Collections Comparator Comparable

package com.wh.util;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

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

public class Test {

    private static final Logger LOGGER = LoggerFactory.getLogger(Test.class);

    public static void main(String[] args) {
        List<String> lists = new ArrayList<String>();
        lists.add("1");
        lists.add("5");
        lists.add("3");
        LOGGER.info("lists 排序前：" + lists.toString());
        //排序，直接调用sort方法排序，排序方式是自燃排序，即升序排序
        Collections.sort(lists);
        LOGGER.info("lists 排序后：" + lists.toString());
    }

}

运行结果：

完美排序。但是在实际开发中排序的集合远比这个复杂，我们来看简单的模拟。

package com.wh.bean;

import java.io.Serializable;

/**
 * Created by WH on 2016/9/17.
 */
public class TestA implements Serializable{

    private static final long serialVersionUID = -8968350748602548887L;

    private String name;

    //变量类型必须要为int的封装类型，因为只有是封装类型才可以调用Integer的方法
    private Integer order;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getOrder() {
        return order;
    }

    public void setOrder(int order) {
        this.order = order;
    }

    @Override
    public String toString() {
        return "TestA{" +
                "name='" + name + '\'' +
                ", order=" + order +
                '}';
    }
}

在调用Collections.sort()给listA排序中，编译不通过，报错了。这就是当时我开发中遇到的小阻碍，为什么编译不通过呢，前面的演示中为什么可以排序呢？这是因为lists中的String本身含有compareTo方法，所以可以直接调用sort方法，所以要让类TestA实现Comparable方法，当时参考了网上资料（这里）。解决代码如下：

package com.wh.util;

import com.wh.bean.TestA;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

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

public class Test {

    private static final Logger LOGGER = LoggerFactory.getLogger(Test.class);

    public static void main(String[] args) {
        List<TestA> listA = new ArrayList<TestA>();
        //TestA 即是上面类
        TestA a1 = new TestA();
        a1.setName("张大胖");
        a1.setOrder(1);
        listA.add(a1);
        TestA a5 = new TestA();
        a5.setName("王晓梅");
        a5.setOrder(5);
        listA.add(a5);
        TestA a3 = new TestA();
        a3.setName("刘晓彤");
        a3.setOrder(3);
        listA.add(a3);
        LOGGER.info("listA 排序前：" + listA.toString());
        Collections.sort(listA, new Comparator<TestA>() {
            @Override
            public int compare(TestA o1, TestA o2) {
                //升序
                return o1.getOrder().compareTo(o2.getOrder());
            }
        });
        LOGGER.info("listB 升序排序后：" +listA.toString());
        Collections.sort(listA, new Comparator<TestA>() {
            @Override
            public int compare(TestA o1, TestA o2) {
                //降序
                return o2.getOrder().compareTo(o1.getOrder());
            }
        });
        LOGGER.info("listB 降序排序后：" +listA.toString());
    }

}

运行结果：

这里可以实现降序和升序排序。除了这个解决方法还有另外一个解决方法：

package com.wh.bean;

import java.io.Serializable;

/**
 * Created by WH on 2016/9/17.
 */
//实现Comparable
public class TestB implements Serializable, Comparable<TestB> {

    private static final long serialVersionUID = -2872189514594810657L;

    private String name;

    private Integer order;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getOrder() {
        return order;
    }

    public void setOrder(Integer order) {
        this.order = order;
    }
　　
    @Override
    public int compareTo(TestB o) {
　　　　　//升序
        return this.order.compareTo(o.getOrder());
    }

    @Override
    public String toString() {
        return "TestB{" +
                "name='" + name + '\'' +
                ", order=" + order +
                '}';
    }
}

 

 

package com.wh.bean;

import java.io.Serializable;

/**
 * Created by WH on 2016/9/17.
 */
public class TestB implements Serializable, Comparable<TestB> {

    private static final long serialVersionUID = -2872189514594810657L;

    private String name;

    private Integer order;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getOrder() {
        return order;
    }

    public void setOrder(Integer order) {
        this.order = order;
    }

    @Override
    public int compareTo(TestB o) {
        return this.order.compareTo(o.getOrder());
    }

    @Override
    public String toString() {
        return "TestB{" +
                "name='" + name + '\'' +
                ", order=" + order +
                '}';
    }
}

运行结果：

就这样解决了问题。