Thinking in Java(7)-Guaranteeing proper cleanup

You can’t know when the garbage collector will be called,or if it will be called. So if you want something cleaned up for the class,you must explicitly write a special method to do it,and make sure that client programmer knows that they must call this method.

Consider an example of a computer-aided design system that draws pictures on the screen:

class Shape{
Shape(int i) {
System.out.println("Shape constructor");
}
void dispose() {
System.out.println("Shape dispose");
}
}

class Circle extends Shape {
Circle(int i) {
super(i);
System.out.println("Drawing Circle");
}
void dispose() {
System.out.println("Erasing Circle");
super.dispose();
}
}

class Triangle extends Shape {
Triangle(int i) {
super(i);
System.out.println("Drawing Triangle");
}
void dispose() {
System.out.println("Erasing Triangle");
super.dispose();
}
}

class Line extends Shape{
private int start,end;
Line(int start,int end) {
super(start);
this.start = start;
this.end = end;
System.out.println("Drawing Line: " + start + ", " + end);
}
void dispose() {
System.out.println("Erasing ;Line: " + start + ", " + end);
super.dispose();
}
}

public class CADSystem extends Shape {
private Circle c;
private Triangle t;
private Line[] lines = new Line[3];
public CADSystem(int i) {
super(i + 1);
for (int j = 0;j < lines.length;j++)
lines[j] = new Line(j,j*j);
c = new Circle(1);
t = new Triangle(1);
System.out.println("Combined constructor");
}

public void dispose() {
System.out.println("CADSystem.dispose()");
t.dispose();
c.dispose();
for (int i = lines.length - 1;i >= 0;i--)
lines[i].dispose();
super.dispose();
}

public static void main(String[] args) {
CADSystem x = new CADSystem(47);
try{

}
finally {
x.dispose();
}

}

}

The finally clause is saying,”Always call dispose() for x,no matter what happens.”

In general,you should follow the same form that is imposed by a C++ complier on its destructors:First perform all of the cleanup work specific to your class,in the reverse order of creation.

result:

Drawing Line: 0, 0
Shape constructor
Drawing Line: 1, 1
Shape constructor
Drawing Line: 2, 4
Shape constructor
Drawing Circle
Shape constructor
Drawing Triangle
Combined constructor
CADSystem.dispose()
Erasing Triangle
Shape dispose
Erasing Circle
Shape dispose
Erasing ;Line: 2, 4
Shape dispose
Erasing ;Line: 1, 1
Shape dispose
Erasing ;Line: 0, 0
Shape dispose
Shape dispose