> fooList = new ArrayList<>();
        fooList.add(new Foo(123, "aiueo"));
        fooList.add(new Foo(234, "ateawgareg"));
        fooList.add(new Foo(456, "faaewfgwaiaef"));

        Map<Integer, Foo> map = fooList.stream()
                .collect(Collectors.toMap(Foo::getId, UnaryOperator.identity()));
        System.out.println(map);
    }

    private static class Foo {
        private int id;
        private String name;
        public Foo(int id, String name) {
            this.id = id;
            this.name = name;
        }

        public int getId() {
            return id;
        }

        public String toString() {
            return "{id:" + id + ", name:" + name + "}";
        }
    }
}

{456={id:456, name:faaewfgwaiaef}, 234={id:234, name:ateawgareg}, 123={id:123, name:aiueo}}