@Test
public void listDistinctByStreamDistinct() {
  // 1. 對于 String 列表去重
  List<String> stringList = new ArrayList<String>() {{
    add("A");
    add("A");
    add("B");
    add("B");
    add("C");
  }};
  out.print("去重前：");
  for (String s : stringList) {
    out.print(s);
  }
  out.println();
  stringList = stringList.stream().distinct().collect(Collectors.toList());
  out.print("去重后：");
  for (String s : stringList) {
    out.print(s);
  }
  out.println();

}

結果如下：

去重前：AABBC
去重后：ABC

1.2 對于實體類列表的去重

注：代碼中我們使用了 Lombok 插件的 @Data注解，可自動覆寫 equals() 以及 hashCode() 方法。

/**
* 定義一個實體類
*/  
@Data
public class Student {
  private String stuNo;
  private String name;
}

@Test
public void listDistinctByStreamDistinct() throws JsonProcessingException {
    ObjectMapper objectMapper = new ObjectMapper();
    // 1. 對于 Student 列表去重
    List<Student> studentList = getStudentList();
    out.print("去重前：");
    out.println(objectMapper.writeValueAsString(studentList));
    studentList = studentList.stream().distinct().collect(Collectors.toList());
    out.print("去重后：");
    out.println(objectMapper.writeValueAsString(studentList));
  }

結果如下：

去重前：[{"stuNo":"001","name":"Tom"},{"stuNo":"002","name":"Mike"},{"stuNo":"001","name":"Tom"}]
去重后：[{"stuNo":"001","name":"Tom"},{"stuNo":"002","name":"Mike"}]

二. 根據 List<Object> 中 Object 某個屬性去重

2.1 新建一個列表出來

@Test
  public void distinctByProperty1() throws JsonProcessingException {
    // 這里第一種方法我們通過新創(chuàng)建一個只有不同元素列表來實現根據對象某個屬性去重
    ObjectMapper objectMapper = new ObjectMapper();
    List<Student> studentList = getStudentList();
    out.print("去重前        :");
    out.println(objectMapper.writeValueAsString(studentList));
    studentList = studentList.stream().distinct().collect(Collectors.toList());
    out.print("distinct去重后:");
    out.println(objectMapper.writeValueAsString(studentList));
    // 這里我們引入了兩個靜態(tài)方法，以及通過 TreeSet<> 來達到獲取不同元素的效果
    // 1. import static java.util.stream.Collectors.collectingAndThen;
    // 2. import static java.util.stream.Collectors.toCollection;
    studentList = studentList.stream().collect(
      collectingAndThen(
        toCollection(() -> new TreeSet<>(Comparator.comparing(Student::getName))), ArrayList::new)
    );
    out.print("根據名字去重后 :");
    out.println(objectMapper.writeValueAsString(studentList));
  }

結果如下：

去重前 :[{"stuNo":"001","name":"Tom"},{"stuNo":"001","name":"Tom"},{"stuNo":"003","name":"Tom"}]
distinct去重后:[{"stuNo":"001","name":"Tom"},{"stuNo":"003","name":"Tom"}]
根據名字去重后 :[{"stuNo":"001","name":"Tom"}]

2.2 通過 filter() 方法

我們首先創(chuàng)建一個方法作為 Stream.filter() 的參數，其返回類型為 Predicate，原理就是判斷一個元素能否加入到 Set 中去，代碼如下：

private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
    Set<Object> seen = ConcurrentHashMap.newKeySet();
    return t -> seen.add(keyExtractor.apply(t));
}

使用如下：

@Test
  public void distinctByProperty2() throws JsonProcessingException {
    // 這里第二種方法我們通過過濾來實現根據對象某個屬性去重
    ObjectMapper objectMapper = new ObjectMapper();
    List<Student> studentList = getStudentList();
    out.print("去重前        :");
    out.println(objectMapper.writeValueAsString(studentList));
    studentList = studentList.stream().distinct().collect(Collectors.toList());
    out.print("distinct去重后:");
    out.println(objectMapper.writeValueAsString(studentList));
    // 這里我們將 distinctByKey() 方法作為 filter() 的參數，過濾掉那些不能加入到 set 的元素
    studentList = studentList.stream().filter(distinctByKey(Student::getName)).collect(Collectors.toList());
    out.print("根據名字去重后 :");
    out.println(objectMapper.writeValueAsString(studentList));
  }

結果如下：

去重前 :[{"stuNo":"001","name":"Tom"},{"stuNo":"001","name":"Tom"},{"stuNo":"003","name":"Tom"}]
distinct去重后:[{"stuNo":"001","name":"Tom"},{"stuNo":"003","name":"Tom"}]
根據名字去重后 :[{"stuNo":"001","name":"Tom"}]

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