有如下Student 对象，  private String name;       private int age;       private int score;   private String classNum;  其中，classNum

package homework003;

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

public class Text {

    public static void main(String[] args) {

        List<Student>  list = new ArrayList<>();
        list.add(new Student("Tom",18,100,"class05"));
        list.add(new Student("Jerry",22,70,"class04"));
        list.add(new Student("Owen",25,90,"class05"));
        list.add(new Student("Jim",30,80,"class05"));
        list.add(new Student("Steve",28,66,"class06"));
        list.add(new Student("Kevin",24,100,"class04"));

        int sum = 0;
        for(int i =0;i<list.size();i++)
        {
            sum+=list.get(i).getAge();
        }
        System.out.println("所有学生的平均年龄为："+sum/list.size());

        int num1=0;
        int num2=0;
        int num3=0;
        int score1=0;
        int score2=0;
        int score3=0;
        for(int i =0;i<list.size();i++)
        {
            Student st =list.get(i);
            if(st.getClassNum()=="class04")
            {
                num1++;
                score1+=st.getScore();
            }
            else if(st.getClassNum()=="class05")
            {
                num2++;
                score2+=st.getScore();
            }
            else
            {
                num3++;
                score3+=st.getScore();
            }
        }
        System.out.println("class04的平均分："+(score1/num1));
        System.out.println("class05的平均分："+(score2/num2));
        System.out.println("class06的平均分："+(score3/num3));

    }

}

package homework003;

public class Student {

        private String name;
        private int age;
        private int score;
        private String classNum;

        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 int getScore() {
            return score;
        }
        public void setScore(int score) {
            this.score = score;
        }
        public String getClassNum() {
            return classNum;
        }
        public void setClassNum(String classNum) {
            this.classNum = classNum;
        }

        public Student(String name, int age, int score, String classNum) {
            super();
            this.name = name;
            this.age = age;
            this.score = score;
            this.classNum = classNum;
        }
        public Student() {
            super();
        }

        @Override
        public String toString() {
            return "Student [name=" + name + ", age=" + age + ", score=" + score + ", classNum=" + classNum + "]";
        }

}