Java programs perform I/O through streams. A stream is an abstraction that either produces or consumes information. A stream is linked to a physical device by the Java I/O system. All streams behave in the same manner, even if the actual physical devices they are linked to differ. Thus, the same I/O classes and methods can be applied to any type of device. For example, the same methods that you use to write to the console can also be used to write to a disk file. Java implements streams within class hierarchies defined in the java.io package.