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}}