Java 常用类库 之 比较类 Comparable

Posted 2020-11-19 verejava

http://www.verejava.com/?id=169930999133100

/**
    知识点： 比较类 Comparable

    题目: 将某班学生按数学成绩从小到大排序

    思路:
        1. 抽象出类:
            1.1 班级(ClassSet)
            1.2 学生(Student)
        2. 找出类关系:
            2.1 学生 属于 班级 Student -> ClassSet(多对1)
        3. 找出类属性:
            3.1 ClassSet(班级名称,班级人数)
            3.2 Student(学生名称,数学成绩)
        4. 找出类方法:
            4.1 学生添加到班级 ClassSet{addStudent(Student s)}
            4.2 学生成绩从小到大排序  ClassSet{sortByScore()}
*/
import java.util.Arrays;
public class TestComparable
{
    public static void main(String[] args)
    {
        //实例化4G班级
        ClassSet c=new ClassSet("4G",4);
        //添加学生
        c.addStudent(new Student("李明",90));
        c.addStudent(new Student("李浩",80));
        c.addStudent(new Student("王涛",95));
        c.addStudent(new Student("张胜",70));

        //获得4G班级学生数组集合
        Student[] students=c.getStudents();
        //输出学生信息
        for(Student s:students)
        {
            if(s!=null)
                System.out.println(s.getName()+","+s.getMathScore());
        }

        System.out.println("
根据学生成绩排序");
        //c.sortByBuble();
        //Arrays.sort(students);
        c.sortByMerge();
        for(Student s:students)
        {
            if(s!=null)
                System.out.println(s.getName()+","+s.getMathScore());
        }

    }
}
class ClassSet
{
    private String className;//班级名称
    private int maxSize;//班级学生人数
    private int currentSize;//当前多少学生
    private Student[] students;//所有学生的数组

    public ClassSet(String className,int maxSize)
    {
        this.className=className;
        this.maxSize=maxSize;
        students=new Student[maxSize];
    }

    public Student[] getStudents()
    {
        return this.students;
    }

    /**
        添加学生
    */
    public void addStudent(Student s)
    {
        for(int i=0;i<students.length;i++)
        {
            if(students[i]==null)
            {
                students[i]=s;
                currentSize++;
                break;
            }
        }
    }
    /**
        按照学生的数学成绩冒泡排序从小到大排序
    */
    public Student[] sortByBuble()
    {
        for(int i=0;i<currentSize-1;i++)
        {
            for(int j=0;j<currentSize-i-1;j++)
            {
                Student s=students[j];
                if(students[j].getMathScore()>students[j+1].getMathScore())
                {
                    students[j]=students[j+1];
                    students[j+1]=s;
                }
            }
        }
        return this.students;
    }

    public void sortByMerge()
    {
        //创建一个临时数组存放分区元素
        Comparable[] tempArray=students.clone();
        merge_sort(students,tempArray,0,students.length);
    }

    private  void merge_sort(Comparable[] array,Comparable[] tempArray, int first, int last) {
        if(first+1<last)
        {
            int mid=(first+last)/2;
            // 对左半部份排序
            merge_sort(array,tempArray,first,mid);
            // 对有半部分排序  
            merge_sort(array,tempArray,mid,last);
            // 合并到一个临时数组，再拷贝到array中 
            merge(array,tempArray,first,mid,last);
        }
    }

    private  void merge(Comparable[] array,Comparable[] tempArray, int first, int mid,int last) {
        int beginLeft=first;
        int beginRight=mid;
        int k=first;
        
        while((beginLeft<mid)&&(beginRight<last)){
            if(array[beginLeft].compareTo(array[beginRight])<0){
                tempArray[k]=array[beginLeft];
                beginLeft++;
            }else{
                tempArray[k]=array[beginRight];
                beginRight++;
            }
            k++;
        }
        while(beginLeft<mid){
            tempArray[k++]=array[beginLeft++];
        }
        while(beginRight<last){
            tempArray[k++]=array[beginRight++];
        }
        for(int i=first;i<last;i++){
            array[i]=tempArray[i];
        }
    }
}
class Student implements Comparable
{
    private String name;//学生姓名
    private int mathScore;//数学成绩

    public Student(String name,int mathScore)
    {
        this.name=name;
        this.mathScore=mathScore;
    }
    public String getName()
    {
        return this.name;
    }
    public void setName(String name)
    {
        this.name=name;
    }
    public int getMathScore()
    {
        return this.mathScore;
    }
    public void setMathScore(int mathScore)
    {
        this.mathScore=mathScore;
    }

    /**
        实现Comparable 接口要复写 compareTo(T o) 方法
        如果从小到大排序
        大于         则返回 1
        小于         则返回 -1
        等于         则返回 0
        如果从大到小排序
        大于         则返回 -1
        小于         则返回 1
        等于         则返回 0
    */
    public int compareTo(Object obj)
    {
        if(obj instanceof Student)
        {
            Student s=(Student)obj;
            if(this.mathScore>s.getMathScore())
                return 1;
            if(this.mathScore<s.getMathScore())
                return -1;
            
        }
        return 0;
    }
    
}

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