Importing methods in the Java library

1
Importing methods in the Java library
2
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

3
Previously discussed (cont.)

• Schematically

4
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)

5
Organization of the Java library (cont.)

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

6
Organization of the Java library (cont.)

• Some commonly used packages

• java.lang Provides classes that are fundamental
  to the design of the Java programming language.
  Official website http//download.oracle.com/java
  se/1.4.2/docs/api/java/lang/package-summary.html
• java.lang 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 http//download.oracle.
  com/javase/1.4.2/docs/api/java/util/package-summar
  y.html

7
Organization of the Java library (cont.)

• Schematical representation of Java's Standard
  library

8
Organization of the Java library (cont.)

• 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

9
Organization of the Java library (cont.)

• 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

10
Importing a method that is available in Java's
standard library importing a class

• Rule of usage
• The import clauses must occur before any class
  definitions

• If a Java program wants to use a method in the
  Java library, the Java program must first import
  the containing class

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Importing a method that is available in Java's
standard library importing a class (cont.)

• Syntax to import a class from the Java library

import className
12
Importing a method that is available in Java's
standard library importing a class (cont.)

• Examples

import java.lang.Math import
java.lang.Double import java.util.Stack
import java.util.Scanner // After the import
clauses, you can write // the class
definition // This program can now use all
methods defined inside // the classes Math,
Double, Stack and Scanner 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

13
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 ...
14
Importing all classes in a package (cont.)

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

import java.lang. // import all class in
java.lang package import java.util. // import
all class in java.util package
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Frequently used methods java.lang

• According to the Rule of usage
• We must import java.lang.Math if we want to use
  the method Math.sqrt()

• If a Java program wants to use a method in the
  Java library, the Java program must first import
  the containing class

16
Frequently used methods java.lang (cont.)

• We should have written

import java.lang.Math // We MUST import this
class to use Math.sqrt 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( bb - 4ac
) ) / (2a) x2 ( -b Math.sqrt( bb -
4ac ) ) / (2a)
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)
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Frequently used methods java.lang (cont.)

• But.... because
• 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.

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

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Summary importing methods in the Java library

• Rule of usage

• If a Java program wants to use a method in the
  Java library, the Java program must first import
  the containing class
• All classes in the java.lang package have
  already been imported into a Java program
• (You can use methods in these classes without the
  import clause)

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Summary importing methods in the Java library
(cont.)

• Analogy
• The reason you need to import the method is
  because the Java compiler needs to know the
  details of the method before it can make the
  correct translation.

• You can think of methods inside your Java
  program and methods inside the Java library as
  subjects (people) from different automonous
  entities (e.g., countries)
• When you want to "bring" a method from the Java
  library into your program, you need to issue an
  import "license"