一直学习java，都没有把构造函数弄清楚，通过昨天的学习，现在有了一点点理解，不知道还有什么不知道的，望大家指导指导。
当父类没有任何构造函数，子类定义了一个不带参数的构造函数，这时候在测试类里面创建一个子类的对象时，程序运行时，首先调用系统默认的父类的构造函数，然后再调用子类的构造函数。例子：
public class SuperClass {
private String name;
private String birthdate;
public void show() {
System.out.println("this is a public function" + name + birthdate);
}
}
public class SubClass extends SuperClass {
String address;
public SubClass() {
System.out.println("SubClass:this is a no parameter constructor");
}
public void personinformation(String _name, String _birth, String _address) {
System.out.println("output information" + _name + _birth + _address);
}
}
public class Test {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
SubClass absence = new SubClass();
}
}
运行结果：SubClass:this is a no parameter constructor
当父类里面定义了几个构造函数，其中至少有一个不带参数，子类定义了一个不带参数的构造函数，运行的时候，调用不带参数的构造函数，然后再运行子类的构造函数。
public class SuperClass {
private String name;
private String birthdate;
public SuperClass() {
name = "chen";
birthdate = "feb";
System.out.println("SuperClass:this is a no parameter constructor");
}
public SuperClass(String chen, String febu) {
name = chen;
birthdate = febu;
System.out.println("this is parameter constructor:" + name + birthdate);
}
public void show() {
System.out.println("this is a public function" + name + birthdate);
}
}
public class SubClass extends SuperClass {
String address;
public SubClass() {
System.out.println("SubClass:this is a no parameter constructor");
}
public void personinformation(String _name, String _birth, String _address) {
System.out.println("output information" + _name + _birth + _address);
}
}
public class Test {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
SubClass absence = new SubClass();
}
}
运行结果：SuperClass:this is a no parameter constructor
SubClass:this is a no parameter constructor
当父类中只定义带参数的构造函数，父类定义的构造函数和上面的一样，这样程序运行就会出现错误，编译不能通过，会出现错误提示。因为在创建子类对象时，由于没有不带参数的构造函数，它不知道要调用父类的那个构造函数。所以要在定义子类构造的第一行加上调用父类的构造函数：super("hhh");根据括号里面的参数类型、个数来确定调用父类里面的那个构造函数。例子：
public class SuperClass {
private String name;
private String birthdate;
public SuperClass(String chen, String febu) {
name = chen;
birthdate = febu;
System.out.println("this is parameter constructor:" + name + birthdate);
}
public SuperClass(String chen) {
name = chen;
System.out.println("this is parameter constructor1:" + name );
}
public void show() {
System.out.println("this is a public function" + name + birthdate);
}
}
public class SubClass extends SuperClass {
String address;
public SubClass() {
super("hhh"); //非常重要！根据他来调用父类的构造函数
System.out.println("SubClass:this is a no parameter constructor");
}
public void personinformation(String _name, String _birth, String _address) {
System.out.println("output information" + _name + _birth + _address);
}
}
public class Test {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
SubClass absence = new SubClass();
}
}
运行结果：this is parameter constructor1:hhh
SubClass:this is a no parameter constructor
父类只有不带参数的构造函数，如果父类里面有一个其他类的对象实例，子类里面同样有一个其他类的对象实例 ，那么运行的时候后，首先运行的是父类里的那个类的构造函数，在运行父类的构造函数，然后运行子类里的那个类的构造函数，最后运行子类的构函数。
public class SuperClass {
private String name;
private String birthdate;
public SuperClass() {
name = "chen";
birthdate = "feb";
Circle2 c2 = new Circle2(); //父类里定义了Circle2的对象c2
System.out.println("SuperClass:this is a no parameter constructor");
}
public SuperClass(String chen, String febu) {
name = chen;
birthdate = febu;
System.out.println("this is parameter constructor:" + name + birthdate);
}
public void show() {
System.out.println("this is a public function" + name + birthdate);
}
}
public class SubClass extends SuperClass {
String address;
Circle c = new Circle(); //父类里定义了Circle的对象c
public SubClass() {
System.out.println("SubClass:this is a no parameter constructor");
}
public void personinformation(String _name, String _birth, String _address) {
System.out.println("output information" + _name + _birth + _address);
}
}
public class Circle {
public static final double PI = 3.14;
public Circle() {
System.out.println("Circle:this is a no parameter constructor");
}
}
public class Circle2 {
public static final double PI = 3.14;
private double banjing;
public Circle2() {
System.out.println("Circle2:this is a no parameter constructor");
}
}
public class Test {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
SubClass absence = new SubClass();
}
}
运行结果：
Circle2:this is a no parameter constructor
SuperClass:this is a no parameter constructor
Circle:this is a no parameter constructor
SubClass:this is a no parameter constructor