集合类TreeSet中关于Comparable接口与Comparator接口实现排序

实现自定义的排序常用有两种方法

一、实体类实现java.lang.Comparable接口且重写compareTo()方法

二、业务排序类实现java.util.Compartor接口且重写compare()方法

第一种方法举例：

实体类为学生类属性有学分，学号，姓名，该类实现了Comparable接口并重写compareTo(）方法本例按照学分降序排列

public class Student implements Comparable<Student>{
private int id;
private int score;
private String name;
public int getId() {
  return id;
}
public int getScore() {
  return score;
}
public Student(int id, int score, String name) {
  super();
  this.id = id;
  this.score = score;
  this.name = name;
}
@Override
public String toString(){
  return " 学号:"+id+" 姓名:"+name+" 学分:"+score;
}
@Override
public int compareTo(Student o) {
  // TODO Auto-generated method stub
  if(this.getScore()>o.getScore())
   return -1;
  else if(this.getScore()<o.getScore())
   return 1;
  else {
   if(this.getId()>o.getId())
    return -1;
   else if(this.getId()>o.getId())
    return 1;
   else return 0;
  }
}
}

接下来建立TreeSet类将实例化学生对象添加到集合

import java.util.TreeSet;
public class MyTree2 {
public static void main(String[] args) {
  TreeSet <Student>t =new TreeSet<>();
  t.add( new Student(1000,33,"java1"));
  t.add( new Student(1001,340,"java2"));
  t.add( new Student(1002,200,"java3"));
  //遍历集合输出
  t.forEach((Student o) -> {
   System.out.println(o);
  });
}
}

运行结果

如果没有在Student类中实现该接口将会抛异常

第二种方法举例

实体类不再实现接口，在业务排序类实现不同的排序需求

实体类：

    public class Student {
private int id;
private int score;
private String name;
public int getId() {
  return id;
}
public int getScore() {
  return score;
}
public Student(int id, int score, String name) {
  super();
  this.id = id;
  this.score = score;
  this.name = name;
}
@Override
public String toString(){
  return " 学号:"+id+" 姓名:"+name+" 学分:"+score;
}
}

排序类（按id排序）：

import java.util.Comparator;
public class SortId implements Comparator<Student>{
@Override
public int compare(Student ob1, Student ob2) {
  // TODO Auto-generated method stub
  if(ob1.getId()>ob2.getId()) {
   return 1;
  }else if(ob1.getId()<ob2.getId()) {
   return -1;
  }
  else {
   if(ob1.getScore()>ob2.getScore()) {
    return 1;
   }else if(ob1.getScore()<ob2.getScore()) {
    return -1;
   }
   else return 0;
  }
}
}

排序类（按学分排序）：

import java.util.Comparator;
public class SortScore implements Comparator{
@Override
public int compare(Object o1, Object o2) {
  // TODO Auto-generated method stub
  Student ob1=(Student)o1;
  Student ob2=(Student)o2;
  if(ob1.getScore()>ob2.getScore()) {
   return 1;
  }else if(ob1.getScore()<ob2.getScore()) {
   return -1;
  }
  else {
   if(ob1.getId()>ob2.getId()) {
    return 1;
   }else if(ob1.getId()<ob2.getId()) {
    return -1;
   }
   else return 0;
  }
}
}

同样建立TreeSet类将实例化学生对象添加到集合

import java.util.TreeSet;
public class MyTree1 {
public static void main(String[] args) {
SortScore c =new SortScore();
SortId c1 = new SortId();

//在new TreeSet对象是指定不同的排序
  TreeSet <Student>t =new TreeSet<>(c);//指定学分排序方式
  TreeSet <Student>t1 =new TreeSet<>(c1);//指定id排序方式
  t.add( new Student(1003,33,"java1"));
  t.add( new Student(1002,34,"java2"));
  t.add( new Student(1001,2,"java3"));
  t1.add( new Student(1003,33,"java1"));
  t1.add( new Student(1002,34,"java2"));
  t1.add( new Student(1001,2,"java3"));
  System.out.println("-------按学分排序---------");
  t.forEach((Student o) -> {
   System.out.println(o);
  });
  System.out.println("-------按id排序---------");
  t1.forEach((Student o) -> {
   System.out.println(o);
  });
}
}

运行结果：

总结

两种方法均可以实现排序，第二种方法在排序方式多种的情况下更实用

集合类TreeSet中关于Comparable接口与Comparator接口实现排序

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