java repaint()无效

Answer 1

1. Before each rendering, the previous canvas must be cleared, otherwise it will be superimposed

2. If you want to display animation, you need to set a sleep time, otherwise you will not be able to see the intermediate process

Answer 2

It’s probably because you are not here x=2, y=2 时清除之前 x=1, y=1 时写的 "hello", so they all overlap and form a thick blue line : )