List对象去重和按照某个字段排序的实现方法

public class People {    private int id;   private String name;   private Date createDate;    public int getId() {     return id;   }    public void setId(int id) {     this.id = id;   }    public String getName() {     return name;   }    public void setName(String name) {     this.name = name;   }    public Date getCreateDate() {     return createDate;   }    public void setCreateDate(Date createDate) {     this.createDate = createDate;   }  } 

public static void main(String[] args) throws Exception{     List<People> list = new ArrayList<People>();     People p1 = new People();     p1.setId(1);     p1.setName("赵");     list.add(p1);     People p2 = new People();     p2.setId(2);     p2.setName("钱");     list.add(p2);     People p3 = new People();     p3.setId(3);     p3.setName("孙");     list.add(p3);     People p4 = new People();     p4.setId(4);     p4.setName("李");     list.add(p4);     People p5 = new People();     p5.setId(5);     p5.setName("周");     list.add(p5);     list.add(p1);     for(People p : list){       System.out.println(p);     }     System.out.println("==========================");     Set<People> set = new HashSet<People>(list);     list.clear();     list.addAll(new ArrayList<People>(set));     for(People p : list){       System.out.println(p);     }   } 

public class MyComparator implements Comparator<People> {    @Override   public int compare(People p1, People p2) {     return p1.getCreateDate().compareTo(p2.getCreateDate());   }  } 

public static void main(String[] args) throws Exception{     List<People> list = new ArrayList<People>();     People p1 = new People();     p1.setId(1);     p1.setName("赵");     p1.setCreateDate(new Date(new Date().getTime() + 1000));     list.add(p1);     People p2 = new People();     p2.setId(2);     p2.setName("钱");     p2.setCreateDate(new Date(new Date().getTime() + 8000));     list.add(p2);     People p3 = new People();     p3.setId(3);     p3.setName("孙");     p3.setCreateDate(new Date(new Date().getTime() + 7000));     list.add(p3);     People p4 = new People();     p4.setId(4);     p4.setName("李");     p4.setCreateDate(new Date(new Date().getTime() + 5000));     list.add(p4);     People p5 = new People();     p5.setId(5);     p5.setName("周");     p5.setCreateDate(new Date(new Date().getTime() + 6000));     list.add(p5);     for(People p : list){       System.out.println(p.getId());     }     //1,2,3,4,5     System.out.println("=======================");     Collections.sort(list, new MyComparator());     //1,4,5,3,2     for(People p : list){       System.out.println(p.getId());     }   }