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Friday, June 27, 2008

import Statements

An import statement is a way of making more of the functionality of Java available to your program. Java can do a lot of things, and not every program needs to do everything. So, to cut things down to size, so to speak, Java has its classes divided into "packages." Your own classes are part of packages, too.

No import Needed

The simple Hello.java program we've used as an example so far doesn't have any import statements:

public class Hello{
public static void main(String arg[]){
System.out.println("Hello.");
}
}

Everything in the program is already available to the compiler. The compiler can access any class in the java.lang package without needing an import statement. It can also access any classes in the "local" package, which is any classes defined in files in the same directory as the program being compiled that aren't part of another package (that is, they don't have a package statement at the start of the file.)

import Required

Anything that isn't in the java.lang package or the local package needs to be imported. An example is the Scanner class. If you look up the Scanner class in the Java API Specification, you'll see that it is in the java.util package. Remember, to look it up you scroll to the class name in the lower left frame then click on it to bring up its definition in the main frame of the browser. Class names are in regular typeface, interfaces are in italics (some classes and interfaces have the same name.)

Here's an example program that uses Scanner, with an import statement:

import java.util.Scanner;

public class ScannerTest{
public static void main(String arg[]){

// scanner gets its input from the console.
Scanner scanner = new Scanner(System.in);
String name = "";

// Get the user's name.
System.out.print("Your name, adventurer? >");
name = scanner.next();
System.out.println();

// Print their name in a a message.
System.out.println("Welcome, " + name + " to Javaland!");
}
}

We imported just the class Scanner from java.util in the import statement in this program. If we'd been using multiple classes from java.util, we could have made all the classes in java.util available to us by using this import statement:

import java.util.*;

The * is a "regular expression operator" that will match any combination of characters. Therefore, this import statement will import everything in java.util. If you have tried entering and running the example program above, you can change the import statement to this one.

If we need multiple classes from different packages, we use an import statement for each package from which we need to import classes (or interfaces, or any other part of that package we need.) It's not unusual to see a series of import statements near the start of a program file:

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import javax.swing.*;
import javax.swing.event.*;

Now, you may wonder why we have statements importing java.awt.* and java.awt.event.*. It seems like if you import java.awt.* that ought to import everything "under" java.awt, right? And the "dot" notation sure makes it look like java.awt.event is under java.awt.

Package Names and Those Misleading Dots

The thing is, each package with its own name is an entirely separate entity. The dot notation and name similarities are just a convention for making it easier to keep track of which packages have functions that are related to each other. One package isn't inside another one. java.awt.event is an entirely separate package from java.awt, it's not inside java.awt.

This can be very confusing, since the other use of the "dot notation" is a way of getting to something that is "inside" or a member of something else. System.out.println() is a way of getting at the println() method that's a member of out which is a member of the System class. But it doesn't work that way with packages. Each package name is a whole different package, and each package needs its own import statement. java.awt.event is entirely different from java.awt.

You can see package names in the API reference in the upper left frame. If you click on a package name, the lower left frame with change to limit what it shows to just the items in the package you've clicked.
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