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Chapter 3 Java Programming With Supplied Classes. 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.
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Chapter 3Java Programming With Supplied Classes Chapter 3 - Java Programming With Supplied Classes
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 Classes
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 Chapter 3 - Java Programming With Supplied Classes
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”); Chapter 3 - Java Programming With Supplied Classes
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() Chapter 3 - Java Programming With Supplied Classes
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 Classes
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 Chapter 3 - Java Programming With Supplied Classes
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 Classes
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 Chapter 3 - Java Programming With Supplied Classes
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 Classes
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 Classes
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 Chapter 3 - Java Programming With Supplied Classes
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( ); Chapter 3 - Java Programming With Supplied Classes
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 Classes
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 Classes
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 Chapter 3 - Java Programming With Supplied Classes
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.* Chapter 3 - Java Programming With Supplied Classes
