Chapter 3
Java Programming With Supplied
Classes
Chapter 3 - Java Programming With Supplied Classes
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Topics
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Packages and classes supplied w/ JDK
The String class and its methods
Declaring and accessing a String array
The Vector, Calendar, and Date classes
Data wrapper classes
Writing and executing an applet
Controlling font and color
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Using the Packages and Classes
Supplied with Java
• Java Development Kit (JDK)
– JDK 1.4 Consists of 135 packages, 2,991 pre-defined
classes and interfaces ~ JDK 1.3: 76 and 1,842
– Package
– Group of related classes (class library)
– Keywords
• import
– Gives the compiler access to the classes
• package
– Assign classes to a particular package
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Using the String Class
• String Class
– Member of java.lang package
• Automatically imported by Java compiler
– Two ways to instantiate a String object
• Similar to primitives:
– String a = Hello World”;
• Use the keyword new:
– String s = new String(“Hello World”);
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Using the String Class
• Method types
– Instance methods (nonstatic)
• Associated with a specific instance of the class
• Use reference variable to invoke
• e.g., s.length()
– Class methods (static)
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Not associated with any instance
Use class name to invoke
Contain keyword static in their headers
e.g., Student.getTotalNumberOfStudents()
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Using the String Class
• NullPointerException
– Results from attempting to invoke an instance method
using a reference variable that hasn’t been initialized
• Immutable
– Refers to the fact that Java Strings cannot be
changed
• Methods that “change” a string value actually return a new
String instance
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Creating a String Array
• Array declaration
– String stringArray[] = new String[4];
• Creates array of 4 String objects
• Array elements are String objects
• Array element instantiation
– stringArray[0] = new String(“Hello”);
• Must be performed for each element
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Creating a String Array
• Comparing String objects
– Reference variables contain references, not
data values
– Cannot use comparison operator (==)
– Must use String methods:
• equals
• equalsIgnoreCase
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Using the Vector Class
• Vector Class
– Contained in java.util package
• must be imported
– Array that is dynamically resizable
• Can contain different class data types
• Cannot contain primitive data types (need wrapper)
– Declaration
• Vector v = new Vector(3);
– Creates vector with three elements
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Working With Dates
• Classes for Working with Date Values
– In java.util package:
• Calendar Class
– Contains methods and constants
• Date Class
– An instance contains the actual date value
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Working With Dates
• Classes for Working with Date Values
– In java.text package:
• DateFormat Class
– An instance provides several data formats for display
purposes
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Using Wrapper Classes
• Wrapper Classes
– Contains primitive data inside an object
instance
– Reside in java.lang package
– Named same as primitive counterpart with the
first letter capitalized
• e.g., double  Double, float  Float, etc.
• Except for Integer: int  Integer
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Using Wrapper Classes
• Converting Primitive to Wrapper and Back
– Primitive to wrapper:
• Instantiate the appropriate wrapper class using the
primitive variable as the argument
Double d;
Double doubleWrapper = new Double( d );
– Wrapper to primitive:
• Use instance method named xxxValue (where xxx
is the primitive data type)
e.g., d = doubleWrapper.doubleValue( );
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Using Wrapper Classes
• Converting String to Primitive and Back
– String to primitive :
• Use instance method named parsexxx (where xxx
is the primitive data type)
– String s1 =new String(“2.2”);
– doublePrimitive = s.parseDouble(s1);
– Wrapper to primitive:
• Use instance method named toString that creates
a String instance containing the primitive value
– String s2 = Double.toString(doublePrimitive);
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Using Wrapper Classes
• Converting String to Wrapper and Back
– String to wrapper:
• Use static wrapper method named valueOf that
creates a wrapper instance from String instance
– doubleWrapper = Double.valueof(s1);
– Wrapper to String:
• Use wrapper method named toString that creates
a String instance from the wrapper instance
– s1 = doubleWrapper.toString( );
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Using the Applet Class
• Writing a Simple Applet
– Import:
• Graphics Class from java.awt package
• Applet Class from java.applet package
– Subclass of Panel:
» GUI window without a title bar
– Executed in a browser window
• Requires HTML file
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Using the Applet Class
• Controlling Color and Font
– Font Class from java.awt package
• Specifies font name, style, and size
– Color Class from java.awt package
• Specifies colors
– Wild card character
• Used with import statements to specify multiple classes
• import java.awt.*
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