构造函数在有继承时的调用
使用下面例子时,修改子类和父类构造里的super的参数列表,可以看到效果
例子 :
CommenPerson.java
public class CommenPerson {
private String name;
private int age;
public CommenPerson(){
System.out.println("CommenPerson.不带参数构造");
}
public CommenPerson(String name, int age) {
super();
this.name = name;
this.age = age;
System.out.println("Student.带参数构造");
}
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 void study(){
System.out.println("CommenPerson.study()");
}
public void eat(){
System.out.println("CommenPerson.eat()");
}
}
Student.java
public class Student extends CommenPerson{
private String stuNo;
public Student(){
super();//调用父类的构造函数,如果没有写会自动添加,必须写在子类构造的第一条语句
System.out.println("Student.不带参数构造");
}
public Student(String name,int age,String stuNo) {
super(name,age);
this.stuNo = stuNo;
System.out.println("Student.带参数构造");
}
public String getStuNo() {
return stuNo;
}
public void setStuNo(String stuNo) {
this.stuNo = stuNo;
}
public void homework(){
System.out.println("Student.homework");
super.study();
}
}
Teacher.java
public class Teacher extends CommenPerson{
private int salary;
public Teacher(){
super();
System.out.println("Teacher.constructor");
}
public Teacher(String name,int age,int salary) {
super(name,age);
this.salary = salary;
}
public int getSalary() {
return salary;
}
public void setSalary(int salary) {
this.salary = salary;
}
public void study(){
System.out.println("Teacher.study()");
}
public void eat(){
System.out.println("Teacher.eat()");
}
public void tech(){
System.out.println("Teacher.tech()");
}
Demo1.java
public class Demo1 {
public static void main(String[] args) {
Student stu1 = new Student("zhangsan",20,"s001");
Teacher tea1 = new Teacher();
}
}
在Student的有参构造函数里,把super()改成无参或者不写super,会去调用父类的无参构造方法,会导致传入参数的值一直是null或0,不能传值
结论
- 在Student stu1,且Student继承于commonPerson时,loadclass student类和commonPerson类进入方法区
- new Student();时 进入Student的构造函数,此时看构造函数第一个语句,如果该构造函数有super(),则super()先调用父类的无参构造函数,如果没有写会自动添加super();如果有super(参数列表),会先去调用父类对应参数列表的构造,执行完父类构造,父类对象出现在堆中,name=null,age=0;
- 执行完父类构造后,会自动回子类构造,执行子类构造,执行完毕,stuNo=null,子类对象在堆中出现
- 如果第二部是 new Student(“zhangsan”,20,”s001”);会先调用Student中的对应含有此参数的构造函数,然后看构造函数中的super(),然后根据参数对应去调用父类的构造函数.
总结
在new一个新对象时,先调用该类对应的构造函数,然后在构造函数里按顺序执行,第一行是super,必定先去父类调用对应的构造方法,根据super()里的参数看调用的是哪个构造函数
内存图
