1. Stream 的distinct()方法

Stream<T> distinct();

1.1 对于 String 列表的去重

@Test
public void listDistinctByStreamDistinct() {
  // 1. 对于 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();
}

去重前：AABBC
去重后：ABC

1.2 对于实体类列表的去重

/**
* 定义一个实体类
*/ 
@Data
public class Student {
  private String stuNo;
  private String name;
}

@Test
public void listDistinctByStreamDistinct() throws JsonProcessingException {
    ObjectMapper objectMapper = new ObjectMapper();
    // 1. 对于 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));
  }

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"}]

2. 根据 List<Object> 中 Object 某个属性去重

2.1 新建一个列表出来

  @Test
  public void distinctByProperty1() throws JsonProcessingException {
    // 这里第一种方法我们通过新创建一个只有不同元素列表来实现根据对象某个属性去重
    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));
    // 这里我们引入了两个静态方法，以及通过 TreeSet<> 来达到获取不同元素的效果
    // 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("根据名字去重后 :");
    out.println(objectMapper.writeValueAsString(studentList));
  }

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"}]
根据名字去重后 :[{"stuNo":"001","name":"Tom"}]

2.2 通过 filter() 方法

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

  @Test
  public void distinctByProperty2() throws JsonProcessingException {
    // 这里第二种方法我们通过过滤来实现根据对象某个属性去重
    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));
    // 这里我们将 distinctByKey() 方法作为 filter() 的参数，过滤掉那些不能加入到 set 的元素
    studentList = studentList.stream().filter(distinctByKey(Student::getName)).collect(Collectors.toList());
    out.print("根据名字去重后 :");
    out.println(objectMapper.writeValueAsString(studentList));
  }

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"}]
根据名字去重后 :[{"stuNo":"001","name":"Tom"}]

3. 总结

 
  END
 


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