Java面向对象6——继承中的构造方法

子类的构造过程中必须调用其基类的构造方法。

子类可以在自己的构造方法中使用super(argument_list)调用基类的构造方法

　- 使用this(argument_list)调用本类的另外的构造方法

　- 如果调用super，必须写在子类构造方法的第一行

如果子类的构造方法中没有显式地调用基类构造方法，则系统默认调用基类无参数的构造方法。

如果子类构造方法中既没有显式调用基类构造方法，而基类中又没有无参的构造方法，则编译出错。

实例1：

class SuperClass {
private int n;

/*
SuperClass() {
System.out.println("SuperClass()");
}
*/

SuperClass(int n) {
System.out.println("SuperClass(" + n + ")");
this.n = n;
}
}

class SubClass extends SuperClass {
private int n;

SubClass(int n) {
//此处相当于调用了super();
System.out.println("SubClass(" + n + ")");
this.n = n;
}

SubClass() {
super(300);//对super的调用必须在第一行
System.out.println("SubClass()");
}
}

public class TestSuperSub {
public static void main(String arg[]) {
//SubClass sc1 = new SubClass();
SubClass sc2 = new SubClass(400);
}
}

　　实例2：

class A{
protected void print(String s){
System.out.println(s);
}
A(){
print("A()");
}
public void f() {
print("A:f()");
}
}

class B extends A{
B(){//子类的构造方法
//自动调用父类无参数的构造方法A();
print("B()");
}
public void f() {
//重写了基类中的方法
print("B:f()");
}
public static void main(String[] args){
B b = new B();
b.f();
}
}

　　运行结果：

　　　　　　


　　

　　实例3：

class Person{
private String name;
private String location;

Person(String name){
this.name = name;
location = "beijing";
}
Person(String name, String location){
this.name = name;
this.location = location;
}
public String info() {
return "name: "+name+" location:"+location;
}
}
class Student extends Person{
private String school;
Student (String name, String school){
//调用了自己这个类另一个构造方法
this(name,"beijing",school);
}
Student (String n,String l, String school){
super(n,l);
this.school = school;
}
public String info() {
return super.info()+" school: "+school;
}
}

class Teacher extends Person{
private String title;
Teacher(String name,String title){
this(name,"beijing",title);
}
Teacher(String n,String l,String title){
super(n,l);
this.title = title;
}
public String info(){
return super.info()+" title: "+ title;
}
}
public class Test{
public static void main (String[] args){
Person p1 = new Person("A");
Person p2 = new Person("B","Shanghai");
Student s1 = new Student ("C","S1");
Student s2 = new Student ("C","shanghai","S2");
Teacher t1 = new Teacher ("D","Professor");
System.out.println(p1.info());
System.out.println(p2.info());
System.out.println(s1.info());
System.out.println(s2.info());
System.out.println(t1.info());
}
}

结果：