Java中的 import

java.util.ArrayList x = new java.util.ArrayList ( 149 );

import java.io.*;import java.util.ArrayList;import java.util.Date;import java.util.Properties;

ArrayList x = new java.util.ArrayList( 149 );

Unlike C or C++ we do not need to include headers to help the compiler determine what sorts of parameters other routines want; it can go look for itself in the source or class files. The import statement is not like the C++ include. So long as you fully qualify your reference in the code to class names with com.mindprod.mypackage.myClass there is no need for imports. They just allow shorthand. Even when you do have an import, you can still fully qualify your references to classes.

1. import com.mindprod.mypackage.MyClass;
2. import com.mindprod.mypackage.*;

• failing to get upper/lower case precisely correct in the import statement package and classnames.
• failing to get upper/lower case precisely correct in the matching directories and *.java names. See classpath for more details.
• failing to name each source module as X.java where X is the name of the public class defined in that source file.
• Your imports must be accessible via the CLASSPATH.
• Mismatch of CLASSPATH and import. The more qualification you specify in the CLASSPATH, the less you are allowed to specify in the import statement. You may not specify a directory both in the import and in the CLASSPATH. The import gives the lower levels of qualification the CLASSPATH the higher. Another way of stating this is your CLASSPATH should point to the directory just above the one named for your package’s highest level of qualification.
• Wildcards are not as wild as you might expect causing you to fail to import each package branch separately. import java.awt.*; does not automatically import java.awt.event.* as well. You must separately import java.awt.event.*.
• You can’t use wildcards on partial names, e.g. import java.awt.Text*;
• You can teach your IDE to handle your imports for you. This save a great deal of mindless busywork.
• It is a good idea to avoid wildcards, unless you have a very large number of classes. The list of explicit imports is great documenation on just what sorts of thing the class is doing/could potentially do.
• If you have a class name used in more than one package, you will save yourself a lot of grief if you always fully qualify references to it, rather than relying on import. The computer may understand but, your fellow programmers will often be confused by the lack of qualification. Beware of java.util.List and java.awt.List.
• Never use a classname (either in your package or in your anonymous package) that is also used in some other package. You are just begging for trouble. In theory the purposes of packages is to protect against name clashes, but as soon as you use import that protection is gone.
• Package names should be pure lower case. Class names should begin with an upper case letter. Violating this convention will confuse the heck out of anyone trying to decipher your code.
• You can write simple programs leaving out the package statement. However, this should only be used for programs that fit on one page. You can’t use classes in the default package from a named package. So except for tiny experiments, always put your classes in a named package of the form com.mindprod.xxx or org.hans.xxx i.e. your domain name backwards, all lower case.
• You must not import classes in the same package as the current class. If you are using the default nameless package, you must not import any other classes in the default package.

It is a great help to understanding someone else’s code, (or even your own), if you refrain from using the .* style of import with the imports giving you an explicit list of the classes you use in that class. The list of explicit imports gives you a rough idea of what the class could do and what it is likely up to. Don’t leave unused imports lying around. Use an import tidier, or Eclipse, to get rid of them, and sort the valid ones into alphabetical order. During development, it is fine to use the lazy .* form, (doing so will save you a ton of compiler errors), but once your work gels, convert over. Bon’t camouflage your use of classes by fully qualifying them and not reporting them in the import. The only time you need qualification is when there is a name clash, e.g. java.awt.List and java.util.List.

The other problem with using wildcards is this. Let’s say you use import java.awt.* to get your Label class. Then later somebody using your code has a com.wombat.superawt. Label class on their classpath. The code won’t compile, or worse, it will use the wombat Label class in place of the awt Label class without telling anyone.