Using 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
Previously discussed (cont.)
• Schematically:
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 standard Java library is named java
• A package named xxx inside the standard Java library is
named java.xxx
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/javase/1.4.2/docs/api/java/lang/packa
ge-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 randomnumber generator, and a bit array).
Official website:
http://download.oracle.com/javase/1.4.2/docs/api/java/util/packag
e-summary.html
Organization of the Java library (cont.)
• Schematical representation of Java's Standard library:
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
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
Using a method that is available in Java's
standard library: importing a class
• 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
The import clauses must occur before any class definitions
• Syntax to import a class from the Java library:
import className ;
Using 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
Using a method that is available in Java's
standard library: importing a class (cont.)
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; ...
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
Frequently used methods: java.lang
• According to the 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
We must import java.lang.Math if we want to use the
method Math.sqrt()
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;
Frequently used methods: java.lang (cont.)
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 = ");
Frequently used methods: java.lang (cont.)
System.out.println(c);
System.out.print("x1 = ");
System.out.println(x1);
System.out.print("x2 = ");
System.out.println(x2); }
Frequently used methods: java.lang
(cont.)M
• 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: using 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)