java集合的工具類Collections中提供了兩種排序的方法,分別是:

1.Collections.sort(List list)

2.Collections.sort(List list,Comparator c)

第一種稱為自然排序,參與排序的對象需實現comparable接口,重寫其compareTo()方法,方法體中實現對象的比較大小規則,示例如下:

實體類:(基本屬性,getter/setter方法,有參無參構造方法,toString方法)

package?test;

public?class?Emp?implements?Comparable?{

????private?String?name;
????private?int?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?Emp()?{
????????super();
????}
????public?Emp(String?name,?int?age)?{
????????super();
????????this.name?=?name;
????????this.age?=?age;
????}
????@Override
????public?String?toString()?{
????????return?"Emp?[name="?+?name?+?",?age="?+?age?+?"]";
????}
????@Override
????public?int?compareTo(Object?o)?{
????????if(o?instanceof?Emp){
????????????Emp?emp?=?(Emp)?o;
//??????????return?this.age-emp.getAge();//按照年齡升序排序
????????????return?this.name.compareTo(emp.getName());//換姓名升序排序
????????}
????????throw?new?ClassCastException("不能轉換為Emp類型的對象...");
????}

}

第二種叫定制排序,或自定義排序,需編寫匿名內部類,先new一個Comparator接口的比較器對象c,同時實現compare()其方法;

然后將比較器對象c傳給Collections.sort()方法的參數列表中,實現排序功能;

說明:第一種方法不夠靈活,實體類實現了comparable接口后,會增加耦合,如果在項目中不同的位置需要根據不同的屬性調用排序方法時,需要反復修改比較規則(按name還是按age),二者只能選擇其一,會起沖突.第二種就很好地解決了這個問題.在需要的地方,創建個內部類的實例,重寫其比較方法即可.

jUnit4單元測試類代碼如下:

package?test;

import?java.util.ArrayList;
import?java.util.Collections;
import?java.util.Comparator;
import?java.util.List;

import?org.junit.BeforeClass;
import?org.junit.Test;

public?class?TestSort?{

????static?List?list?=?new?ArrayList();
????//@BeforeClass注解標注的方法會在其它測試方法執行之前先執行,
????//且只執行一次.@Before注解標注的方法會在每個測試方法之前執行;
????//此處初始化集合只需要一次,因此使用@BeforeClass.
????@BeforeClass
????public?static?void?init(){
????????list.add(new?Emp("tom",18));
????????list.add(new?Emp("jack",20));
????????list.add(new?Emp("rose",15));
????????list.add(new?Emp("jerry",17));
????????System.out.println("排序前:");
????????for(Object?o?:?list){
????????????System.out.println(o);
????????}
????}

????/**按age升序排序*/
//??@Test
//??public?void?testSortAge(){
//??????Collections.sort(list);
//??????System.out.println("自然排序按age排序后:");
//??????for(Object?o?:?list){
//??????????System.out.println(o);
//??????}
//??}
//
????/**按name升序排序*/
????@Test
????public?void?testSortName(){
????????Collections.sort(list);
????????System.out.println("自然排序按name升序排序后:");
????????for(Object?o?:?list){
????????????System.out.println(o);
????????}
????}

????/**使用Comparator比較器按age升序排序*/
????@Test
????public?void?testComparatorSortAge(){
????????Collections.sort(list,new?Comparator?()?{
????????????@Override
????????????public?int?compare(Object?o1,?Object?o2)?{
????????????????if(o1?instanceof?Emp?&&?o2?instanceof?Emp){
????????????????????Emp?e1?=?(Emp)?o1;
????????????????????Emp?e2?=?(Emp)?o2;
????????????????????return?e1.getAge()?-?e2.getAge();
????????????????}
????????????????throw?new?ClassCastException("不能轉換為Emp類型");
????????????}
????????});
????????System.out.println("使用Comparator比較器按age升序排序后:");
????????for(Object?o?:?list){
????????????System.out.println(o);
????????}
????}
????/**使用Comparator比較器按name升序排序*/
????@Test
????public?void?testComparatorSortName(){
????????Collections.sort(list,new?Comparator?()?{
????????????@Override
????????????public?int?compare(Object?o1,?Object?o2)?{
????????????????if(o1?instanceof?Emp?&&?o2?instanceof?Emp){
????????????????????Emp?e1?=?(Emp)?o1;
????????????????????Emp?e2?=?(Emp)?o2;
????????????????????return?e1.getName().compareTo(e2.getName());
????????????????}
????????????????throw?new?ClassCastException("不能轉換為Emp類型");
????????????}
????????});
????????System.out.println("使用Comparator比較器按name升序排序后:");
????????for(Object?o?:?list){
????????????System.out.println(o);
????????}
????}

}


右鍵空白位置—>Run As—>JUnit Test—>

運行結果如下:

排序前:
Emp?[name=tom,?age=18]
Emp?[name=jack,?age=20]
Emp?[name=rose,?age=15]
Emp?[name=jerry,?age=17]
自然排序按name升序排序后:
Emp?[name=jack,?age=20]
Emp?[name=jerry,?age=17]
Emp?[name=rose,?age=15]
Emp?[name=tom,?age=18]
使用Comparator比較器按age升序排序后:
Emp?[name=rose,?age=15]
Emp?[name=jerry,?age=17]
Emp?[name=tom,?age=18]
Emp?[name=jack,?age=20]
使用Comparator比較器按name升序排序后:
Emp?[name=jack,?age=20]
Emp?[name=jerry,?age=17]
Emp?[name=rose,?age=15]
Emp?[name=tom,?age=18]

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