Using other components
Whenever you want to use a predefined class in your program, the compiler must know how to locate it. Of course, the class might already exist in the same source code file that it’s being called from. In that case, you simply use the class—even if the class doesn’t get defined until later in the file (Java eliminates the “forward referencing” problem, so you don’t need to think about it).
What about a class that exists in some other file? You might think that the compiler should be smart enough to simply go and find it, but there is a problem. Imagine that you want to use a class with a particular name, but more than one definition for that class exists (presumably these are different definitions). Or worse, imagine that you’re writing a program, and as you’re building it you add a new class to your library that conflicts with the name of an existing class.
To solve this problem, you must eliminate all potential ambiguities. This is accomplished by telling the Java compiler exactly what classes you want by using the import keyword. import tells the compiler to bring in a package, which is a library of classes. (In other languages, a library could consist of functions and data as well as classes, but remember that all code in Java must be written inside a class.)
Most of the time you’ll be using components from the standard Java libraries that come with your compiler. With these, you don’t need to worry about long, reversed domain names; you just say, for example:
to tell the compiler that you want to use Java’s ArrayList class. However, util contains a number of classes and you might want to use several of them without declaring them all explicitly. This is easily accomplished by using ‘*’ to indicate a wild card:
It is more common to import a collection of classes in this manner than to import classes individually.