1.按班级计算List <Student>中学生的数量 2.按班级将学生排序

面试题：其中涉及到了对容器、泛型、以及冒泡排序

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;

/**
 * 题目要求:1.按班级计算List<Student>中学生的数量 2.按班级将学生排序
 * 
 * @author huang
 * 
 */
public class Student {

    private int id;

    private String name;

    private int classRate;

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getClassRate() {
        return classRate;
    }

    public Student(int id, String name, int classRate) {
        this.id = id;
        this.name = name;
        this.classRate = classRate;
    }

    /**
     * 按班级计算学生数量
     * 
     * @param students
     * @return
     */
    public static Map<Integer, List<Student>> getStudentCountByClassRate(
            List<Student> students) {
        Map<Integer, List<Student>> map = new HashMap<Integer, List<Student>>();
        if (students != null && students.size() > 0) {
            for (Student s : students) {
                List<Student> stus = null;
                if (map.containsKey(s.getClassRate())) {
                    stus = map.get(s.getClassRate());
                } else {
                    stus = new ArrayList<Student>();
                }
                stus.add(s);
                map.put(s.getClassRate(), stus);
            }
        }
        return map;
    }

    /**
     * 冒泡交换
     * 
     * @param x
     * @param y
     * @param array
     */
    public static void convert(int x, int y, Integer[] array) {
        int temp = array[x];
        array[x] = array[y];
        array[y] = temp;
    }

    /**
     * 按classRate排序
     * 
     * @param students
     * @return
     */
    public static List<Student> sortStudentByClassRate(List<Student> students) {
        Map<Integer, List<Student>> map = getStudentCountByClassRate(students);
        Set<Integer> keySet = map.keySet();
        Integer[] array = keySet.toArray(new Integer[] {});
        if (array != null && array.length > 0) {
            for (int i = 0; i < array.length; i++) {
                for (int j = 0; j < array.length; j++) {
                    if (j + 1 < array.length) {
                        if (array[j] > array[j + 1]) {
                            convert(j, j + 1, array);
                        }
                        // convert(j, j + 1, array);
                    }
                }
            }
        }
        List<Student> stus = new ArrayList<Student>();
        for (int i : array) {
            stus.addAll(map.get(i));
        }
        return stus;
    }

    public static void main(String[] args) {
        Student s1 = new Student(1, "a1", 1);
        Student s2 = new Student(2, "a2", 2);
        Student s3 = new Student(3, "a3", 3);
        Student s4 = new Student(4, "a4", 1);
        Student s5 = new Student(5, "a5", 3);
        Student s6 = new Student(6, "a6", 3);
        Student s7 = new Student(7, "a7", 1);
        Student s8 = new Student(8, "a8", 2);
        Student s9 = new Student(9, "a9", 1);
        List<Student> stus = new ArrayList<Student>();
        stus.add(s1);
        stus.add(s2);
        stus.add(s3);
        stus.add(s4);
        stus.add(s5);
        stus.add(s6);
        stus.add(s7);
        stus.add(s8);
        stus.add(s9);
        Map<Integer, List<Student>> map = getStudentCountByClassRate(stus);
        Set<Integer> keySet = map.keySet();
        for (int i : keySet) {
            System.out.println("班级" + i + "有学生"
                    + ((List<Student>) map.get(i)).size());
        }
        List<Student> sts = sortStudentByClassRate(stus);
        for (Student s : sts) {
            System.out.println("学生姓名：" + s.getName() + ",属于" + s.getClassRate()
                    + "班");
        }

    }

}

测试结果：

班级1有学生4
班级2有学生2
班级3有学生3
学生姓名：a1,属于1班
学生姓名：a4,属于1班
学生姓名：a7,属于1班
学生姓名：a9,属于1班
学生姓名：a2,属于2班
学生姓名：a8,属于2班
学生姓名：a3,属于3班
学生姓名：a5,属于3班
学生姓名：a6,属于3班