Working with Images and Resources in Java

Suppose your program already has an image. The next question is where that image can go. In Java, an image can be copied onto two kinds of destination: the screen itself, or an off-screen canvas that exists only in memory. Both destinations are reached through the same tool, the set of drawing methods defined in the Graphics class. Once you see that the image is the source and a graphics context is the route to the destination, most of this topic falls into place.

This article approaches the topic from the destination side. First we look at how an image gets copied through a Graphics object. Then we look at how an off-screen canvas can be prepared in advance. Finally we follow a small piece of drawing from start to finish so you can track what changes at each step.

An Image of either type can be copied onto the screen or onto an off-screen canvas by using methods defined in the Graphics class. The most common place to do this is inside the paintComponent() method of a JComponent. The Graphics object that Java passes to paintComponent() is the handle you use for drawing. In that case you do not create the destination yourself. You draw through the Graphics object you were given.

The diagram shows the central structure. The image is the thing being copied, the g.drawImage() call performs the copy, and the Graphics object decides where the result lands. The same call works in both cases. What differs is whether the result is visible to a user the moment it is drawn, and whether the drawing persists in the program for later use.

The source introduces the copying operation with two names. g is the Graphics object passed into paintComponent(), and img is a variable of type Image. The copy itself is written as a call to one of the g.drawImage() methods. Notice that the call names the Graphics object first. That is how the destination is chosen: whatever surface g refers to receives the pixels.

Here is a short scenario you can trace on paper. Assume a component's paintComponent() method has been called by Java, so a Graphics object g is available, and an Image named img was loaded earlier. Work through the steps in order and keep track of which object is being used at each point.

Copying images is only half of the picture. The source also describes creating images by drawing to an off-screen canvas. The canvas is represented by an object of the class BufferedImage, which is defined in the package java.awt.image. Because it is an off-screen canvas, drawing into it does not put anything on the screen by itself. The result is a picture held in the program, ready to be used later.

Put the two halves together and the full path becomes clear. First you draw into the BufferedImage through its own Graphics object, which changes the canvas contents in memory. Later, you copy that canvas onto a visible surface by calling g.drawImage() with the screen's Graphics object. Nothing is visible during the first stage. The copy in the second stage is what makes the result appear.