绘制形状时在JPanel中为我的画布清除按钮

Question

I am drawing shapes and the shapes store into an arraylist. When I hit clear, all the shapes clear out of the arraylist except for the last one I have drawn.

I even did

Arraylist list = new Arraylist();
list.clear();
JOptionPane.showMessageDialog(null, "Size is: " + list.size());

just to prove the size was 0 and a similar test to make sure the last added object was added into the array list. What could be the possible reasons for this?

Answer 1

call the repaint() method in the event handler of the clear button.

list.clear();
repaint();

it will refresh your canvas.

Answer 2

Arraylist list = new Arraylist();
list.clear();

Well, that doesn't do anything because all did was create a local variable which is empty to begin with and then clear the empty List. Your painting code will still be referencing the instance variable from your class. Your code should just be:

//Arraylist list = new Arraylist();
list.clear();