Java中比较器小结

        比较器指的是集合存储的元素的特性，如果元素是可比较的则可以进行相应的排序，否则不行。但是在Java中有Comparable接口和Comparator接口，具体有什么区别呢？

        对于Comparable接口来说，它往往是进行比较类需要实现的接口，它仅包含一个有compareTo()方法，只有一个参数，返回值为int，返回值大于0表示对象大于参数对象；小于0表示对象小于参数对象；等于0表示两者相等

public class TestComparable implements Comparable{

private String name; //主键
private int age;     //年龄
public TestComparable(String name, int age){
this.name = name;
this.age = age;
}
public String getName(){
return name;
}
public void setName(String name){
this.name = name;
}
public int getAge(){
return age;
}
public void setAge(int age){
this.age = age;
}
@Override
public int compareTo(Object obj) {//覆盖Comparable接口的compareTo()接口
return this.age - ((TestComparable)obj).getAge();
}

public static void main(String[] args) {
TestComparable user1 = new TestComparable("user001", 24);
TestComparable user2 = new TestComparable("user002", 20);
if(user1.compareTo(user2) > 0)//具体使用就是通过对象调用普通方法一样，返回值是对象值与参数值比较的结果
System.out.println("用户1年龄较大.");
else if(user1.compareTo(user2) < 0)
System.out.println("用户2年龄较大.");
else System.out.println("两用户年龄一样大.");
}
}

        对于Comparator接口来说，它的实现者被称为比较器，它包含一个compare()方法，有两个参数，返回值与Comparable的compareTo()方法一样，不同之处是Comparator接口一般不会被集合元素类所实现，而是单独实现或者匿名内部类方式实现

public class TestComparator{

private String name; //主键
private int age;     //年龄
public TestComparator(String name, int age){
this.name = name;
this.age = age;
}
public String getName(){
return name;
}
public void setName(String name){
this.name = name;
}
public int getAge(){
return age;
}
public void setAge(int age){
this.age = age;
}

public static void main(String[] args){
TestComparator user1 = new TestComparator("user001", 24);
TestComparator user2 = new TestComparator("user002", 20);
Comparator cmp = new Comparator(){//以内部类的形式实现Comparator接口
@Override
public int compare(Object obj1, Object obj2) {
TestComparator user1 = (TestComparator)obj1;//类型转换
TestComparator user2 = (TestComparator)obj2;
return user1.getAge() - user2.getAge();//两参数对象进行比较
}
};
int res = cmp.compare(user1, user2);//比较结果是第一个对象与第二个对象的比较值
if(res > 0)
System.out.println("用户1年龄较大.");
else if(res < 0)
System.out.println("用户2年龄较大.");
else System.out.println("两用户年龄一样大.");
}
}

        比较器是把集合或数组的元素强行按照指定方法进行排序的对象，它是实现了Comparator接口类的实例。如果一个集合元素是可比较的（实现了Comparable接口），那么就具有了默认排序方法，比较器则是强行改变它默认的比较方式来进行排序。或者有的集合元素不可比较（没有实现Comparable接口），则可用比较器来实现动态的排序。

        下面举例写一个HashMap分别按照Key排序和按照Value排序的程序：

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.TreeMap;

//对Map按key排序和按value排序

public class MapSortTest {
//sort by key.
public static Map<String, String> sortMapByKey(Map<String, String> oriMap){
if(oriMap==null || oriMap.isEmpty())
return null;
Map<String, String>sortedMap = new TreeMap<String, String>(new Comparator<String>(){
@Override
public int compare(String key1, String key2) {
//				int intKey1 = Integer.valueOf(key1);
//				int intKey2 = Integer.valueOf(key2);
//				return intKey1 - intKey2;
return key1.compareTo(key2);
}

});
sortedMap.putAll(oriMap);
return sortedMap;
}

//sort by value
public static Map<String, String> sortMapByValue(Map<String, String> oriMap){
Map<String, String> sortedMap = new LinkedHashMap<String, String>();
if(oriMap!=null || !oriMap.isEmpty()){
List<Map.Entry<String, String>> entryList = new ArrayList<Map.Entry<String, String>>(oriMap.entrySet());
Collections.sort(entryList, new Comparator<Map.Entry<String, String>>(){
@Override
public int compare(Entry<String, String> entry1, Entry<String, String> entry2) {
//					int value1 = Integer.valueOf(entry1.getValue());
//					int value2 = Integer.valueOf(entry2.getValue());
//					return value1 - value2;
return entry1.getValue().compareTo(entry2.getValue());
}
});
Iterator<Map.Entry<String, String>> iter = entryList.iterator();
Map.Entry<String, String> tmpEntry = null;
while(iter.hasNext()){
tmpEntry = iter.next();
sortedMap.put(tmpEntry.getKey(), tmpEntry.getValue());
}
}

return sortedMap;
}

public static void main(String[] args) {
Map<String, String> map = new HashMap<>();
map.put("zhangsan", "29");
map.put("lisi", "35");
map.put("wangwu", "40");
map.put("mazi",	"24");

System.out.println("The original map: " + map);
System.out.println("Sort by key: " + sortMapByKey(map));
System.out.println("Sort by value: " + sortMapByValue(map));
}

}