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Importing methods in the Java library

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Title: 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

11
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
15
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)
17
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.

18
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)

19
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"
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