【27】java中正确的清除

首先说明，java的垃圾回收机制只能回收内存里的数据，如果程序运行过程中向非内存的某些地方写入了数据，那末程序结束时我们需要将他们删除。

比如，一个画图程序，运行过程中，它向屏幕上“写”了一些数据（这些数据不是存储在内存中），那么我们需要多写一些代码将他们删除：

public class Shape {

    public Shape(int i){
        System.out.println("Shape constructor");
    }
    
    public void cleanup(){
        System.out.println("Shape cleanup");
    }
}

public class Circle extends Shape {

    public Circle(int i){
        super(i);
        System.out.println("Circle constructor");
    }
    
    public void cleanup(){
        System.out.println("Circle cleanup");
        super.cleanup();
    }
}

public class Triangle extends Shape {
    public Triangle(int i){
        super(i);
        System.out.println("Triangle constructor");
    }
    
    public void cleanup(){
        System.out.println("Triangle cleanup");
        super.cleanup();
    }
}

public class Line extends Shape {

    private int start,end;
    
    public Line(int start,int end){
        super(start);
        
        this.start = start;
        this.end = end;
        
        System.out.println("Drawing a line from " + start + " to " + end);
    }
    
    public void cleanup(){
        System.out.println("Earsing the line from " + start + " to " + end);
        super.cleanup();
    }
}

public class CADSystem extends Shape {

    private Circle circle;
    private Triangle triangle;
    
    private Line[] lines = new Line[10];//句柄数组，未绑定对象
    /**
     * @param i
     */
    public CADSystem(int i) {
        super(i + 1);//一次初始化父类，组合类，自己
        circle = new Circle(1);
        triangle = new Triangle(1);
        
        for(int j = 0;j <10;++j){//一次为数组里的每个句柄绑定对象
            lines[j] = new Line(j, j*j);
        }
        
        System.out.println("CADSystem constructor");
    }
    
    public void cleanup(){//以与初始化相反的顺序进行删除
        System.out.println("CADSystem cleanup");
        
        for(int i = 0;i <10;++i){
            lines[i].cleanup();
        }

        circle.cleanup();
        triangle.cleanup();
        
        super.cleanup();
    }

    /**
     * @param args
     */
    public static void main(String[] args) {

        CADSystem xCadSystem = new CADSystem(36);
        
        try {
            
        }finally{//finally，最终肯定要被执行的
            xCadSystem.cleanup();
        }
    }
}