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, "a
![List<T> を IDをキーにしたMap に変換 - Qiita](https://cdn-ak-scissors.b.st-hatena.com/image/square/36556d608ea343710d9bbf503e1ecc1091129ce6/height=288;version=1;width=512/https%3A%2F%2Fqiita-user-contents.imgix.net%2Fhttps%253A%252F%252Fcdn.qiita.com%252Fassets%252Fpublic%252Farticle-ogp-background-412672c5f0600ab9a64263b751f1bc81.png%3Fixlib%3Drb-4.0.0%26w%3D1200%26mark64%3DaHR0cHM6Ly9xaWl0YS11c2VyLWNvbnRlbnRzLmltZ2l4Lm5ldC9-dGV4dD9peGxpYj1yYi00LjAuMCZ3PTk3MiZoPTM3OCZ0eHQ9TGlzdCUzQ1QlM0UlMjAlRTMlODIlOTIlMjBJRCVFMyU4MiU5MiVFMyU4MiVBRCVFMyU4MyVCQyVFMyU4MSVBQiVFMyU4MSU5NyVFMyU4MSU5Rk1hcCUyMCVFMyU4MSVBQiVFNSVBNCU4OSVFNiU4RiU5QiZ0eHQtYWxpZ249bGVmdCUyQ3RvcCZ0eHQtY29sb3I9JTIzMjEyMTIxJnR4dC1mb250PUhpcmFnaW5vJTIwU2FucyUyMFc2JnR4dC1zaXplPTU2JnM9OTU4MTkzOGQ4YmMxMmYzY2I5OTIxZGQ4NDM4MWEwYWM%26mark-x%3D142%26mark-y%3D57%26blend64%3DaHR0cHM6Ly9xaWl0YS11c2VyLWNvbnRlbnRzLmltZ2l4Lm5ldC9-dGV4dD9peGxpYj1yYi00LjAuMCZoPTc2Jnc9NzcwJnR4dD0lNDBBcHBsZVBlZGxhciZ0eHQtY29sb3I9JTIzMjEyMTIxJnR4dC1mb250PUhpcmFnaW5vJTIwU2FucyUyMFc2JnR4dC1zaXplPTM2JnR4dC1hbGlnbj1sZWZ0JTJDdG9wJnM9MTNkYjJiNTdlNDkzOTIwMTI2ZmEwMzJkMDY2YWNkNTM%26blend-x%3D142%26blend-y%3D486%26blend-mode%3Dnormal%26s%3Db089df102dc3976d53a6287a920bfdd2)