---

Importing methods in the Java library

---

• Previously discussed
  • Method = a collection of statements that performs a complex (useful) task

    A method is identified by a method name

    ---

  • Class = a container for methods

    Methods that serves a similar purpose are stored in the same class

    A class is identified by a class name

    ---

  • Schematically:

  ---

  ---

  ---

  ---

  For the lecturer:

  Motivate the need for organization using a hierarchy Example 1: USA, states, countries, districts Example 2: Store, departments (toy, tools), sections (board games, electronic games)... The Java library also uses a hierarchical structure based on functionality

  ---

  ---

  ---

  ---

• Organization of the Java library
  • To find "things" more easily, humans usually organize (= group) the "things" in a hierarchical manner

    Examples:

    When you look for cheese in a Super market, you first find the dairy section and then look for the cheese section The USA is divided into states, which in turn are divided into counties and there are different districts inside a county.

    ---

  • The organization of the Java library:

  • The Standard Java library consists of a number of packages

    Each package consists of a number of classes (that provide similar functionality)

    The classes are organized into packages according to the functionality of the methods in a class. In other words: A package contains classes with methods that solve a related problem.

    ---

  • The standard Java library is named java

  • A package named xxx inside the standard Java library is named java.xxx

    Some commonly used packages:

    java.lang: Provides classes that are fundamental to the design of the Java programming language. Official website: click here java.util: Contains the collections framework, legacy collection classes, event model, date and time facilities, internationalization, and miscellaneous utility classes (a string tokenizer, a random-number generator, and a bit array). Official website: click here

    ---

  • Schematical representation of Java's Standard library:

    ---

  • A class named yyy inside the package java.xxx is named java.xxx.yyy

    Example:

    The class Math inside the package java.lang is known as java.lang.Math The class Double inside the package java.lang is known as java.lang.Double The class Stack inside the package java.util is known as java.util.Stack The class Scanner inside the package java.util is known as java.util.Scanner

    Note:

    It is a Java convention that the name of a Java class begins with a upper case letter It is also a Java convention that the name of a method begins with a lower case letter

  ---

  ---

  ---

  ---

• The class path name of a class inside the Java library
  • Each class inside the Java library has a unique class path name in the form:

    java.PackageName.ClassName

    (The package name can be composite (e.g. name1.name2), but the most commonly used packages has a non-composite name)

    ---

  • The following figure show the class path names of some classes:

  ---

  ---

  ---

  ---

  ---

  ---

• Using classes in the Java library
  • In order to use (the programs stored inside) the classes in the Java library, we must identify the class within the Java library

  • One way to identify a class in the Java library is:

    use the class path

    ---

  • Example: we can refer to the sin() method stored in the Math class (inside the package lang as:

    java.lang.Math.sin( ... )

    ---

  • Example: Java program using the class path to identify a class

    public class ClassPath1 { public static void main(String[] args) { double x = 1.234; double r; r = java.lang.Math.sin( x ); System.out.print("Sin of " + x); System.out.println(" = " + r ); } }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac ClassPath1.java To run:          java ClassPath1

  ---

  ---

  ---

  ---

• Making programs (methods) in Java's standard library more accessible: importing a class
  • Rule of usage:

    If you want to make a class inside the Java library accessible without using the class path, a Java program must first import the class (with an import clause)

    The import clauses must occur before any class definitions

    ---

  • Syntax to import a class from the Java library:

    import classPathName ;

    Examples:

    import java.lang.Math; // After the import clauses, you can use the class // by its name, without the entire classPath // This program can now use all things defined inside // the class java.lang.Math without the classPathName public class MyProgram { public static void main(String[] args) { double a; a = Math.sqrt(2.0); // Save computed value in variable System.out.println(a); // You can print the saved value later } }

  ---

  ---

  ---

  ---

• Importing all classes in a package
  • Some complex Java program may use many different methods contained in many different classes in the same package

    It would be a pain to write a long list of import clauses

    Example:

    import java.lang.Math; import java.lang.Double; import java.lang.Integer; ... import java.util.Scanner; import java.util.Stack; ...

    ---

  • There is a short hand to import all classes contained inside a package:

    import java.lang.* ; // import all classes in java.lang package import java.util.* ; // import all classes in java.util package

  ---

  ---

  ---

  ---

  ---

  ---

• The frequently used class: java.lang
  • According to the Rule of usage:

    If you want to make a class inside the Java library accessible without using the class path, a Java program must first import the class (with an import clause)

    we must:

    import java.lang.Math if we want to use methods like Math.sqrt(), Math.sin(), and so on (without the class path).

    ---

    In other words, we should have written:

    import java.lang.Math; // We MUST import this class to use Math.sqrt // without the class path public class Abc { double a, b, c, x1, x2; // Define 5 variable a = 1.0; b = 0.0; c = -4.0; x1 = ( -b - Math.sqrt( b*b - 4*a*c ) ) / (2*a); x2 = ( -b + Math.sqrt( b*b - 4*a*c ) ) / (2*a); System.out.print("a = "); System.out.println(a); System.out.print("b = "); System.out.println(b); System.out.print("c = "); System.out.println(c); System.out.print("x1 = "); System.out.println(x1); System.out.print("x2 = "); System.out.println(x2); }

    ---

  • But.... because:

    The package java.lang contains classes that are fundamental to the design of the Java programming language.

    all classes in the java.lang package are automatically included in every Java program (the Java compiler is programmed to do this)

    That is why we did not need to import java.lang.Math in our program.

  ---

  ---

  ---

  ---

  ---

  ---

• Summary: importing methods in the Java library
  • Rule of usage:

    You can use a class inside the Java library by identifying the class by its (very long and awkward) class path If a Java program wants to use a class in the Java library without having to specify the (very long and awkward) class path the Java program must first import the class All classes in the java.lang package have already been imported into a Java program (You can use the classes (and the methods inside these classes) in java.lang without the import clause)