Most applets and windows applications want to change what is drawn on the screen over its lifetime. This can be a sequenced animation, response to user input or mouse events, and so on.
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Whenever you want to redraw the screen, call
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public void repaint(); // note no arguments
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Repaint gets the graphics context g and creates a thread to call update(g), which calls your paint method. So all your drawing changes can also be put in paint.
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One draws a sequence of text and shapes to define the screen, where the position of the object in the screen is given by pixel coordinates. If one object overlaps another, the latest one drawn covers up the area of overlap.
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The exception to this is XOR graphics, which may be used to temporarily highlight a particular color. This is an advanced technique as other colors will also be affected.
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