java comparable comparator 比较器的使用 及 自定义比较器

1. 原始类

/**

* 将对象作为TreeMap的key,此时需要给对象类需要具备比较功能、需要实现Comparable接口

* @author bruce

* 此类对象主要是用要用于TreeMap的key

*/

public class Student implements Comparable<Student> {

private int age;

private String name;



public Student(String name, int age) {

this.age = age;

this.name = name;

}

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}



/**

* 先按age比较，如果age相等比较name

*/

@Override

public int compareTo(Student o) {



int res = this.age - o.age;

return res == 0 ? this.name.compareTo(o.name) : res ;

}



}

2. 自定义比较器

import java.util.Comparator;

public class MyComparator implements Comparator<Student> {

@Override

public int compare(Student o1, Student o2) {

int res = o1.getName().compareTo(o2.getName());

return res == 0 ? o1.getAge() - o2.getAge() : res;

}

}

3. demo

import java.util.Comparator;

import java.util.Iterator;

import java.util.Set;

import java.util.TreeMap;

import org.junit.Test;

/**

* 将对象作为TreeMap的key,此时需要给对象类需要具备比较功能、需要实现Comparable接口

* @author bruce

*

*/

public class TreeMapDemo {



/**

* 按照Student中定义的默认比较方法<先比较年龄，如果年龄相等比较姓名>进行比较

*/

// @Test

public void defaultComparable(){

TreeMap<Student, String> map = new TreeMap<Student, String>();

map.put(new Student("xiaosan", 23), "上海");

map.put(new Student("xiaosi", 27), "北京");

map.put(new Student("xiaosan", 25), "上海");

map.put(new Student("xiaowu", 26), "深圳");



Set<Student> set = map.keySet();

Iterator<Student> iterator = set.iterator();

while(iterator.hasNext()){

Student student= iterator.next();

System.out.println(" key = " + student + " value = " + map.get(student));

}

}



/**

* 使用比较器，先比较姓名、如果姓名相等比较年龄

*/

@Test

public void createComparable(){

TreeMap<Student, String> map = new TreeMap<>(new Comparator<Student>() {

@Override

public int compare(Student o1, Student o2) {

//先按照姓名比较

int res = o1.getName().compareTo(o2.getName());

return res == 0 ? o1.getAge() - o2.getAge() : res;

}



});



map.put(new Student("5", 26), "上海");

map.put(new Student("1", 26), "北京");

map.put(new Student("3", 26), "上海");

map.put(new Student("4", 26), "深圳");



Iterator<Student> iterator = map.keySet().iterator();

while(iterator.hasNext()){

Student student = iterator.next();

System.out.println(" key = " + student + " value = " + map.get(student));

}

}

}