Change color of JComponent after paintComponent has finished

问题

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.

回答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:

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

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.

来源：https://stackoverflow.com/questions/20387349/change-color-of-jcomponent-after-paintcomponent-has-finished

标签

java

swing

colors

paintcomponent

jcomponent