Post on 19-Dec-2015
Fall 2007 CS 225 2
Chapter Topics
• Inheritance and how it facilitates code reuse• Polymorphism• Abstract classes• The Object class and its methods• Method overriding• Cloning and the difference between a true
clone (deep copy) and a shallow copy
Fall 2007 CS 225 3
Chapter Topics (continued)
• Single versus multiple inheritance
• Interfaces and delegation to simulate multiple inheritance
• Object factory design pattern
• Packages and visibility
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Inheritance and Class Hierarchies
• One advantage of OOP is that it enables programmers to reuse previously written code saved as classes
• All Java classes are arranged in a hierarchy, starting with Object, which is the superclass of all Java classes
• Inheritance in OOP is analogous to inheritance in humans
• Inheritance and hierarchical organization allow you to capture the idea that one thing may be a refinement or extension of another
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Is-a Versus Has-a Relationships
• One misuse of inheritance is confusing the has-a relationship with the is-a relationship
• The has-a relationship means that one class has the second class as an attribute
• We can combine is-a and has-a relationships• The keyword extends specifies that one class
is a subclass of another
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A Superclass and a Subclass
• Consider two classes: Computer and Laptop
• A laptop is a kind of computer and is therefore a subclass of computer
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Where would you add
• Cost
• Battery
• Time to discharge
• Number of expansion slots
• Wireless card present
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Initializing Superclass Data
• Private data fields belonging to a base class must be initialized by invoking the base class’s constructor with the appropriate parameters
• If the execution of any constructor in a subclass does not invoke a superclass constructor, Java automatically invokes the no-parameter constructor for the superclass– Initializes that part of the object inherited from the
superclass before the subclass starts to initialize its part of the object
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Protected Visibility
• Private data fields are not accessible to derived classes
• Protected visibility allows data fields to be accessed either by the class defining it or any subclass
• In general, it is better to use private visibility because subclasses may be written by different programmers and it is always good practice to restrict and control access to the superclass data fields
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Method Overriding
• If a derived class has a method found within its base class, that method will override the base class’s method
• The keyword super can be used to gain access to superclass methods overridden by the base class
• A subclass method must have the same return type as the corresponding superclass method
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Method Overloading
• Method overloading: having multiple methods with the same name but different signatures in a class
• Constructors are often overloaded
• Example:– MyClass(int inputA, int inputB)– MyClass(int inputA, int inputB, double
inputC)
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Polymorphism
• A variable of a superclass type can reference an object of a subclass type
• Polymorphism means many forms or many shapes
• Polymorphism allows the JVM to determine which method to invoke at run time
• At compile time, the Java compiler can’t determine what type of object a superclass may reference but it is known at run time
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Example
Computer [] comp = new Computer[3];comp[0] = new Computer( …);comp[1] = new LapTop( …);comp[2] = new Computer( …)for (int i=0; i<comp.length; i++) System.out.println(
comp[I].getRamSize() + "\n" + comp[I].toString();
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Interfaces
• An interface can declare methods but does not provide an implementation of those methods– Methods declared in an interface are called
abstract methods
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Abstract Classes
• An abstract class can have abstract methods, data fields, and concrete methods
• Abstract class differs from a concrete class in that– it cannot be instantiated– it can declare abstract methods, which must be
implemented in its subclasses
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Abstract Classes and Interfaces
• Like an interface, an abstract class can’t be instantiated
• An abstract class can have constructors to initialize its data fields when a new subclass is created– Subclass uses super(…) to call the constructor
• May implement an interface but it doesn’t have to define all of the methods declared in the interface– Implementation is left to its subclasses
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Class Object
• Object is the root of the class hierarchy
• All classes inherit the methods defined in class Object but may be overridden
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The Method toString
• You should always override the toString method if you want to represent an object’s state
• If you do not override it, the toString method for class Object will return a string…just not the string you want or are expecting
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Operations Determined by Type of Reference Variable
• A variable can reference an object whose type is a subclass of the variable type– The type of reference, not the type of the object
referenced, determines what operations can be performed
• Java is strongly typed so the compiler needs to be able to verify that the type of the expression being assigned is compatible with the variable type
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Casting in a Class Hierarchy
• Java provides casting to enable us to process an object referenced by one type through a reference variable of its actual type
• Casting does not change the object referenced; it creates an anonymous reference to that object
• Downcast: cast a higher type to a lower type• The instanceof operator can guard against
ClassCastException errors• You can downcast an interface reference to the
specific implementation type
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Java 5.0 Reduces Need for Casting
• Two new features that reduce the need for casting:– Autoboxing/unboxing– Generics
• Autoboxing/unboxing eases the conversion between a primitive type and its corresponding wrapper type
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The Method Object.equals
• The Object.equals method has a parameter of type Object
• Compares two objects to determine whether they are equal
• You must override the equals method if you want to be able to compare two objects of a class
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Cloning
• The purpose of cloning in object-oriented programming is analogous to cloning in biology– Create an independent copy of an object
• Initially, both objects will store the same information• You can change one object without affecting the other
• Will cause both e1.name and e2.name to reference “Jim”
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Cloning
• The purpose of cloning in object-oriented programming is analogous to cloning in biology– Create an independent copy of an object
• Initially, both objects will store the same information• You can change one object without affecting the other
Will cause both e1.name and e2.name to reference “Jim”
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• The statement e1.setAddressLine1("Room 224"); creates a new String object that is referenced by e1.address.line1 and e2.address.line1
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The Object.clone method
• Java provides the Object.clone method to help solve the shallow copy problem
• The initial copy is a shallow copy as the current object’s data fields are copied
• To make a deep copy, you must create cloned copies of all components by invoking their respective clone methods
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Cloning
• After e1.setAddressLine1("Room 224"); only e1.address.line1 references the new String object.
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Employee.clone()
public Object clone() { try { Employee cloned = (Employee) super.clone(); cloned.address = (Address)address.clone(); return cloned; } catch (CloneNotSupportedException ex){ throw new InternalError(); }
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Address.clone()
public Object clone() { try { Address cloned = (Address) super.clone(); return cloned; } catch (CloneNotSupportedException ex){
throw new InternalError(); }
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Multiple Inheritance
• Multiple inheritance: the ability to extend more than one class
• Multiple inheritance is a language feature that is difficult to implement and can lead to ambiguity– Therefore, Java does not allow a class to
extend more than one class
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Reuse Through Delegation• Delegation is used to reduce
– duplication of modifications • problems associated with version control
– A method of one class accomplishes an operation by delegating it to a method of another class
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Packages
• The Java API is organized into packages• The package to which a class belongs is declared
by the first statement in the file in which the class is defined using the keyword package followed by the package name
• All classes in the same package are stored in the same directory or folder
• All the classes in one folder must declare themselves to be in the same package
• Classes that are not part of a package may access only public members of classes in the package
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Package Visibility
• There exists a default package– Files that do specify a package are considered part
of the default package
• If you don’t declare packages, all of your packages belong to the same, default package
• Package visibility sits between private and protected
• Classes, data fields, and methods with package visibility are accessible to all other methods of the same package but are not accessible to methods outside of the package
• Classes, data fields, and methods that are declared protected are visible to all members of the package
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Visibility Supports Encapsulation
• The rules for visibility control how encapsulation occurs in a Java program
• Private visibility is for members of a class that should not be accessible to anyone but the class, not even the classes that extend it
• Package visibility allows the developer of a library to shield classes and class members from classes outside the package
• Use of protected visibility allows the package developer to give control to other programmers who want to extend classes in the package
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Case Study: Problem Statement
• We want to draw some standard geometric shapes on the screen– rectangle, circle, right triangle
• Each shape has properties– position on the screen– interior color– border color
• Methods for– area, perimeter
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Object Factories
• An object factory is a method that creates instances of other classes
• Object factories are useful when:– The necessary parameters are not known
or must be derived via computation– The appropriate implementation of an
interface or abstract class should be selected as the result of some computation