THE JAVA API The Java API which stands for Application Programming Interface is a repository of prewritten classes provided by the creators of Java to enhance its usefulness. There are classes for creating windows with menus and buttons, for drawing two-dimensional figures, for downloading pages from the Internet all kinds of stuff. Classes within the API are organized into libraries called packages, each of which has a like theme. Some of the Java API Packages Package Theme Some Classes Within It java.lang java.util Classes that are considered essential to every Java program Classes that provide helpful utilities Math String Calendar Random Object System Date Scanner java.io Class that perform system input and output File PrintStream The classes in the API are already compiled and available to your Java programs. To use any class other than those in java.lang, specify its full name package name. class identifier. This allows the compiler to resolve the external reference by finding the class file in the Java library. The Scanner class is in package java.util. The following statement correctly references it. java.util.scanner input = new java.util.scanner( System.in ); The import Statement You can inform the Java compiler that you are using a class from a package by placing an import statement at the top of your Java file. The syntax is: import package name. class identifier ; Using this import statement allows you to refer to the class by its shorter class identifier. The Java API Page 1
1 2 3 4 5 6 7 8 9 10 import java.util.scanner; public static void main( String [] args ) Scanner input = new Scanner( System.in ); You can also use an asterisk (*) as a wildcard in the import statement to tell the compiler to import the whole package. The syntax is: import package name. * ; 1 2 3 4 5 6 7 8 9 10 import java.util.*; public static void main( String [] args ) Scanner input = new Scanner( System.in ); Using the wildcard to import the whole package does not reduce the run-time efficiency of your code because the import statement creates no executable operations and it doesn t add unnecessary machine code to your program. It is a compiler declaration that simply allows the compiler to refine its search pattern when resolving the external references. The only machine code added to your program is that for the classes that your program actually references. The Java API Page 2
The java.lang Package The classes in package java.lang are considered so essential that you can use their shorter class identifiers without an import statement. The Java statement: java.lang.string name = new java.lang.string( "JAVA" ); Can be replaced by the following without having to import java.lang.string: String name = new String( "JAVA" ); The import static Statement Class variables and methods can be imported so that you can use their shorter class identifiers. Use Java s import static statement. The syntax is: import static package name. class identifier. * ; Java s String class contains this static method: static String valueof( double d ) // Returns the string representation of the double argument. Without an import static statement, this method is called using: String.valueOf( d ) Or, you can import the static items in String and dispense with the qualifier: import static java.lang.string.*; valueof( d ) The Java API Page 3
Exercises For each application below, circle what's wrong and explain. Two of them are correct. 1. import Scanner; import Math; import System; Scanner in = new Scanner( System.in ); java.lang.system. area = Math.PI * Math.pow( radius, 2 ); 2. Scanner in = new Scanner( System.in ); java.lang.system. area = Math.PI * Math.pow( radius, 2 ); The Java API Page 4
3. java.util.scanner in = new java.util.scanner( System.in ); System. area = Math.PI * Math.pow( radius, 2 ); 4. import java.util.scanner; import java.lang.math; import java.lang.system; Scanner in = new Scanner( in ); The Java API Page 5
5. import java.util.scanner; import static java.lang.math; import static java.lang.system; Scanner in = new Scanner( in ); 6. import java.util.scanner; import static java.lang.math.*; import static java.lang.system.*; Scanner in = new Scanner( in ); The Java API Page 6
7. import java.util.scanner; import static java.lang.math.*; import static java.lang.system.*; Scanner input = new Scanner( in ); The Java API Page 7