判断List集合中对象的某个字段是否存在重复值

判断泛型集合中的对象的某个字段是否有重复值的需求，特在此对方法进行记录。
这里我们使用了Java8的新特性Strem流，详细代码如下。
实体类如下：

/**  
 * @Title: Person.java
 * @Description: TODO(描述)
 * @author Soul
 */
public class Person {

  private String name;
  private Integer age;
  private String gradle;
  
  public Person() {
    super();
  }
  
  public Person(String name, Integer age, String gradle) {
    super();
    this.name = name;
    this.age = age;
    this.gradle = gradle;
  }
  
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
  public Integer getAge() {
    return age;
  }
  public void setAge(Integer age) {
    this.age = age;
  }
  public String getGradle() {
    return gradle;
  }
  public void setGradle(String gradle) {
    this.gradle = gradle;
  }
  
  @Override
  public String toString() {
    return "Person [name=" + name + ", age=" + age + ", gradle=" + gradle + "]";
  } 
  
}

测试代码如下：

public static void main(String[] args) {
     Person person1 = new Person("小刘", 24, "男");
        Person person2 = new Person("小贺", 18, "女");
        Person person3 = new Person("小袁", 18, "女");
        Person person4 = new Person("小李", 21, "男");
        Person person5 = new Person("小子", 21, "男");
        List<Person> persons = Arrays.asList(person1, person2, person3, person4,person5);

        // 根据指定属性分组，并统计数量（key：指定属性，value：数量）
        
        Map<Object, Long> mapGroup = persons.stream().collect(Collectors.groupingBy(person -> person.getAge(), Collectors.counting()));
        System.out.println(mapGroup.toString());

        // 筛选Map中value大于1的key
        Stream<Object> stringStream = mapGroup.entrySet().stream().filter(entry -> entry.getValue() > 1).map(entry -> entry.getKey());
        System.out.print("重复的数据：{ ");
        stringStream.forEach(str -> {
            System.out.print(str + " ");
        });
        System.out.println("}");
  }

输出结果：

1 2	{18=2.21=2,24=1} 重复的数据：{ 18 21 }