Notes

Outline

Chapter 5:  Enhancing Classes Presentation slides for Java Software Solutions Foundations of Program Design Third Edition by John Lewis and William Loftus Java Software Solutions is published by Addison-Wesley Presentation slides are copyright 2002 by John Lewis and William Loftus. All rights reserved. Instructors using the textbook may use and modify these slides for pedagogical purposes.

Enhancing Classes Now we can explore various aspects of classes and objects in more detail Chapter 5 focuses on: object references and aliases passing objects references as parameters the static modifier wrapper classes nested classes and inner classes interfaces dialog boxes GUI components, events, and listeners

References Recall from Chapter 2 that an object reference variable holds the memory address of an object Rather than dealing with arbitrary addresses, we often depict a reference graphically as a “pointer” to an object      ChessPiece bishop1 = new ChessPiece();

The null Reference An object reference variable that does not currently point to an object is called a null reference An attempt to follow a null reference causes a NullPointerException to be thrown For example String name; declares an object reference variable, but does not create a String object for it to refer to. Therefore, the variable name contains a null reference

The this Reference The this reference allows an object to refer to itself. Inside a method, the this reference can be used to refer to the currently executing object For example, if(this.position == piece2.position) result = false; clarifies which position is being referenced The this reference refers to the object through which the method containing the code was invoked

The this reference The this reference can also distinguish the parameters of a constructor from the corresponding instance variables with the same names

Assignment Revisited The act of assignment takes a copy of a value and stores it in a variable For primitive types: num2 = num1;

Reference Assignment For object references, assignment copies the memory location: bishop2 = bishop1;

Aliases Two or more references that refer to the same object are called aliases of each other One object (and its data) can be accessed using different reference variables Aliases can be useful, but should be managed carefully Changing the object’s state (its variables) through one reference changes it for all of its aliases

Testing Objects for Equality The == operator compares object references for equality, returning true if the references are aliases of each other A method called equals is defined for all objects, but unless we redefine it when we write a class, it has the same semantics as the == operator bishop1.equals(bishop2); returns true if both references refer to the same object We can redefine the equals method to return true under whatever conditions we think are appropriate

Garbage Collection When an object no longer has any valid references to it, it can no longer be accessed by the program It is useless, and is called garbage Java performs automatic garbage collection periodically, returning an object's memory to the system for future use

Passing Objects as Parameters Parameters in a Java method are passed by value This means that a copy of the actual parameter (the value passed in) is stored into the formal parameter (in the method header) Passing parameters is essentially like an assignment statement When an object is passed to a method, the actual parameter and the formal parameter become aliases of each other

Passing Objects to Methods What you do using a parameter inside a method may or may not have a permanent effect (outside the method) See ParameterPassing.java (page xxx) See ParameterTester.java (page xxx) See Num.java (page xxx) Note the difference between changing the reference and changing the object that the reference points to

ParameterPassing.java

ParameterTester.java

Num.java

The static Modifier In Chapter 2 we discussed static methods (also called class methods) that can be invoked through the class name rather than through a particular object For example, the methods of the Math class are static To make a method static, we apply the static modifier to the method definition The static modifier can be applied to variables as well It associates a variable or method with the class rather than with an object

Static Variables Static variables sometimes are called class variables Normally, each object has its own data space If a variable is declared as static, only one copy of the variable exists           private static float price; Memory space for a static variable is created when the class in which it is declared is loaded

Static Variables All objects created from the class share access to the static variable Changing the value of a static variable in one object changes it for all others

Static Methods

Static Methods The order of the modifiers can be interchanged, but by convention visibility modifiers come first Recall that the main method is static;  it is invoked by the system without creating an object Static methods cannot reference instance variables, because instance variables don't exist until an object exists However, they can reference static variables or local variables

Static Methods and Static Variables Static methods and static variables often work together See CountInstances.java (page xxx) See Slogan.java (page xxx)

CountInstances.java

Slogan.java

Wrapper Classes A wrapper class represents a particular primitive type For example Integer ageObj = new Integer (20); uses the Integer class to create an object which effectively represents the integer 20 as an object This is useful when a program requires an object instead of a primitive type

Wrapper classes There is a wrapper class in the Java.lang package for each primitive type (figure 5.4 here)

Some Methods of the Integer class (figure 5.5 here) The Java wrapper classes often contain useful static constants For example, the Integer class contains MIN_VALUE and MAX_VALUE which hold the smallest and largest int values

Keyboard Input Revisited The Keyboard class was introduced in Chapter 2 to facilitate input from the keyboard Input can be read from the keyboard without using the Keyboard class See Wages2.java (page xxx)

Wages2.java

Java I/O Java I/O is accomplished using objects that represent streams of data A stream is an ordered sequence of bytes The System.out object represents a standard output stream, which defaults to the monitor screen Reading keyboard input is more complicated

Reading Keyboard Input First, we must establish an input stream The System.in object is used to create an InputStreamReader object The InputStreamReader object is used to create a BufferedReader object This creates an input stream that treats input as characters and buffers them so that input can be read a line at a time The readLine method of the BufferedReader class reads an entire line of input as a String the parseInt method of the Integer wrapper class and the parseDouble method of the Double class convert the input string

Reading Keyboard Input Problems that arise in reading or converting a value manifest themselves as exceptions The main method in Wages2 may throw an IOException, as acknowledged in the throws clause in the method header The Keyboard class hides these aspects of keyboard input I/O and exceptions are explored further in Chapter 8

Nested Classes In addition to containing data and methods, a class can contain other classes A class declared within another class is called a nested class

Nested Classes A nested class has access to the variables and methods of the enclosing class, even if they are declared private In certain situations this makes the implementation of the classes easier because they can share information easily Furthermore, the nested class can be protected by the enclosing class from external use This is a special relationship and should be used with care

Nested Classes A nested class produces a separate bytecode file If a nested class called Inside is declared in an outer class called Outside, two bytecode files will be produced: Outside.class Outside$Inside.class Nested classes can be declared as static, in which case they cannot refer to instance variables or methods

Inner Classes A nonstatic nested class is called an inner class An inner class is associated with each instance of the enclosing class An instance of an inner class can exist only within an instance of an enclosing class See TestInner.java (page xxx) See Outer.java (page xxx)

TestInner.java

Outer.java

Interfaces

Interfaces A Java interface is a collection of abstract methods and constants An abstract method is a method header without a method body An abstract method can be declared using the modifier abstract, but because all methods in an interface are abstract, usually it is left off An interface is used to define a set of methods formally that a class will implement See Complexity.java (page xxx)

Complexity.java

Interfaces

Interfaces An interface cannot be instantiated Methods in an interface have public visibility by default A class formally implements an interface by stating so in the class header providing implementations for each abstract method in the interface If a class asserts that it implements an interface, it must define all methods in the interface or the compiler will produce errors. See Question.java (page xxx)

Question.java

Interfaces A class that implements an interface can implement other methods as well A class can implement multiple interfaces class ManyThings implements interface 1, inerface2, interface3 { // all methods of all interfaces } The interfaces are listed in the implements clause, separated by commas The class must implement all methods in all interfaces listed in the header

Interfaces See MiniQuiz.java (page xxx)

Interfaces (Figure 5.6 here)

Interfaces In addition to, or instead of abstract methods, an interface can contain constants When a class implements an interface, it gains access to all its constants

Interfaces The Java standard class library contains many helpful interfaces The Comparable interface contains an abstract method called compareTo, which is used to compare two objects The String class implements Comparable which gives us the ability to put strings in alphabetical order The Iterator interface contains methods that allow the user to move easily through a collection of objects

The Comparable Interface The Comparable interface provides a common mechanism for comparing one object to another if (obj1.compareTo(obj2) < 0) System.out.println (“obj1 is less than obj2”); The result is negative is obj1 is less that obj2, 0 if they are equal, and positive if obj1 is greater than obj2

The Iterator Interface The Iterator interface provides a means of moving through a collection of objects, one at a time The hasNext method returns a boolean result (true if there are items left to process) The next method returns an object The remove method removes the object most recently returned by the next method

Dialog Boxes A dialog box is a graphical window that pops up on top of any currently active window for the user The Swing package contains a class called JOptionPane that simplifies the creation and use of basic dialog boxes (Figure 5.7 here)

Dialog Boxes There are three categories of JOptionPane dialog boxes A message dialog displays an output string An input dialog presents a prompt and a single input text field A confirm dialog presents the user with a simple “yes-or-no” question See EvenOdd.java (page xxx)

EvenOdd.java

Graphical User Interfaces A Graphical User Interface (GUI) is created with at least three kinds of objects components events listeners A GUI component defines a screen element to display information or to allow the user to interact with the program push buttons text fields labels etc.

Containers A container is a special component that holds and organizes other components a dialog box an applet

Events An event is an object that represents some activity to which we may want to respond For example, we may want our program to perform some action when the following occurs: the mouse is moved a mouse button is clicked the mouse is dragged a graphical button is clicked a keyboard key is pressed a timer expires Events often correspond to user actions, but not always

Events and Listeners The Java standard class library contains several classes that represent typical events Certain objects, such as an applet or a graphical button, generate (fire) an event when it occurs Other objects, called listeners, wait for events to occur We can write listener objects to do whatever we want when an event occurs

Events and Listeners

Listener Interfaces We can create a listener object by writing a class that implements a particular listener interface The Java standard class library contains several interfaces that correspond to particular event categories For example, the MouseListener interface contains methods that correspond to mouse events After creating the listener, we add the listener to the component that might generate the event to set up a formal relationship between the generator and listener

Creating GUIs To create a program with a GUI define and set up the components create listener objects create the relationships between the listeners and the components which generate events of interest define what happens in response to each event of interst See PushCounter.java (page xxx)

PushCounter.java

Creating GUIs A push button is a component that allows the user to initiate an action with the press of the mouse button A label is a component that displays a line of text The init method of an applet sets up the GUI and adds each component to the applet container (the content pane) The content pane is retrieved using the getContentPane method A JButton generates an action event

Creating GUIs A listener object commonly is created by defining a class that implements a listener interface For example, the interface for an action event is called ActionListener, which defines only the actionPerformed method The ButtonListener class implements the ActionListener interface When the button is pushed, the JButton object invokes the actionPerformed method, generating an ActionEvent

GUI Applications A frame is a container component used for stand-alone GUI-based applications A panel is a container, but, unlike a frame, it cannot be displayed on its own it must be added to another container it helps organize the components in a GUI See Fahrenheit.java (page xxx) See FahrenheitGUI.java (page xxx)

Summary Chapter 5 has focused on: object references and aliases passing objects references as parameters the static modifier wrapper classes nested classes and inner classes interfaces dialog boxes GUI components, events, and listeners