Java学习笔记7---父类构造方法有无参数对子类的影响
子类不继承父类的构造方法，但父类的构造方法对子类构造方法的创建有影响。
具体来说就是：
①.当父类没有无参构造方法时：
1).子类也不能有无参构造方法；
2).且必须在子类构造方法中显式以super(参数)的形式调用父类构造方法；
否则会出现如下的错误：
Implicit super constructor Person() is undefined for default constructor. Must define an explicit constructor
3).子类在有参构造方法中显式调用super(参数)后，如果再写一个无参构造方法，则会出现下面的错误：
Implicit super constructor Person() is undefined. Must explicitly invoke another constructor
②.父类有无参构造方法时：
1).子类可以有无参构造方法；
2).也可以有有参构造方法；在有参构造方法中，可以用super显式调用父类构造方法也可以不调用；
也就是说，这时候，子类在构造方法的创建上是比较自由的。
下面是简单示例：
有两个类，Person类和Student类，Student类继承自Person类。两个类的构造方法详见代码。
Person类：
package human;
public class Person {
String name;
int age;
String gender;
private String hobby;
public Person() {
}
public Person(String n, String g) {
this.name = n;
this.gender = g;
}
public Person(String n, int a, String g, String h) {
this.name = n;
this.age = a;
this.gender = g;
this.hobby = h;
}
public void setName(String n) {
this.name = n;
}
public void setAge(int a) {
this.age = a;
}
public void setGender(String g) {
this.gender = g;
}
public void setHobby(String h) {
this.hobby = h;
}
public String getName() {
return this.name;
}
public int getAge() {
return this.age;
}
public String getGender() {
return this.gender;
}
public String getHobby() {
return this.hobby;
}
public void informationPrint() {
System.out.println("My name is " +getName());
System.out.println("I am " + getAge() +" years old");
if(getGender() == "female")
System.out.println("I am a girl");
else
if(getGender() == "male")
System.out.println("I am a boy");
else
System.out.println("Something is wrong!");
System.out.println("My hobby is " + hobby);
}
}
Student类：
package human;
public class Student extends Person {
String stuNumber;
int score;
public Student() {
}
public Student(String n, String g) {
super(n,g);
}
public Student(String n, int a, String g, String h) {
super(n,a,g,h);
}
public Student(String sN, int s) {
this.stuNumber = sN;
this.score = s;
}
public Student(String n, String g, String sN, int s) {
super(n,g);
this.stuNumber = sN;
this.score = s;
}
public Student(String n, int a, String g, String h, String sN, int s) {
super(n,a,g,h);
this.stuNumber = sN;
this.score = s;
}
public void setStuNumber(String num) {
this.stuNumber = num;
}
public void setScore(int s) {
this.score = s;
}
public String getStuNumber() {
return this.stuNumber;
}
public int getScore() {
return this.score;
}
public void informationPrint() {
super.informationPrint();
System.out.println("My number is " + stuNumber);
System.out.println("My score is " + score);
}
}
测试类：
package human;
public class TestMain {
public static void main(String[] args) {
Person xiaoxiP = new Person("xiaoxiP",29,"female","piano");
Person xiaonanP = new Person("xiaonanP","male");
Student xiaoxiS = new Student("xiaoxiS",28,"female","piano","124",90);
Student xiaonanS = new Student("xiaonanS","male","123",98);
xiaoxiP.informationPrint();
xiaoxiS.informationPrint();
xiaonanP.informationPrint();
xiaonanS.informationPrint();
}
}
结果：
My name is xiaoxiP I am 29 years old I am a girl My hobby is piano My name is xiaoxiS I am 28 years old I am a girl My hobby is piano My number is 124 My score is 90 My name is xiaonanP I am 0 years old I am a boy My hobby is null My name is xiaonanS I am 0 years old I am a boy My hobby is null My number is 123 My score is 98
总结（追加）：
①.父类没有无参构造方法时，子类也不能有无参构造方法，必须用super显式调用父类构造方法。
②.父类没有无参构造方法且有多个有参构造方法时，子类可以只显式调用父类的一个构造方法；
子类也可以构造多个构造方法，只要保证每个构造方法都显式调用了父类构造方法就可以，对于调用父类哪一个构造方法则没有要求。
③.父类有无参构造方法时，子类可以有无参构造方法，也可以有有参构造方法；
既可以用super显式调用父类构造方法，也可以不用super显式调用。
待学习：访问权限修饰符的问题。
