Change color of JComponent after paintComponent has finished

Question

I am testing one small widget class that extends JComponent
the constructor for the widget contains one vector and sets PreferredSize of the component, then there is the paintComponent:

public void paintComponent(Graphics g){
        g.setColor(Color.RED);
        g.drawString("this is text one", 10, 10);
            //here I draw some shapes based on the 
            //vector size and integers
        }
    }

the component is drawn correctly and after that i call some other methods in main, when the methods finish their jobs i call widget.methodsFinished():

methodsFinished(){
        g.setColor(Color.GREEN);
        g.drawString("this is text two", 30, 30);
                this.update(g);
}

I get nullpointer exception by doing this, can you tell me how to correctly update the color of already drawn shapes in this component, thank you in advance.

Answer 1

can you tell me how to correctly update the color of already drawn shapes in this component, thank you in advance.

Not really so tough:

1. Declare a private Color field in your class context.
2. Declare a public setShapeColor(Color color) to set the color to the component
3. invoke repaint() to reflect the color changes

4. And as a warning: don't forget to call super.paintComponent(g); inside the paitnComponent(Graphics) function, which you haven't done.

    class MyComponent extends JPanel
    {
        private Color shapeColor = Color.RED;
   
        public void setShapeColor(Color color)
        {
          this.shapeColor = color; 
        }
   
       @Override
      public void paintComponent(Graphics g){
       super.paintComponent(g);
       g.setColor(shapeColor);
       g.drawString("this is text one", 10, 10);
           //here I draw some shapes based on the 
           //vector size and integers
          }
      }
    } 
   

Though as OOP principle, you should actually declare a MyShape class with Color attribute and before drawing use the setter method as the example to set the color to the shape.