I want to draw a rect when Player click on the GUI here is my code :

Quote

@Override
public void drawScreen(int mouseX, int mouseY, float partialTicks) {
    this.drawDefaultBackground();
    super.drawScreen(mouseX, mouseY, partialTicks);
}

@Override
protected void mouseClicked(int mouseX, int mouseY, int mouseButton) throws IOException {
    drawRect(1, 1, 20, 20, Color.RED.getRGB());
    super.mouseClicked(mouseX, mouseY, mouseButton);
}

 

Drawing must happen each frame, as at the beginning of each frame the screen gets cleared of anything that is drawn on it. Create a variable to hold if the player clicked the mouse or not and draw your rectangle in the drawScreen method if that variable is true/hold whatever value you need it to. 

If you want something to be drawn 'after draw screen' just put your drawing calls after the super.drawScreen 

Thanks !