Java集合的兩種排序方法分享，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(duì)象需實(shí)現(xiàn)comparable接口,重寫(xiě)其compareTo()方法,方法體中實(shí)現(xiàn)對(duì)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(wú)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(duì)象...");

    }

}

第二種叫定制排序,或自定義排序,需編寫(xiě)匿名內(nèi)部類,先new一個(gè)Comparator接口的比較器對(duì)象c,同時(shí)實(shí)現(xiàn)compare()其方法;

然后將比較器對(duì)象c傳給Collections.sort()方法的參數(shù)列表中,實(shí)現(xiàn)排序功能;

說(shuō)明:第一種方法不夠靈活,實(shí)體類實(shí)現(xiàn)了comparable接口后,會(huì)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(shí),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(huì)起沖突.第二種就很好地解決了這個(gè)問(wèn)題.在需要的地方,創(chuàng)建個(gè)內(nèi)部類的實(shí)例,重寫(xiě)其比較方法即可.

jUnit4單元測(cè)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(huì)在其它測(cè)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(huì)在每個(gè)測(cè)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);

        }

    }

    /**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(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);

        }

    }

    /**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(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]