5.3. The Java API

You will need to be able to use that Java class library, also called the Java Application Program Interface, or API.

The API is broken up into packages. For example, one of the packages is called java.math. You refer to a class by the package that it belongs to. Class java.math.Math is the class called Math in package java.math.

In a Java program, you refer to classes by their full names. For example,

  double q = java.math.Math.sqrt(2.0);

Pay attention to the case of letters. In the Java API, package names start with lower case letters. Class names start with upper case letters. Most method names start with lower case letters

Imports

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It is clumsy for the sqrt function to have such a long name, java.math.Math.sqrt. You can shorten it by doing an import Import

  import java.math.Math;

  double q = Math.sqrt(2.0);

  import java.math.*;

Write imports at the beginning of a Java file, before the first class.

Package java.lang

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Every Java program implicitly imports all classes in package java.lang. For example, class java.lang.String can be refered to as String, without the package name.

Static imports

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You can shorten some method names further. Import line

  import static java.math.Math.sqrt;

  double q = sqrt(2.0);

  import static java.math.Math.*;

Restrictions on imports

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You can import a class name from a package or a static method in a class. But you cannot import an object.

For example, package java.lang contains a class called System. You can use it for writing to the standard output, as follows.

  System.out.println("A string");

  import static java.lang.System;

  out.println("A string");