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Java Class Libraries & API’s
A class library is a set of classes that supports the
development of programs.
Java comes with a standard class library, but libraries can
be obtained separately from third party vendors.
Classes in class libraries contain methods that are valuable
to the programmer.
API’s (Application Programming Interfaces) are clusters
of related classes.
• Java Database API contains classes that help
programmers use databases
• Java Swing API contains classes that help
programmers use graphical components.
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Java Packages
Classes are also grouped in packages.
Packages are more fundamental and language based than
API’s.
See text appendix “M” for classes and packages.
Example packages from Java’s standard class library:
Package
Class
Use
java.util
Scanner
Random
Inputting from keyboard and files
Generates random numbers
java.lang
Math
String
Various math constants and functions
Handling text
java.text
DecimalFormat
Formatting decimal numbers
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Java Class Packages
To use a package in a program, it must first be imported.
Add the import declaration at the beginning of a program,
after the package statement and before any other
executable code.
• import packageName.*;
Imports all classes in packageName.
• import packageName.className
Imports on className from packageName
• Example: import java.text.DecimalFormat;
Imports only the DecimalFormat class from the
java.text package.
The java.lang package does not have to be imported.
• String and Math methods are always available.
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Declaring An Object
To declare an object, use this statement:
ClassName objectName = new ClassName (parameters);
Where:
• ClassName is the name of the class of the object.
• objectName is your choice of name for the variable
object.
• “=“ is the assignment operator.
• new is a Java reserved word.
• ( parameters) is a set of data values use to initialize
or setup the object.
Except for Math class objects, all others must be declared
before they are used.
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String Class
The String class provides a convenient way to manage and
manipulate text.
The String class is in the java.lang package and is
automatically imported.
See the chart below for a partial summary of available
methods or appendix “M” for a complete list:
String Manipulation
Method
length of string
length()
character at position n
charAt(int index)
find substring: start m and end n-1
substring(int m, int n)
change to upper case
toUpperCase()
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String Class Ex.
Example from Program “StringMutation”
(See text CDROM Chapter 3)
// Variable object declaration
String phrase = "Change is inevitable";
String mutation1, mutation2, mutation3, mutation4;
System.out.println ("Original string: \"" + phrase + "\"");
System.out.println ("Length of string: " + phrase.length());
... Outputs
Original string: "Change is inevitable"
Length of string: 20
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String Class Ex.
Example from Program “StringMutation”
(See text CDROM Chapter 3)
mutation1 = phrase.concat (", except from vending machines.");
mutation2 = mutation1.toUpperCase();
mutation3 = mutation2.replace ('E', 'X');
mutation4 = mutation3.substring (3, 30);
...Outputs
Mutation #1: Change is inevitable, except from vending machines.
Mutation #2: CHANGE IS INEVITABLE, EXCEPT FROM
VENDING MACHINES.
Mutation #3: CHANGX IS INXVITABLX, XXCXPT FROM
VXNDING MACHINXS.
Mutation #4: NGX IS INXVITABLX, XXCXPT F
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Scanner Class
The Scanner class provides a convenient way to read input
values of various types.
The Scanner class accepts input from various sources
including the keyboard, a file, or a string.
Must be imported from java.util package
For keyboard, use “System.in” in the constructor.
See the chart below for a partial summary of available
methods or appendix “M” for a complete list:
Data Type
Method
String
nextLine()
Byte
nextByte()
Integer
nextInteger()
Double
nextDouble()
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Scanner Class
An object declaration looks like this:
Scanner scan = new Scanner (System.in);
• Declares a “new” object variable named “scan” (your
choice of object variable name).
• The Scanner class has a special constructor method that
constructs or creates this new object.
• The parameter “System.in” tells the constructor to use the
PC’s keyboard to accept data entry.
The input stream may contain more than one data item, called a
token.
Tokens are separated by white space, the default delimiter.
To change the delimiter use useDelimiter(String pattern).
• Example: scan.useDelimiter(“,”); changes the delimiter to
a comma.
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Scanner Class Ex. 1
Example Program “Echo” (See text CDROM Chapter 2)
//******************************************************
// Echo.java
Author: Lewis/Loftus
//
// Demonstrates the use of the nextLine method of the Scanner class
// to read a string of text from the user.
//******************************************************
import java.util.Scanner; // First import the Scanner class
Package
Class
Dot Operator
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Scanner Class Ex. 1
Example 1 Program “Echo”
public class Echo
{
//----------------------------------------------------------------// Reads a character string from the user and prints it.
//----------------------------------------------------------------public static void main (String[] args) {
String message;
Scanner scan = new Scanner (System.in);
// Class objectName = new Class (constructor parameters);
System.out.println ("Enter a line of text:");
message = scan.nextLine();
System.out.println ("You entered: \"" + message + "\"");
}
Object
Method
}
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Scanner Class Ex. 2
Example 2 Program “CalculateSum”
Input tokens on separate line.
import java.util.Scanner;
public class CalculateSum {
public static void main(String[] args) {
double firstNum, secondNum, sum;
Scanner scan = new Scanner(System.in);
System.out.println("Enter first number to add:");
firstNum = scan.nextDouble();
System.out.println("Enter second number to add:");
secondNum = scan.nextDouble();
sum = firstNum + secondNum;
System.out.println(firstNum + " + " + secondNum + " = " + sum );
}
}
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Scanner Class Ex. 3
Example 3 Program “CalculateSumMultiple”
Input tokens on same line with white space as delimiter.
import java.util.Scanner;
public class CalculateSumMultiple {
public static void main(String[] args) {
double firstNum, secondNum, sum;
Scanner scan = new Scanner(System.in);
System.out.println("Enter first and second number to add:");
firstNum = scan.nextDouble();
secondNum = scan.nextDouble();
sum = firstNum + secondNum;
System.out.println(firstNum + " + " + secondNum + " = " + sum );
}
}
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Scanner Class Ex. 4
import java.util.Scanner;
public class Average2Nums {
Designates the
public static void main(String[] args) {
keyboard
int firstNum, secondNum;
double average;
Scanner scan = new Scanner(System.in);
System.out.println("Enter first number to average:");
firstNum = scan.nextInt();
System.out.println("Enter second number to average:");
secondNum = scan.nextInt();
average = (double) (firstNum + secondNum) / 2;
System.out.println("Average of " + firstNum + " and " +
secondNum + " is " + average );
}
Casts the sum to double before dividing by 2
}
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Scanner Class Ex. 5
import java.util.Scanner;
public class ConvertSeconds {
public static void main(String[] args) {
Why?
int totalSeconds, hours, minutes, seconds;
Because
Scanner scan = new Scanner(System.in); hours & seconds
are integer!
totalSeconds = scan.nextInt();
seconds = totalSeconds;
hours = seconds / 3600; // Integer division
seconds = seconds % 3600; //modulo
minutes = seconds / 60;
seconds = seconds % 60;
System.out.println(totalSeconds + " sec is " + hours + " hours, "
+
minutes + " minutes, and " + seconds + " seconds");
}
}
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