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# week4 practice Inheritance

## Java 繼承練習：extends, super, constructor

附上兩個檔案，Person.java與Student.java

## Person.java

```
 /* 
 Person.java

 **/

 class Person {
    String name;
    int age;

    static int x;

    Person( String n, int a ){ // z構造方法是不能繼承的，子類要重新定義構造方法(爸爸有，兒子不一定有)
        name = n;
        age = a;
    }

    Person( String n ){
        name = n;
        age = -1;
    }

    Person( int age, String name )
    {
        this.age = age;
        this.name = name;
    }

    Person(  ){
        this( 0, "" );
    }

    void sayHello(){
        System.out.println("Hello!  My name is " + name );
    }

    void sayHello( Person another ){
        System.out.println("Hello," + another.name + "! My name is " + name );
    }

    boolean isOlderThan( int anAge ){
        boolean flg;
        if( age > anAge ) flg = true; else flg=false;
        return flg;
    }

    public static void main(String[] args) 
    {
        System.out.println("Hello World!");
    }
}
```

## Student.java

```
class Student extends Person
{
    String school;
    int score;

    void sayHello(Student another){
        System.out.println("Hi!");
        if (school== another.school) System.out.println(" Schoolmates");
    }

    boolean isGoodStudent(){
        return score >=90;
    }

    void testThisSuper(){
        int a;
        a =age;
        a = this.age;
        a = super.age;// this.age is same as super
    }

    void sayHello(){
        super.sayHello();// 調用父親類的sayhello，然後加一些行為
        System.out.println( "My school is " +school);

    }

    Student(String name, int age, String school){ //構造方法是不能繼承的，子類要重新定義構造方法
        super(name, age); // 調用父類的constructor，在Person.java 第7行，必須放在第一句
        this.school =school;
    }

    //Student(){} // 27行已經定義，所以33行就不用默認

    public static void main(String args[]){ // 父類對象與子類對象的轉換
        Person p = new Person("Tom",50);
        Student s = new Student("Andy",20, "NTU");
        Student s3 = new Student("Mini",20, "NTU");
        Person p2 = new Student("Bob",18, "NCKU");
        Student s2 = (Student)p2; // 強制類型轉換(casting)

        //Student s3 =(Student) p; // 編譯可以，執行不可行

        s.sayHello();
        System.out.println("--------------------");
        p2.sayHello();

        System.out.println("--------------------");
        s.sayHello(p2);
        s.sayHello(p);

        System.out.println("--------------------");
        p2.sayHello(s);
        p2.sayHello(p);

        System.out.println("--------------------");
        s.sayHello(s3);
        System.out.println("--------------------");
        s3.sayHello(s);
        Person []manypeople =new Person[100];
        manypeople[0] = new Person("Li",18);
        manypeople[1] = new Student("wong",18,"PKU");
    }
}
```

## Result:

![](https://507785812-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LcGjO1sZzjRFghWCQRi%2F-LcGjQTYKUTEC3BuDpV6%2F-LcGkSdrzG5WDagqKmk-%2F%E8%9E%A2%E5%B9%95%E5%BF%AB%E7%85%A7%202017-07-04%20%E4%B8%8B%E5%8D%885.22.59.png?generation=1555076080017816\&alt=media)