7: OBJECT-ORIENTED PROGRAMMING in Java

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CHAPTER 7: OBJECT-ORIENTED PROGRAMMING
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Covariant return in Overriding Methods
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In Example 72, the definition of the method makeInstance() at (8) overrides the method definition at (3) Note that the method signatures are the same, but the return type at (8) is a subtype of the return type at (3) The method at (8) returns an object of the subtype TubeLight, whereas the method at (3) returns an object of the supertype Light This is an example of covariant return Depending on whether we call the method makeInstance() on an object of the subtype TubeLight or that of the supertype Light, the respective method definition will be executed The code at (17) and (18) illustrates what object is returned by the method, depending on which method definition is executed Note that covariant return only applies to reference types, not to primitive types For example, changing the return type of the getBill() method at (6) to float, will result in a compile-time error There is no subtype relationship between primitive types
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Example 72
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Overriding, Overloading, and Hiding
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//Exceptions class InvalidHoursException extends Exception {} class NegativeHoursException extends InvalidHoursException {} class ZeroHoursException extends InvalidHoursException {} class Light { protected String billType = "Small bill"; // (1) Instance field
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protected double getBill(int noOfHours) throws InvalidHoursException { // (2) Instance method if (noOfHours < 0) throw new NegativeHoursException(); double smallAmount = 100, smallBill = smallAmount * noOfHours; Systemoutprintln(billType + ": " + smallBill); return smallBill; } public Light makeInstance() { return new Light(); } public static void printBillType() { Systemoutprintln("Small bill"); } // (3) Instance method
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// (4) Static method
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} //______________________________________________________________________________ class TubeLight extends Light { public static String billType = "Large bill"; @Override public double getBill(final int noOfHours) // (5) Hiding field at (1)
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72: OVERRIDING METHODS
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throws ZeroHoursException { // (6) Overriding instance method at (2) if (noOfHours == 0) throw new ZeroHoursException(); double largeAmount = 1000, largeBill = largeAmount * noOfHours; Systemoutprintln(billType + ": " + largeBill); return largeBill; } public double getBill() { Systemoutprintln("No bill"); return 00; } @Override public TubeLight makeInstance() { return new TubeLight(); } // (7) Overloading method at (6)
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// (8) Overriding instance method at (3)
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public static void printBillType() { // (9) Hiding static method at (4) Systemoutprintln(billType); } } //______________________________________________________________________________ public class Client { public static void main(String[] args) throws InvalidHoursException { // (10) TubeLight tubeLight = new TubeLight(); Light light1 = tubeLight; Light light2 = new Light(); // (11) // (12) Aliases // (13)
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Systemoutprintln("Invoke overridden instance method:"); tubeLightgetBill(5); // (14) Invokes method at (6) light1getBill(5); // (15) Invokes method at (6) light2getBill(5); // (16) Invokes method at (2) Systemoutprintln( "Invoke overridden instance method with covariant return:"); Systemoutprintln( light2makeInstance()getClass()); // (17) Invokes method at (3) Systemoutprintln( tubeLightmakeInstance()getClass()); // (18) Invokes method at (8) Systemoutprintln("Access hidden field:"); Systemoutprintln(tubeLightbillType); // (19) Accesses field at (5) Systemoutprintln(light1billType); // (20) Accesses field at (1) Systemoutprintln(light2billType); // (21) Accesses field at (1) Systemoutprintln("Invoke hidden static method:"); tubeLightprintBillType(); // (22) Invokes method at (9) light1printBillType(); // (23) Invokes method at (4) light2printBillType(); // (24) Invokes method at (4) Systemoutprintln("Invoke overloaded method:"); tubeLightgetBill(); // (25) Invokes method at (7) } }
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CHAPTER 7: OBJECT-ORIENTED PROGRAMMING
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Output from the program:
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Invoke overridden instance method: Large bill: 5000 Large bill: 5000 Small bill: 500 Invoke overridden instance method with covariant return: class Light class TubeLight Access hidden field: Large bill Small bill Small bill Invoke hidden static method: Large bill Small bill Small bill Invoke overloaded method: No bill
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Here are a few more facts to note about overriding A subclass must use the keyword super in order to invoke an overridden method in the superclass (see p 295) An instance method in a subclass cannot override a static method in the superclass The compiler will flag this as an error A static method is class-specific and not part of any object, while overriding methods are invoked on behalf of objects of the subclass However, a static method in a subclass can hide a static method in the superclass (see below) A final method cannot be overridden, because the modifier final prevents method overriding An attempt to override a final method will result in a compile-time error An abstract method, on the other hand, requires the non-abstract subclasses to override the method, in order to provide an implementation The accessibility modifier private for a method means that the method is not accessible outside the class in which it is defined; therefore, a subclass cannot override it However, a subclass can give its own definition of such a method, which may have the same signature as the method in its superclass
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