在GeOb類里加個 Colorable 介面字在子類裡實現就好～～定義陣列的時候要注意數組裡的元素要逐個 new 例項，父類是抽象類的話，要用子類實現～～JAVA越來越好用了嘛～～怪不得JAVA比較好找工作

GeOb類～～多加個了藉口

public abstract class GeOb{
    private String color = "white";
    private boolean filled;
    protected java.util.Date da;
    protected GeOb(){
        da = new java.util.Date();
    }
    public
 
 GeOb(String color,boolean filled){
        da = new java.util.Date();
        this.color = color;
        this.filled = filled;
    }
    public void setGe(String color,boolean filled){
        da = new java.util.Date();
        this.color = color;
        this.filled = filled;
    }
    public String getcolor
 
(){
        return color;
    }
    public void setcolor(String color){
        this.color = color;
    }
    public boolean getfillrd(){
        return this.filled;
    }
    public void setfilled(boolean filled){
        this.filled = filled;
    }
    public java.util.Date getda(){
        return
 
 da;
    }
    public String toString(){
        return "created on " + da + "\ncolor : " + color + " and filled : " + filled;
    }
    public void howToColor() {
        System.out.println("Color all four sides ");
    }
    interface Colorable{
        public void howToColor();
    }
    public abstract double getArea();
    public abstract double getPerimeter();
}

Square 類～也即子類

class Square extends GeOb{

    public double getArea() {
        return 0;
    }

    public double getPerimeter() {

        return 0;
    }
    public void howToColor() {
        System.out.println("Color all four sides ");
    }
}

Text1 實現程式碼 ：

public class Text1 {

    public static void main(String[] args) {
        GeOb[] a = new GeOb[10]; // 定義陣列物件
        for(int i = 0 ; i < 5 ; i++)
        a[i] = new Square(); // 把數組裡的元素逐一 new 成例項
        a[0].setGe("write",true);
        a[1].setGe("write",true);
        a[2].setGe("write",false);
        a[3].setGe("write",false);
        a[4].setGe("write",true);
       for(int i = 0 ; i < 5 ; i++){
         System.out.println(a[i].getfillrd());
         if(a[i].getfillrd())
             a[i].howToColor();
      }
    }
}