JaVa
   

Importing Packages

To use a package within our Java app or applet, we need to import it. We do this by means of the import keyword. So, for example, if we wish to include the I/O package, which is called java.io, we would have the following statement at the top of our code (before we define any classes):

import java.io.*;


Note how we have appended an extra decimal point and star to the end of the package name. This means that it will include all of the classes within the package (i.e., the asterisk is used as a wildcard). Another example of this would be if we wished to include the utility package, which is called java.util. This would be done with the following statement:

import java.util.*;


Again, note the use of the asterisk to include all the classes from the package. However, if we only wished to include a single class from the package, we could do this too. Within the utility package, there is an ArrayList class. If we simply wish to use the ArrayList class from the utility package and no others, we could import just the ArrayList class using the following statement at the top of our code.

import java.util.ArrayList;


Of course, if we used the asterisk, the ArrayList package would be included automatically. So once we do this, we could then create a reference to an ArrayList object within a class or method using the following statement:

ArrayList myArrayList;


Also, it is good to know that it is possible to access the ArrayList class (or any other class out of a package) by using its fully qualified name. For example, without any import statements, we could create the myArrayList object as we did before with the following line of code.

java.util.ArrayList myArrayList;


As we mentioned in the introduction, packages provide namespace management, so it is therefore possible that two packages could both have a class with the same name in it. Obviously, this could cause problems if both the packages were imported, so in this case it would make sense to use the fully qualified package name:

package1.MyClass firstReference;
package2.MyClass secondReference;


JaVa
   
Comments