为什Set里的元素打印输出有有顺序
package com.imooc;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
public class SetTest {
public List<Course> coursesToSelect;
public SetTest() {
this.coursesToSelect = new ArrayList<Course>();
}
public void testAdd() {
Course cr1 = new Course("1","数据结构");
coursesToSelect.add(cr1);
Course temp = (Course) coursesToSelect.get(0);
// System.out.println("添加了课程"+":"+temp.id+":"+temp.name);
Course cr2 = new Course("2","C语言");
coursesToSelect.add(0,cr2);
Course temp2 = (Course) coursesToSelect.get(0);
// System.out.println("添加了课程"+":"+temp2.id+":"+temp2.name);
// coursesToSelect.add(cr1);
// Course temp0 = (Course) coursesToSelect.get(2);
// System.out.println("添加了课程"+":"+temp0.id+":"+temp0.name);
Course[] course = {new Course("3","离散数学"),new Course("4","汇编语言")};
coursesToSelect.addAll(Arrays.asList(course));
Course temp3 = (Course) coursesToSelect.get(2);
Course temp4 = (Course) coursesToSelect.get(3);
// System.out.println("添加了课程"+":"+temp3.id+":"+temp3.name+";"+temp4.id+":"+temp4.name);
Course[] course2 = {new Course("5","高等数学"),new Course("6","大学语言")};
coursesToSelect.addAll(2, Arrays.asList(course2));
Course temp5 = (Course)coursesToSelect.get(2);
Course temp6 = (Course)coursesToSelect.get(3);
// System.out.println( "添加了课程"+temp5.id+":"+temp5.name+temp6.id+":"+temp6.name);
}
public void testForEach() {
System.out.println("有如下课程待选（通过foreach访问）:");
for(Object obj : coursesToSelect) {
Course cr = (Course)obj;
System.out.println("课程:"+cr.id+":"+cr.name);
}
}
public static void main(String[] args) {
// TODO Auto-generated method stub
SetTest st = new SetTest();
st.testAdd();
st.testForEach();
//创建一个学生呢个对象
Student student = new Student("1","小明");
System.out.println("欢迎学生:"+student.name+"选课");
//创建一个Scanner对象，用来接收从键盘输入的课程ID
Scanner console = new Scanner(System.in);
for(int i = 0; i<3; i++) {
System.out.println("请输入课程id");
String courseId = console.next();
for(Course cr : st.coursesToSelect) {
if(cr.id.equals(courseId)) {
student.courses.add(cr);
}
}
}
st.testForEachForSet(student);
}
public void testForEachForSet(Student student) {
//打印输出，学生所选的课程！
System.out.println("一共输出了几门课程:"+student.courses.size());
for(Course cr : student.courses) {
System.out.println("选择了课程:"+cr.id+":"+cr.name);
}
}
}
有如下课程待选（通过foreach访问）:
课程:2:C语言
课程:1:数据结构
课程:5:高等数学
课程:6:大学语言
课程:3:离散数学
课程:4:汇编语言
欢迎学生:小明选课
请输入课程id
1
请输入课程id
2
请输入课程id
4
一共输出了几门课程:3
选择了课程:1:数据结构
选择了课程:2:C语言
选择了课程:4:汇编语言
