Java 8 Stream.distinct() 列表去重示例
在这篇文章里,我们将提供Java8 Stream distinct()示例。 distinct()返回由该流的不同元素组成的流。distinct()是Stream接口的方法。distinct()使用hashCode()和equals()方法来获取不同的元素。因此,我们的类必须实现hashCode()和equals()方法。如果distinct()正在处理有序流,那么对于重复元素,将保留以遭遇顺序首先出现的元素,并且以这种方式选择不同元素是稳定的。在无序流的情况下,不同元素的选择不一定是稳定的,是可以改变的。distinct()执行有状态的中间操作。在有序流的并行流的情况下,保持distinct()的稳定性是需要很高的代价的,因为它需要大量的缓冲开销。如果我们不需要保持遭遇顺序的一致性,那么我们应该可以使用通过BaseStream.unordered()方法实现的无序流。
1. Stream.distinct()
Stream<T> distinct()
它是Stream接口的方法。在此示例中,我们有一个包含重复元素的字符串数据类型列表
DistinctSimpleDemo.java
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package com.concretepage;
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import java.util.Arrays;
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import java.util.List;
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import java.util.stream.Collectors;
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public class DistinctSimpleDemo {
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public static void main(String[] args) {
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List<String> list = Arrays.asList("AA", "BB", "CC", "BB", "CC", "AA", "AA");
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long l = list.stream().distinct().count();
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System.out.println("No. of distinct elements:"+l);
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String output = list.stream().distinct().collect(Collectors.joining(","));
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System.out.println(output);
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}
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}
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No. of distinct elements:3
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AA,BB,CC
2. Stream.distinct() with List of Objects
在此示例中,我们有一个Book对象列表。 为了对列表进行去重,该类将重写hashCode()和equals()。
Book.java
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package com.concretepage;
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public class Book {
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private String name;
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private int price;
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public Book(String name, int price) {
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this.name = name;
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this.price = price;
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}
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public String getName() {
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return name;
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}
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public int getPrice() {
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return price;
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}
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public boolean equals(final Object obj) {
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if (obj == null) {
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return false;
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}
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final Book book = (Book) obj;
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if (this == book) {
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return true;
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} else {
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return (this.name.equals(book.name) && this.price == book.price);
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}
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}
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public int hashCode() {
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int hashno = 7;
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hashno = 13 * hashno + (name == null ? 0 : name.hashCode());
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return hashno;
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}
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}
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package com.concretepage;
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import java.util.ArrayList;
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import java.util.List;
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public class DistinctWithUserObjects {
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public static void main(String[] args) {
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List<Book> list = new ArrayList<>();
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{
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list.add(new Book("Core Java", 200));
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list.add(new Book("Core Java", 200));
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list.add(new Book("Learning Freemarker", 150));
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list.add(new Book("Spring MVC", 300));
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list.add(new Book("Spring MVC", 300));
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}
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long l = list.stream().distinct().count();
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System.out.println("No. of distinct books:"+l);
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list.stream().distinct().forEach(b -> System.out.println(b.getName()+ "," + b.getPrice()));
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}
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}
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No. of distinct books:3
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Core Java,200
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Learning Freemarker,150
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Spring MVC,300
3. Distinct by Property
distinct()不提供按照属性对对象列表进行去重的直接实现。它是基于hashCode()和equals()工作的。如果我们想要按照对象的属性,对对象列表进行去重,我们可以通过其它方法来实现。如下代码段所示:
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static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
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Map<Object,Boolean> seen = new ConcurrentHashMap<>();
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return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
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}
list.stream().filter(distinctByKey(b -> b.getName()));
distinctByKey()方法返回一个使用ConcurrentHashMap 来维护先前所见状态的 Predicate 实例,如下是一个完整的使用对象属性来进行去重的示例。
DistinctByProperty.java
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package com.concretepage;
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import java.util.ArrayList;
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import java.util.List;
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import java.util.Map;
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import java.util.concurrent.ConcurrentHashMap;
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import java.util.function.Function;
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import java.util.function.Predicate;
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public class DistinctByProperty {
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public static void main(String[] args) {
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List<Book> list = new ArrayList<>();
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{
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list.add(new Book("Core Java", 200));
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list.add(new Book("Core Java", 300));
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list.add(new Book("Learning Freemarker", 150));
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list.add(new Book("Spring MVC", 200));
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list.add(new Book("Hibernate", 300));
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}
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list.stream().filter(distinctByKey(b -> b.getName()))
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.forEach(b -> System.out.println(b.getName()+ "," + b.getPrice()));
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}
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private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
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Map<Object,Boolean> seen = new ConcurrentHashMap<>();
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return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
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}
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}
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Core Java,200
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Learning Freemarker,150
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Spring MVC,200
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Hibernate,300
posted on 2022-06-17 17:22 ExplorerMan 阅读(269) 评论(0) 编辑 收藏 举报