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。