在學習本篇以前,最好對java8新特性有必定的瞭解。能夠參考:Java8新特性--流(Stream)html
場景:有一個實體的List集合,須要根據實體中的某個字段對List去重java
要想去重,能夠考慮使用TreeSet。先來看看TreeSet的用法:post
@Getter @Setter @NoArgsConstructor @AllArgsConstructor public class Dish { private String id; private String name; }
private static List<Dish> dishList = new ArrayList<Dish>(); static { Dish dish1 = new Dish("001", "張三"); dishList.add(dish1); Dish dish2 = new Dish("001", "李四"); dishList.add(dish2); Dish dish3 = new Dish("002", "王五"); dishList.add(dish3); }
/** * 測試TreeSet默認比較器及傳入比較器 */ @Test public void testTreeSet() { // TreeSet不傳比較器,則默認按照天然順序排序 TreeSet<Integer> treeSet = new TreeSet<>(); treeSet.add(3); treeSet.add(1); treeSet.add(4); System.out.println(treeSet); // [1, 3, 4] // TreeSet中的元素,若是是實體,必須得傳比較器(或者實體類須要實現Comparable中的compareTo方法),否則就會報錯 // TreeSet是Set的子類,裏面的元素有序且不能重複,能夠去重 TreeSet<Dish> treeSet2 = new TreeSet<>(Comparator.comparing(Dish::getId)); treeSet2.addAll(dishList); treeSet2.forEach(d -> System.out.println("id:" + d.getId() + ", name:" + d.getName())); // id:001, name:張三 // id:002, name:王五 }
從上面能夠看到TreeSet能夠根據實體中的某個字段(這裏是id)排序後去重。若是再將TreeSet轉爲List就能夠達到咱們的目的:學習
List<Dish> newDishList = new ArrayList<>(treeSet2); newDishList.forEach(d -> System.out.println("id:" + d.getId() + ", name:" + d.getName())); // id:001, name:張三 // id:002, name:王五
雖然大功告成,可是寫這麼一堆代碼,好麻煩。能夠使用java8中的收集器簡化一下:測試
/** * 測試Collectors.toCollection方法:將結果收集到其它類型的集合中(這裏是TreeSet) */ @Test public void testToCollection() { TreeSet<Dish> treeSet2 = dishList.stream() .collect(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Dish::getId)))); List<Dish> newDishList = new ArrayList<>(treeSet2); newDishList.forEach(d -> System.out.println("id:" + d.getId() + ", name:" + d.getName())); // id:001, name:張三 // id:002, name:王五 }
還能夠再簡單一點:url
/** * 測試Collectors.collectingAndThen方法:將流中的數據經過Collector計算,計算的結果再經過Function處理一下 * (這裏是將TreeSet轉爲ArrayList。即至關於將最終結果又通過了new ArrayList<>(treeSet)) */ @SuppressWarnings("unchecked") @Test public void testCollectingAndThen() { List<Dish> newDishList = dishList.stream().collect(Collectors.collectingAndThen( Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Dish::getId))), ArrayList::new)); newDishList.forEach(d -> System.out.println("id:" + d.getId() + ", name:" + d.getName())); // id:001, name:張三 // id:002, name:王五 }