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(shí)現(xiàn)comparable接口,重寫其compareTo()方法,方法體中實(shí)現(xiàn)對象的比較大小規(guī)則,示例如下:
實(shí)體類:(基本屬性,getter/setter方法,有參無參構(gòu)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(zhuǎn)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(nèi)部類,先new一個Comparator接口的比較器對象c,同時實(shí)現(xiàn)compare()其方法;
然后將比較器對象c傳給Collections.sort()方法的參數(shù)列表中,實(shí)現(xiàn)排序功能;
說明:第一種方法不夠靈活,實(shí)體類實(shí)現(xiàn)了comparable接口后,會增加耦合,如果在項(xiàng)目中不同的位置需要根據(jù)不同的屬性調(diào)用排序方法時,需要反復(fù)修改比較規(guī)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(chuàng)建個內(nèi)部類的實(shí)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(biāo)注的方法會在其它測試方法執(zhí)行之前先執(zhí)行,????//且只執(zhí)行一次.@Before注解標(biāo)注的方法會在每個測試方法之前執(zhí)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);????????}????}????//??@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);//??????}//??}//????????@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);????????}????}????????@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(zhuǎn)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);????????}????}????????@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(zhuǎn)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(yùn)行結(jié)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]以上就是長沙達(dá)內(nèi)教育java培訓(xùn)機(jī)構(gòu)的小編針對“Java集合的兩種排序方法”的內(nèi)容進(jìn)行的回答，希望對大家有所幫助，如有疑問，請?jiān)诰€咨詢，有專業(yè)老師隨時為你服務(wù)。
