Java 1.5 Tiger A Developers Notebook [Electronic resources] نسخه متنی

8.1 Importing Static Members

If you've ever done any output


in Java, you've typed System.out at least
a few times. While there are lots of better ways to handle output, there is perhaps none as simple, direct, and to-the-point as good old System.out.println( ) and System.err.println( ). However, it's annoying to type System.out and System.err time and time again (at least, it is for me). In Tiger, this annoyance is gone, via static imports.

NOTE

If you try and
import something non-static with this syntax, all you get is a "cannot find symbol" compiler
errorit's not very descriptive.

8.1.1 How do I do that?

Anytime you have a static member, like out and err in the java.lang.
System class, you can import those static members into your code:

import static java.lang.System.err;
import static java.lang.System.out;

You just use import static instead

of
import. Magically, your code can
now use the methods of these static members without prefacing those
methods with the static member's package name, as seen in Example 8-1.

Example 8-1. Using statically imported member variables

package com.oreilly.tiger.ch08;
import static java.lang.System.err;
import static java.lang.System.out;
public class StaticImporter {
public static void main(String[] args) {
if (args.length < 2) {
err.println(
"Incorrect usage: java com.oreilly.tiger.ch08 [arg1] [arg2]");
return;
}
out.println("Good morning, " + args[0]);
out.println("Have a " + args[1] + " day!");
}
}

As said before, this isn't anything particularly revolutionary, but it's certainly
a nice bonus to the language.

As an added bonus, this same thing applies to static methods, in addition
to static variables. For example, consider the java.util.Arrays class,
which has several static methods, such as binarySearch( ), sort( ), and
equals( ). These methods are just static members of the class, and can be
imported, just like the err and out members in Example 8-1:

NOTE

Note that Arrays.sort( ) is a heavily
overloaded method. This import directive imports the method by its name, and not any particular overloading of it.

     import static java.util.Arrays.sort;

You can now just code as shown here:

     sort(myObjectArray); // no need to use Arrays.sort( )

This is a particularly nice improvement to the language, I think.

8.1.2 What about...

...static member types? A member type is a type defined within another
class. For example, the java.lang.Character class defines a class within
its body, called Subset. The formal name of this class, because it does not
stand on its own, is Character.Subset (and it appears that way within
the JavaDocs). You can import a member type, in Tiger and in previous
versions, with a normal import statement:

     import java.lang.Character.Subset;

This is a little-known, little-used feature of Java that has nothing to do
with Tiger. However, where Tiger does come in is adding the ability to
do static imports. In the case of Character.Subset, not only is Subset a
member type, it's a static member typemeaning that it could also be imported as seen here:

NOTE

Anonymous inner
classes defined
within a method
are not members,
and cannot be
imported.

     import static java.lang.Character.Subset;

So, which is correct? Actually, both. However, you shouldn't have both in
the same codejust one or the other. Otherwise, compilers will gripe
about a name conflict (two identically named types). Personally, I like
the clarity of import static for this sort of thing, as it reminds me that
Character.Subset is staticbut that's style more than any real best
practice.