Using Predefined Classes

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Strings
and
Pre-Defined Classes
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Using Predefined Classes

• Static variables and methods
• Fundamentals of Strings
• Browsing class packages

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Static Modifiers

• The keyword static associates a variable or
  method with a
• class, rather than with an object of a class.
• A static variable/method is not referenced
  through a particular
• instance of a class, but through the class
  itself.
• A static variable is also known as a class
  variable. Other variables are known as instance
  variables.
• A static method is also known as a class
  method. Other
• methods are known as instance methods.
• A class method cannot reference instance
  variables, which only
• exist in an instance of a class.

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Static Modifiers
public class MyMath public static final
double PI 3.14 private int maxint
public static double square(double n)
return( n n ) // end
square public void setmaxint(int i)
maxint i // end setnextint // end
MyMath
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import javax.swing. public class AreaTest
public static void main(String args)
String input double area,radius
input JOptionPane.showInputDialog("What is
radius") radius Double.parseDouble(input)
area MyMath.PI MyMath.square(radius)
JOptionPane.showMessageDialog(null,"The
area is " area " square in.","Area",
JOptionPane.PLAIN_MESSAGE)
System.exit(0) // end main // end
AreaTest

• Variable PI and method square are referenced
  through the Class.

Run Rrogram
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Java Strings

• A string is a series of characters treated as
• a single unit
• Examples hello Florida
• 245-90
• A string in Java is an object of class String.
• String FirstName
• String School, j

8
Java Strings

• There are two ways to create a String object.

- Instantiation (use one of the
constructors) String LastName new
String(Allen)
- Dynamically String FirstName Barney

• The characters in the String are number 0
  length - 1.

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STRING METHODS
public char charAt( int index) - returns the
character located at position index. - Example
FAMU The character at position 0 is
F
the character at position 3 is U
public String concat( String str) - returns a
string that is str added to the object string -
Example Jack be concatentated with
nimble gives Jack be
nimble
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public class Vertical public static void
main(String a ) int j String
name new String(JANE) char ch
for(j0 j lt name.length( ) j)
ch name.charAt( j )
System.out.println( ch ) // end for
// end main // end Vertical
Run Program
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• Create a String that has some text in it.
• Display the first and last characters in the
  String.

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STRING METHODS
public boolean endsWith(String suf) - returns
true if the object string has suf as a suffix -
Example er is a suffix of rattler
ttl is not a suffix of
rattler
public boolean startsWith(String pre) - returns
true if the object string has pre as a prefix -
Example ratt is a prefix of rattler
le is not a prefix of
rattler
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import javax.swing. public class CheckWord
public static void main(String a )
String N1, N2 String input
N1 JOptionPane.showInputDialog("Enter a
word ") if( N1.endsWith("er") )
JOptionPane.showMessageDialog(null,"This
word ends with er","CheckWord",
JOptionPane.PLAIN_MESSAGE)
else JOptionPane.showMes
sageDialog(null,"This word does not end with
er","CheckWord", JOptionPane.PLAIN_MESSA
GE) System.exit(0) // end
main // end CheckWord
Run Program
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• The String class has a set of static methods
  (overloaded) for converting other data types to
  strings

public static String valueOf(int i) - returns
integer i as a String - Example 3467
valueOf gives 3467

public static String valueOf(char c) - returns
c as a String - Example c valueOf gives
c
16

• More valueOf methods...

public static String valueOf(double d) -
returns double d as a String - Example
3467.98 valueOf gives 3467.98

public static String valueOf(boolean b) -
returns boolean value b as a String - Example
true valueOf gives true

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• Define double and assign it to a value.
• Determine if the last digit in the number is a 3.

18

• Define an integer and assign a value to it.
• Convert the integer to a String.

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Browsing the JAVA API - to look for classes - to
look for methods - to just look
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