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But what if you use an Animal reference to a Horse object
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Animal animalRefToHorse = new Horse(); ua.doStuff(animalRefToHorse);
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Which of the overloaded versions is invoked You might want to say, The one that takes a Horse, since it s a Horse object at runtime that s being passed to the method. But that s not how it works. The preceding code would actually print
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in the Animal version
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Even though the actual object at runtime is a Horse and not an Animal, the choice of which overloaded method to call is not dynamically decided at runtime. Just remember, the reference type (not the object type) determines which overloaded method is invoked! To summarize, which overridden method to call (in other words, from which class in the inheritance tree) is decided at runtime based on object type, but which overloaded version of the method to call is based on the reference type passed at compile time. Polymorphism in Overloaded and Overridden Methods How does polymorphism work with overloaded methods From what we just looked at, it doesn t appear that polymorphism matters when a method is overloaded. If you pass an Animal reference, the overloaded method that takes an Animal will be invoked, even if the actual object passed is a Horse. Once the Horse masquerading as Animal gets in to the method, however, the Horse object is still a Horse despite being passed into a method expecting an Animal. So it s true that polymorphism doesn t determine which overloaded version is called; polymorphism does come into play when the decision is about which overridden version of a method is called. But sometimes, a method is both overloaded and overridden. Imagine the Animal and Horse classes look like this:
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public class Animal { public void eat() { System.out.println("Generic Animal Eating Generically");
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5: Object Orientation, Overloading and Overriding, Constructors, and Return Types
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} } public class Horse extends Animal { public void eat() { System.out.println("Horse eating hay "); } public void eat(String s) { System.out.println("Horse eating " + s); } }
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Notice that the Horse class has both overloaded and overridden the eat() method. Table 5-2 shows which version of the three eat() methods will run depending on how they are invoked.
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TABLE 5-2
Overloaded and Overridden Method Invocations
Method Invocation Code Animal a = new Animal(); a.eat(); Horse h = new Horse(); h.eat(); Animal ah = new Horse(); ah.eat();
Result Generic Animal Eating Generically Horse eating hay Horse eating hay Polymorphism works the actual object type (Horse), not the reference type (Animal), is used to determine which eat() is called. Horse eating Apples The overloaded eat(String s) method is invoked. Compiler error! Compiler sees that Animal class doesn t have an eat() method that takes a String. Compiler error! Compiler still looks only at the reference type, and sees that Animal doesn t have an eat() method that takes a string. Compiler doesn t care that the actual object might be a Horse at runtime.
Horse he = new Horse(); he.eat("Apples"); Animal a2 = new Animal(); a2.eat("treats"); Animal ah2 = new Horse(); ah2.eat("Carrots");
Overridden and Overloaded Methods (Exam Objective 6.2)
Don t be fooled by a method that s overloaded but not overridden by a subclass. It s perfectly legal to do the following:
public class Foo { void doStuff() { } } class Bar extends Foo { void doStuff(String s) { } }
The Bar class has two doStuff() methods: the no-arg version it inherits from Foo (and does not override), and the overloaded doStuff(String s) defined in the Bar class. Code with a reference to a Foo can invoke only the no-arg version, but code with a reference to a Bar can invoke either of the overloaded versions.
Table 5-3 summarizes the difference between overloaded and overridden methods.
TABLE 5-3
Difference Between Overloaded and Overridden Methods
Overloaded Method argument list return type exceptions Must change Can change Can change
Overridden Method Must not change Must not change Can reduce or eliminate. Must not throw new or broader checked exceptions Must not make more restrictive (can be less restrictive) Object type (in other words, the type of the actual instance on the heap) determines which method is selected. Happens at runtime.
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