java中类的对象比较大小进行排序，不像C++那样可以重载运算符比较大小，java中需要实现特定的接口或者自己给类加上逻辑大小比较函数，个人认为有以下三种方式吧：
一、类实现java.util.Comparator<T>接口
Comparator中有个接口，如下：
int compare（T o1, T o2）;
比较用来排序的两个参数。根据第一个参数小于、等于或大于第二个参数分别返回负整数、零或正整数。
举个栗子：
有一个Person类，需要根据年龄比较大小或者排序，可以实现Comparator接口如下：
import java.util.Comparator;
public class Person implements Comparator<Person> {
private int age;
private String name;
public Person(){
}
public Person(int age, String name) {
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;
}
@Override
public int compare(Person o1, Person o2) {
return o1.getAge() - o2.getAge();
}
public static void main(String [] args){
Person p1 = new Person(27, "LiuBei");
Person p2 = new Person(25, "ZhangFei");
Person p3 = new Person(26, "GuanYu");
System.out.println(p1.compare(p1,p2));
System.out.println(p1.compare(p2,p3));
}
}
输出结果如下：
2
0
-1
可以根据返回结果int值得正负性判断对象的逻辑大小。
二、类实现java.lang.Comparable<T>接口，该接口中有一个方法：
public int compareTo(T o);
含义与上面Comparator接口中的int compare（T o1, T o2）一样，也是根据返回值正负性判断两个对象的大小，
仍以上面的例子展示：
public class Person implements Comparable<Person>{
private int age;
private String name;
public Person(){
}
public Person(int age, String name) {
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;
}
@Override
public int compareTo(Person o) {
return this.age - o.getAge();
}
public static void main(String [] args){
Person p1 = new Person(27, "LiuBei");
Person p2 = new Person(25, "ZhangFei");
Person p3 = new Person(26, "GuanYu");
System.out.println(p2.compareTo(p1));
System.out.println(p1.compareTo(p1));
System.out.println(p1.compareTo(p3));
}
}
输出结果如下：
-2
0
1三、自己在类中增加比较方法，其实与上面两种思想差不多，如下：
public class Person {
private int age;
private String name;
public Person(){
}
public Person(int age, String name) {
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;
}
public int compareTo(Person o) {
return this.age - o.getAge();
}
public static void main(String [] args){
Person p1 = new Person(27, "LiuBei");
Person p2 = new Person(25, "ZhangFei");
Person p3 = new Person(26, "GuanYu");
System.out.println(p2.compareTo(p1));
System.out.println(p1.compareTo(p1));
System.out.println(p1.compareTo(p3));
}
}
可以自己在类中加一个compareTo（Person o）方法，但是一般不推荐这样做，因为像TreeMap等有序集合，在使用key进行排序时，一般都是按照前两种方式要求key类必须实现Comparable接口或者提供key类的Comparator比较器，我们自己写的比较函数，在TreeMap中是不起作用的。如下，按照第三种这样写，是会报异常的：
import java.util.TreeMap;
public class Person {
private int age;
private String name;
public Person(){
}
public Person(int age, String name) {
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;
}
public int compareTo(Person o) {
return this.age - o.getAge();
}
public static void main(String [] args){
Person p1 = new Person(27, "LiuBei");
Person p2 = new Person(25, "ZhangFei");
Person p3 = new Person(26, "GuanYu");
TreeMap map = new TreeMap();
map.put(p1,"LiuBei");
}
}
程序运行结果：
Exception in thread "main" java.lang.ClassCastException: Person cannot be cast to java.base/java.lang.Comparable
at java.base/java.util.TreeMap.compare(TreeMap.java:1291)
at java.base/java.util.TreeMap.put(TreeMap.java:536)
at Person.main(Person.java:40)
改成第一种或者第二种，就可以了，以第二种方式为例如下：
import java.util.TreeMap;
public class Person implements Comparable<Person>{
private int age;
private String name;
public Person(){
}
public Person(int age, String name) {
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;
}
@Override
public int compareTo(Person o) {
return this.age - o.getAge();
}
public static void main(String [] args){
Person p1 = new Person(27, "LiuBei");
Person p2 = new Person(25, "ZhangFei");
Person p3 = new Person(26, "GuanYu");
TreeMap map = new TreeMap();
map.put(p1,"LiuBei");
map.put(p2,"ZhangFei");
map.put(p3,"GuanYu");
System.out.println(map);
}
}
程序的输出结果为：
{Person@13b6d03=ZhangFei, Person@f5f2bb7=GuanYu, Person@73035e27=LiuBei}这样就输出正确的人物顺序了，此例中的自然顺序是按照年龄递增的，没有问题。
第二种构造比较器的方式，大家可以自己亲自动手试一下TreeMap的使用哈。