当集合List中放入的是类对象时，打印出现首地址问题

?List集合中如果放入的是Object（），直接打印object对象的话，控制台输出的是首地址。

?

Object obj=new Object();
System.out.println(obj.toString());

?创建一个student类，包括年龄，姓名，性别，学号四个属性，并再TestStudent测试类中创建几个student类对象放入List集合中

Student类

public class Student {
    private int age;
    private String name;
    private String sex;
    private int id;


    public Student() {
    }

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

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
}

TestStudent测试

public class TestStudent {
    public static void main(String[] args) {
        Student stu1=new Student(18,"张三","男",101);
        Student stu2=new Student(15,"李四","男",102);
        Student stu3=new Student(25,"王五","男",103);
        Student stu4=new Student(56,"赵六","男",104);
        Student stu5=new Student(23,"张飞","男",105);
        List list=new ArrayList();
        list.add(stu1);
        list.add(stu2);
        list.add(stu3);
        list.add(stu4);
        list.add(stu5);
        for (int i=0;i<list.size();i++){
            System.out.println(list.get(i));//想要遍历List集合得到student对象的结果，必须重写toString方法，否则打印的是首地址

        }

       System.out.println("------------===========");
       for (int i=0;i<list.size();i++){ 
           Object o = list.get(i);
           Student s=(Student)o;
           System.out.println(s);//默认调用toString方法
       }

       System.out.println("---------");
       Iterator it=list.iterator(); //迭代器遍历List集合
       while (it.hasNext()){
           Student s=(Student) it.next();
           if (s.getAge()>=20){
              System.out.println(s.toString());
           }
       }

       }
}

?如果用上面for循环遍历list集合或者迭代器，控制台打印的是集合每个元素的首地址

?

?可以通过student构造器来打印出list集合中的student对象

  for (Object ob: list){
      Student s=(Student) ob;//向下转型
            ??????System.out.println(s.getName()+"\t"+s.getAge()+"\t"+s.getSex()+"\t"+s.getId()); 
        }

?

?还有一个更简单的方法，就是在Student类中重写toString方法，这样不管用哪种方法遍历List集合，得到的都是student对象。

?

public class TestStudent {
    public static void main(String[] args) {
        Student stu1=new Student(18,"张三","男",101);
        Student stu2=new Student(15,"李四","男",102);
        Student stu3=new Student(25,"王五","男",103);
        Student stu4=new Student(56,"赵六","男",104);
        Student stu5=new Student(23,"张飞","男",105);
        List list=new ArrayList();
        list.add(stu1);
        list.add(stu2);
        list.add(stu3);
        list.add(stu4);
        list.add(stu5);
        for (int i=0;i<list.size();i++){
            System.out.println(list.get(i));//想要遍历student结果，必须重写toString方法

        }
        System.out.println("------------===========");
        for (int i=0;i<list.size();i++){
            Object o = list.get(i);
            Student s=(Student)o;
            System.out.println(s);//默认调用toString方法
        }

        for (Object ob: list){
            Student s=(Student) ob;//向下转型
//            System.out.println(s.getName()+"\t"+s.getAge()+"\t"+s.getSex()+"\t"+s.getId()); //没有重写toString方法，那么除了这一行，其他的都是打印的地址
            System.out.println(s.toString());
        }

        System.out.println("---------");
        Iterator it=list.iterator();
        while (it.hasNext()){
            Student s=(Student) it.next();
            if (s.getAge()>=20){
                System.out.println(s.toString());//重写toString方法后，toString可以省略不写
            }
        }
    }
}

?

?总结：

当List集合中放入类对象时，在类中重写toString方法就可以解决控制台打印出首地址的问题。

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