Image I/O in Java

Buttons, labels, and menus in a Java graphical interface often show a small picture next to their text. That picture is not simply dropped onto the component. Java expects the picture to be wrapped in a particular kind of object first, and then the graphical interface knows how to display that wrapped object. This article follows that wrapping step from start to finish, so you can see which piece of code is responsible for what.

The name Icon refers to a type that describes what an icon is in a Java GUI. It is the general description. Because it is a general description, the name Icon by itself does not tell you how a particular icon is built from a picture. A programmer who declares a variable as Icon is saying that the variable will hold something that behaves as an icon. The specific object behind it can be chosen separately.

This separation is why the general type appears on the left of many declarations. The variable promises only that it holds an icon-like object. The code that decides which concrete class to use can change without changing the declared type of the variable. This is a common reason to prefer writing the interface name in a declaration rather than the concrete class name.

Creating an icon from a picture happens in four steps. First, you have an Image object, which holds a picture. Second, you pass that image to the constructor of ImageIcon by writing new ImageIcon(image). Third, Java produces an ImageIcon object whose picture is the image you supplied. Finally, you can store that result in a variable of type Icon, because ImageIcon implements the interface.

The class ImageIcon implements the Icon interface. That relationship has two sides. The interface Icon is the contract: it describes what an icon is. The class ImageIcon is the concrete implementation: it is one specific way of satisfying that contract, built from a picture. Because ImageIcon implements Icon, an ImageIcon object can be stored in a variable whose declared type is Icon.

When a variable is declared as Icon but refers to an ImageIcon object, the object that actually exists in memory is the ImageIcon. The variable's declared type limits what you can rely on through that variable. You can use the operations that the Icon type promises. The variable does not change the object itself, and it does not reveal that the object was built from an Image unless you check its class. The specific class is chosen when the object is created, and it stays the same object behind the general name.