Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Chapter 11 Abstract Classes and Interfaces (continued)

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved The ActionListener Interfaces HandleEvent Run

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Handling GUI Events Source object (e.g., button) Listener object contains a method for processing the event.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Trace Execution public class HandleEvent extends JFrame { public HandleEvent() { … OKListenerClass listener1 = new OKListenerClass(); jbtOK.addActionListener(listener1); … } public static void main(String[] args) { … } class OKListenerClass implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println("OK button clicked"); } 1. Start from the main method to create a window and display it animation

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Trace Execution public class HandleEvent extends JFrame { public HandleEvent() { … OKListenerClass listener1 = new OKListenerClass(); jbtOK.addActionListener(listener1); … } public static void main(String[] args) { … } class OKListenerClass implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println("OK button clicked"); } animation 2. Click OK

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Trace Execution public class HandleEvent extends JFrame { public HandleEvent() { … OKListenerClass listener1 = new OKListenerClass(); jbtOK.addActionListener(listener1); … } public static void main(String[] args) { … } class OKListenerClass implements ActionListener { public void actionPerformed(ActionEvent e) { System.out.println("OK button clicked"); } animation 3. Click OK. The JVM invokes the listener’s actionPerformed method

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved The Cloneable Interfaces package java.lang; public interface Cloneable { } Marker Interface: An empty interface. A marker interface does not contain constants or methods. It is used to denote that a class possesses certain desirable properties. A class that implements the Cloneable interface is marked cloneable, and its objects can be cloned using the clone() method defined in the Object class.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Examples Many classes (e.g., Date and Calendar) in the Java library implement Cloneable. Thus, the instances of these classes can be cloned. For example, the following code Calendar calendar = new GregorianCalendar(2003, 2, 1); Calendar calendarCopy = (Calendar)calendar.clone(); System.out.println("calendar == calendarCopy is " + (calendar == calendarCopy)); System.out.println("calendar.equals(calendarCopy) is " + calendar.equals(calendarCopy)); displays calendar == calendarCopy is false calendar.equals(calendarCopy) is true

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Implementing Cloneable Interface To declare a custom class that implements the Cloneable interface, the class must override the clone() method in the Object class. The following code declares a class named House that implements Cloneable and Comparable. House

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Shallow vs. Deep Copy House house1 = new House(1, ); House house2 = (House)house1.clone();

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Interfaces vs. Abstract Classes In an interface, the data must be constants; an abstract class can have all types of data. Each method in an interface has only a signature without implementation; an abstract class can have concrete methods. VariablesConstructorsMethods Abstract class No restrictionsConstructors are invoked by subclasses through constructor chaining. An abstract class cannot be instantiated using the new operator. No restrictions. InterfaceAll variables must be public static final No constructors. An interface cannot be instantiated using the new operator. All methods must be public abstract instance methods

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Interfaces vs. Abstract Classes, cont. Suppose that c is an instance of Class2. c is also an instance of Object, Class1, Interface1, Interface1_1, Interface1_2, Interface2_1, and Interface2_2. All classes share a single root, the Object class, but there is no single root for interfaces. Like a class, an interface also defines a type. A variable of an interface type can reference any instance of the class that implements the interface. If a class extends an interface, this interface plays the same role as a superclass. You can use an interface as a data type and cast a variable of an interface type to its subclass, and vice versa.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Caution: conflict interfaces In rare occasions, a class may implement two interfaces with conflict information (e.g., two same constants with different values or two methods with same signature but different return type). This type of errors will be detected by the compiler.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Whether to use an interface or a class? Abstract classes and interfaces can both be used to model common features. How do you decide whether to use an interface or a class? In general, a strong is-a relationship that clearly describes a parent- child relationship should be modeled using classes. For example, a staff member is a person. So their relationship should be modeled using class inheritance. A weak is-a relationship, also known as an is-kind-of relationship, indicates that an object possesses a certain property. A weak is-a relationship can be modeled using interfaces. For example, all strings are comparable, so the String class implements the Comparable interface. You can also use interfaces to circumvent single inheritance restriction if multiple inheritance is desired. In the case of multiple inheritance, you have to design one as a superclass, and others as interface. See Chapter 10, “Object- Oriented Modeling,” for more discussions.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Wrapper Classes F Boolean F Character F Short F Byte F Integer F Long F Float F Double NOTE: (1) The wrapper classes do not have no-arg constructors. (2) The instances of all wrapper classes are immutable, i.e., their internal values cannot be changed once the objects are created.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved The toString, equals, and hashCode Methods Each wrapper class overrides the toString, equals, and hashCode methods defined in the Object class. Since all the numeric wrapper classes and the Character class implement the Comparable interface, the compareTo method is implemented in these classes.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved The Number Class Each numeric wrapper class extends the abstract Number class, which contains the methods doubleValue, floatValue, intValue, longValue, shortValue, and byteValue. These methods “convert” objects into primitive type values. The methods doubleValue, floatValue, intValue, longValue are abstract. The methods byteValue and shortValue are not abstract, which simply return (byte)intValue() and (short)intValue(), respectively.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved The Integer and Double Classes

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved The Integer Class and the Double Class F Constructors  Class Constants MAX_VALUE, MIN_VALUE F Conversion Methods

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Numeric Wrapper Class Constructors You can construct a wrapper object either from a primitive data type value or from a string representing the numeric value. The constructors for Integer and Double are: public Integer(int value) public Integer(String s) public Double(double value) public Double(String s)

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Numeric Wrapper Class Constants Each numerical wrapper class has the constants MAX_VALUE and MIN_VALUE. MAX_VALUE represents the maximum value of the corresponding primitive data type. For Byte, Short, Integer, and Long, MIN_VALUE represents the minimum byte, short, int, and long values. For Float and Double, MIN_VALUE represents the minimum positive float and double values. The following statements display the maximum integer (2,147,483,647), the minimum positive float (1.4E-45), and the maximum double floating-point number ( e+308d).

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Conversion Methods Each numeric wrapper class implements the abstract methods doubleValue, floatValue, intValue, longValue, and shortValue, which are defined in the Number class. These methods “convert” objects into primitive type values.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved The Static valueOf Methods The numeric wrapper classes have a useful class method, valueOf(String s). This method creates a new object initialized to the value represented by the specified string. For example: Double doubleObject = Double.valueOf("12.4"); Integer integerObject = Integer.valueOf("12");

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved The Methods for Parsing Strings into Numbers You have used the parseInt method in the Integer class to parse a numeric string into an int value and the parseDouble method in the Double class to parse a numeric string into a double value. Each numeric wrapper class has two overloaded parsing methods to parse a numeric string into an appropriate numeric value.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved NOTE Arrays are objects. An array is an instance of the Object class. Furthermore, if A is a subclass of B, every instance of A[] is an instance of B[]. Therefore, the following statements are all true: new int[10] instanceof Object new GregorianCalendar[10] instanceof Calendar[]; new Calendar[10] instanceof Object[] new Calendar[10] instanceof Object

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved CAUTION Although an int value can be assigned to a double type variable, int[] and double[] are two incompatible types. Therefore, you cannot assign an int[] array to a variable of double[] or Object[] type.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Sorting an Array of Objects Objective: The example presents a generic method for sorting an array of objects. The objects are instances of the Comparable interface and they are compared using the compareTo method. GenericSort Run

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved TIP Java provides a static sort method for sorting an array of Object in the java.util.Arrays class. So you can use the following code to sort arrays in this example: java.util.Arrays.sort(intArray); java.util.Arrays.sort(doubleArray); java.util.Arrays.sort(charArray); java.util.Arrays.sort(stringArray);

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Automatic Conversion Between Primitive Types and Wrapper Class Types JDK 1.5 allows primitive type and wrapper classes to be converted automatically. For example, the following statement in (a) can be simplified as in (b): Integer[] intArray = {1, 2, 3}; System.out.println(intArray[0] + intArray[1] + intArray[2]); Unboxing

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved BigInteger and BigDecimal If you need to compute with very large integers or high precision floating-point values, you can use the BigInteger and BigDecimal classes in the java.math package. Both are immutable. Both extend the Number class and implement the Comparable interface.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved BigInteger and BigDecimal BigInteger a = new BigInteger(" "); BigInteger b = new BigInteger("2"); BigInteger c = a.multiply(b); // * 2 System.out.println(c); BigDecimal a = new BigDecimal(1.0); BigDecimal b = new BigDecimal(3); BigDecimal c = a.divide(b, 20, BigDecimal.ROUND_UP); System.out.println(c); LargeFactorial Run

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Chapter 12 Object-Oriented Design and Patterns

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Motivations The preceding chapters introduced objects, classes, class inheritance, and interfaces. You learned the concepts of object-oriented programming. This chapter focuses on the analysis and design of software systems using the object-oriented approach. You will learn class-design guidelines, and the techniques and patterns for designing reusable classes.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Software Development Process

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Requirement Specification A formal process that seeks to understand the problem and document in detail what the software system needs to do. This phase involves close interaction between users and designers. Most of the examples in this book are simple, and their requirements are clearly stated. In the real world, however, problems are not well defined. You need to study a problem carefully to identify its requirements.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved System Analysis Seeks to analyze the business process in terms of data flow, and to identify the system’s input and output. Part of the analysis entails modeling the system’s behavior. The model is intended to capture the essential elements of the system and to define services to the system.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved System Design The process of designing the system’s components. This phase involves the use of many levels of abstraction to decompose the problem into manageable components, identify classes and interfaces, and establish relationships among the classes and interfaces.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Implementation The process of translating the system design into programs. Separate programs are written for each component and put to work together. This phase requires the use of a programming language like Java. The implementation involves coding, testing, and debugging.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Testing Ensures that the code meets the requirements specification and weeds out bugs. An independent team of software engineers not involved in the design and implementation of the project usually conducts such testing.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Deployment Deployment makes the project available for use. For a Java applet, this means installing it on a Web server; for a Java application, installing it on the client's computer.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Maintenance Maintenance is concerned with changing and improving the product. A software product must continue to perform and improve in a changing environment. This requires periodic upgrades of the product to fix newly discovered bugs and incorporate changes.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Discovering Classes One popular way for facilitating the discovery process is by creating CRC cards. CRC stands for classes, responsibilities, and collaborators. Use an index card for each class, as shown in Figure 12.2.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Discovering Class Relationships F Association F Aggregation and Composition F Dependency F Inheritance

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Association Association represents a general binary relationship that describes an activity between two classes. An association is usually represented as a data field in the class.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Translation is not Unique NOTE: If you don’t need to know the courses a student takes or a faculty teaches, the data field coureList in Student or Faculty can be omitted.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Association Between Same Class Association may exist between objects of the same class. For example, a person may have a supervisor.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Aggregation and Composition Aggregation is a special form of association, which represents an ownership relationship between two classes. Aggregation models the has-a relationship. If an object is exclusively owned by an aggregated object, the relationship between the object and its aggregated object is referred to as composition.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Dependency A dependency describes a relationship between two classes where one (called client) uses the other (called supplier). In UML, draw a dashed line with an arrow from the client class to the supplier class. For example, the ArrayList class uses Object because you can add objects to an ArrayList. The relationship between ArrayList and Object can be described using dependency, as shown in Figure 12.7(a). The relationship between Calendar and Date can be described using dependency, as shown in Figure 12.7(b).

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Dependency vs. Association Both association and dependency describe one class as depending on another. Association is stronger than dependency. In association, the state of the object changes when its associated object changes. In dependency, the client object and the supplier object are loosely coupled. The association relationship is implemented using data fields and methods. There is a strong connection between two classes. The dependency relationship is implemented using methods.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Coupling Dependency, association, aggregation, and composition all describe coupling relationships between two classes. The difference is the degree of coupling with composition being the strongest, followed by aggregation, association, and dependency in this order.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Representing Aggregation in Classes An aggregation relationship is usually represented as a data field in the aggregated class.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Inner Classes Translation If Name or Address is used in the Person class only, they can be declared as an inner class in Person. For example, public class Person { private Name name; private Address address;... class Name {... } class Address {... }

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Inheritance Inheritance models the is-an-extension-of relationship between two classes.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Weak Inheritance Relationship A weak is-an-extension-of relationship can be represented using interfaces. For example, the weak is-an-extension-of relationship “students are comparable based on their grades” can be represented by implementing the Comparable interface, as follows:

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Class Design 1. Identify classes for the system. 2. Describe attributes and methods in each class. 3. Establish relationships among classes. 4. Create classes.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Borrowing Loans Name BorrowerPersonLoanAddress

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Borrowing Loans, cont. The following is a test program that uses the classes Name, Person, Address, Borrower, and Loan. BorrowLoan Run

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved The Rational Class RationalRunTestRationalClass

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Class Design Guidelines F Designing a Single Class. F Using Modifiers public, protected, private and static F Using Inheritance or Aggregation F Using Interfaces or Abstract Classes

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Designing a Class F A class should describe a single entity or a set of similar operations. A single entity with too many responsibilities can be broken into several classes to separate responsibilities. The String class, StringBuffer class, and StringTokenizer class all deal with strings, for example, but have different responsibilities.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Designing a Class, cont. F Classes are usually designed for use by many different customers. To make a class useful in a wide range of applications, the class should provide a variety of ways for customization through properties and methods.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Designing a Class, cont. F Classes are designed for reuse. Users can incorporate classes in many different combinations, orders, and environments. Therefore, you should design a class that imposes no restrictions on what or when the user can do with it, design the properties to ensure that the user can set properties in any order, with any combination of values, and design methods to function independently of their order of occurrence.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Designing a Class, cont. F Provide a public no-arg constructor and override the equals method and the toString method defined in the Object class whenever possible.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Designing a Class, cont. F Follow standard Java programming style and naming conventions. Choose informative names for classes, data fields, and methods. Always place the data declaration before the constructor, and place constructors before methods. Always provide a constructor and initialize variables to avoid programming errors.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Using Visibility Modifiers F Each class can present two contracts – one for the users of the class and one for the extenders of the class. Make the fields private and accessor methods public if they are intended for the users of the class. Make the fields or method protected if they are intended for extenders of the class. The contract for the extenders encompasses the contract for the users. The extended class may increase the visibility of an instance method from protected to public, or change its implementation, but you should never change the implementation in a way that violates that contract.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Using Visibility Modifiers, cont. F A class should use the private modifier to hide its data from direct access by clients. You can use get methods and set methods to provide users with access to the private data, but only to private data you want the user to see or to modify. A class should also hide methods not intended for client use. The gcd method in the Rational class in Example 11.2, “The Rational Class,” is private, for example, because it is only for internal use within the class.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Using the static Modifier F A property that is shared by all the instances of the class should be declared as a static property.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Using Inheritance or Aggregation In general, the difference between inheritance and aggregation is the difference between the is-an-extension-of relationship and the has-a relationship. For example, an apple is fruit; thus, you would use inheritance to model the relationship between the classes Apple and Fruit. A person has a name; thus, you would use aggregation to model the relationship between the classes Person and Name.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Using Inheritance or Aggregation, cont. Sometimes, the choice between inheritance and aggregation is not obvious. For example, you have used inheritance to model the relationship between the classes Circle and Cylinder. One could argue that a cylinder consists of circles; thus, you might use aggregation to define the Cylinder class as follows:

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Using Inheritance or Composition, cont. public class Cylinder { private Circle circle; /** Constructors */ /** Methods */ }

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Using Inheritance or Aggregation, cont. Both designs are fine. Which one is preferred? If polymorphism is desirable, you need to use the inheritance design. If you don’t care about polymorphism, the aggregation design gives more flexibility because the classes are less dependent using aggregation than using inheritance.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Using Interfaces or Abstract Classes Both interfaces and abstract classes can be used to generalize common features. How do you decide whether to use an interface or a class? In general, a strong is-an-extension-of relationship that clearly describes a parent- child relationship should be modeled using classes.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Using Interfaces or Abstract Classes, cont. For example, since an orange is a fruit, their relationship should be modeled using class inheritance. A weak is-an- extension-of relationship, also known as an is-kind-of relationship, indicates that an object possesses a certain property. A weak is-an-extension-of relationship can be modeled using interfaces. For example, all strings are comparable, so the String class implements the Comparable interface. A circle or a rectangle is a geometric object, for example, so Circle can be designed as a subclass of GeometricObject. Circles are different and comparable based on their radius, for example, so Circle can implement the Comparable interface.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Using Interfaces or Abstract Classes, cont. Interfaces are more flexible than abstract classes, because a subclass can extend only one superclass, but implement any number of interfaces. However, interfaces cannot contain concrete methods. You can combine the virtues of interfaces and abstract classes by creating an interface with a companion abstract class that implements the interface. So you can use the interface or its companion class whichever is more convenient.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved The Java API The Java API (Application Program Interface, Application Programming Interface, or Application Programmer interface) consists of numerous classes and interfaces grouped into more than a dozen of packages. You have used classes and interfaces in the java.lang, javax.swing, and java.util packages.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Framework-Based Programming To create comprehensive projects, you have to use more classes and interfaces in the Java API. The classes and interfaces in the Java API establish a framework for programmers to develop applications using Java. For example, the classes and interfaces in the Java GUI API establish a framework for developing GUI programs. You have to use these classes and interfaces and follow their conventions and rules to create applications. This is referred to as framework-based programming.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Framework-Based Programming, cont. Once you understand the concept of Java and object-orient programming, the most important lesson from now on is learning how to use the API to develop useful programs. The most effective way to achieve it is to imitate good examples. The book provides many carefully designed examples to demonstrate the concept of the framework-based programming using the Java API.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Framework-Based Programming, cont.

Liang, Introduction to Java Programming, Seventh Edition, (c) 2009 Pearson Education, Inc. All rights reserved Design Patterns One important benefit of object-oriented programming is to reuse code. For example, to create a button, you simply use the JButton class to create an instance of JButton. The JButton class is already defined in the Java API so you can use it without having to reinvent the wheel. Design patterns are proven sound software strategies for designing classes. Applying design patterns is like reusing experience. You can apply successful patterns to develop new software without reinventing new solution strategies.