Comparator::compare设定排序的升序 降序

java.util.Comparator中 compare(T o1, T o2) 函数，其实现决定升序降序。举例如下：对某个对象的var类例进行排序

int compare(T o1, T o2) {
    return o1.var - o2.var;
}

理解升序、降序的三步走：

1 明确 两个变量o1 o2表示序列中前 后的两个变量；位置关系：o1 在前；o2居后；

2 明确返回值表达是否交换位置，正数：交换两者位置； 负数： 不交换；

3 判断是降序还是升序。

升序

int compare(T o1, T o2) {
    return o1.var - o2.var;
}

 分析如下：

1 明确o1在前， o2在后；

2 若o1.var-o2.var为正，则交换位置；为负，不交换位置；

3 若上述为正条件成立，则表明o1.var为大数，大数置后，故为升序；

降序

int compare(T o1, T o2) {
    return o2.var - o1.var;
}

 分析如下：

1 明确o1在前， o2在后；

2 若o2.var-o1.var为正，则交换位置；为负，不交换位置；

3 若上述为正条件成立，则表明o2.var为大数，大数置前，故为降序；

默认升序/降序

int compare(T o1, T o2) {
    return -1;//默认不交换，表示升序
    return 1;//默认交换，表示降序
}

demo代码

package bob.util;
import java.lang.*;
import java.util.*;
 
class Student {
    int rollno;
    String name, address;
 
    public Student(int rollno, String name, String address) {
        this.rollno = rollno;
        this.name = name;
        this.address = address;
    }
 
    public String toString() {
        return this.rollno + " " + this.name + " " + this.address;
    }
}
 
//自定义升序
class SortbyrollAsc implements Comparator {
    public int compare(Student a, Student b) {
        return a.rollno - b.rollno;
    }
}
 
//自定义降序
class SortbyrollDsc implements Comparator {
    public int compare(Student a, Student b) {
        return b.rollno - a.rollno;
    }
}
 
//default asc
//默认强制降序
class DefaultDsc implements Comparator {
    public int compare(Student a, Student b) {
        return 1;
    }
}
 
//default dsc
//默认强制升序
class DefaultAsc implements Comparator {
    public int compare(Student a, Student b) {
        return -1;
    }
}
 
// Main class
public class SortDemo {
    public static void main(String[] args) {
        ArrayList ar = new ArrayList();
        ar.add(new Student(141, "Mayank", "london"));
        ar.add(new Student(131, "Anshul", "nyc"));
        ar.add(new Student(161, "Solanki", "jaipur"));
        ar.add(new Student(151, "Aggarwal", "Hongkong"));
        // Display message on console for better readability
        System.out.println("初始化顺序");
        // Iterating over entries to print them
        for (int i = 0; i < ar.size(); i++)
            System.out.println(ar.get(i));
 
        // Sorting student entries by roll number
        Collections.sort(ar, new SortbyrollAsc());
        // Display message on console for better readability
        System.out.println("\n自定义升序排序");
        // Again iterating over entries to print them
        for (int i = 0; i < ar.size(); i++)
            System.out.println(ar.get(i));
 
        // Sorting student entries by roll number
        Collections.sort(ar, new SortbyrollDsc());
        // Display message on console for better readability
        System.out.println("\n自定义降序排序");
        // Again iterating over entries to print them
        for (int i = 0; i < ar.size(); i++)
            System.out.println(ar.get(i));
 
        Collections.sort(ar, new DefaultDsc());
        // Display message on console for better readability
        System.out.println("\n默认强制降序排序");
        // Again iterating over entries to print them
        for (int i = 0; i < ar.size(); i++)
            System.out.println(ar.get(i));
 
        Collections.sort(ar, new DefaultAsc());
        // Display message on console for better readability
        System.out.println("\n默认强制升序排序");
        // Again iterating over entries to print them
        for (int i = 0; i < ar.size(); i++)
            System.out.println(ar.get(i));
    }
}