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Chapter 3 - Java Programming With Supplied Classes2 Topics 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

Chapter 3 - Java Programming With Supplied Classes3 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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5 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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7 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) Not associated with any instance Use class name to invoke Contain keyword static in their headers e.g., Student.getTotalNumberOfStudents()

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9 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

Chapter 3 - Java Programming With Supplied Classes10 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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Chapter 3 - Java Programming With Supplied Classes12 Creating a String Array Comparing String objects –Reference variables contain references, not data values –Cannot use comparison operator (==) –Must use String methods: equals equalsIgnoreCase

Chapter 3 - Java Programming With Supplied Classes13 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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Chapter 3 - Java Programming With Supplied Classes15 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

Chapter 3 - Java Programming With Supplied Classes16 Working With Dates Classes for Working with Date Values –In java.text package: DateFormat Class –An instance provides several data formats for display purposes

Chapter 3 - Java Programming With Supplied Classes17 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 Using Wrapper Classes

Chapter 3 - Java Programming With Supplied Classes18 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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Chapter 3 - Java Programming With Supplied Classes20 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);

Chapter 3 - Java Programming With Supplied Classes21 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( );

Chapter 3 - Java Programming With Supplied Classes22 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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Chapter 3 - Java Programming With Supplied Classes25 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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