visual basic barcode printing 7: Objects and Collections in Java

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7: Objects and Collections
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Implementing an equals() Method
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So let s say you decide to override equals() in your class. It might look something like this:
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public class EqualsTest { public static void main (String [] args) { Moof one = new Moof(8); Moof two = new Moof(8); if (one.equals(two)) { System.out.println("one and two are equal"); } } } class Moof { private int moofValue; Moof(int val) { moofValue = val; } public int getMoofValue() { return moofValue; } public boolean equals(Object o) { if ((o instanceof Moof) && (((Moof)o).getMoofValue() == this.moofValue)) { return true; } else { return false; } } }
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Let s look at this code in detail. In the main method of EqualsTest, we create two Moof instances, passing the same value (8) to the Moof constructor. Now look at the Moof class and let s see what it does with that constructor argument it assigns the value to the moofValue instance variable. Now imagine that you ve decided two Moof objects are the same if their moofValue is identical. So you override the equals() method and compare the two moofValues. It is that simple. But let s break down what s happening in the equals() method:
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1. public boolean equals(Object o) { 2. if ((o instanceof Moof) && (((Moof)o).getMoofValue() == this.moofValue)) { 3. return true;
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Overriding hashCode() and equals() (Exam Objective 9.2)
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} else { return false; } }
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First of all, you must observe all the rules of overriding, and in line 1 we are indeed declaring a valid override of the equals() method we inherited from Object. Line 2 is where all the action is. Logically, we have to do two things in order to make a valid equality comparison:
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1. Be sure that the object being tested is of the correct type! It comes in polymorphically as type Object, so you need to do an instanceof test on it. Having two objects of different class types be considered equal is usually not a good idea, but that s a design issue we won t go into here. Besides, you d still have to do the instanceof test just to be sure that you could cast the object argument to the correct type so that you can access its methods or variables in order to actually do the comparison. Remember, if the object doesn t pass the instanceof test, then you ll get a runtime ClassCastException if you try to do, for example, this:
public boolean equals(Object o) { if (((Moof)o).getMoofValue() == this.moofValue){ // the preceding line compiles, but it's BAD! return true; } else { return false; } }
The (Moof)o cast will fail if o doesn t refer to something that IS-A Moof.
2. Compare the attributes we care about (in this case, just moofValue). Only the developers can decide what makes two instances equal. (For performance you re going to want to check the fewest number of attributes.)
By the way, in case you were a little surprised by the whole ((Moof)o).getMoofValue() syntax, we re simply casting the object reference, o, just-in-time as we try to call a method that s in the Moof class but not in Object. Remember without the cast, you can t compile because the compiler would see the object referenced by o as simply, well, an Object. And since the Object class doesn t have a moofvalue() method, the compiler would squawk (technical
7: Objects and Collections
term). But then as we said earlier, even with the cast the code fails at runtime if the object referenced by o isn t something that s castable to a Moof. So don t ever forget to use the instanceof test first. Here s another reason to appreciate the short circuit && operator if the instanceof test fails, we ll never get to the code that does the cast, so we re always safe at runtime with the following:
if ((o instanceof Moof) && (((Moof)o).getMoofValue() == this.moofValue)) { return true; } else { return false; }
Remember that the equals(), hashCode(), and toString() methods are all public. The following would not be a valid override of the equals() method, although it might appear to be if you don t look closely enough during the exam:
class Foo { boolean equals(Object o) { } } }
And watch out for the argument types as well. The following method is an overload, but not an override of the equals() method:
class Boo { public boolean equals(Boo b) { } }
Be sure you re very comfortable with the rules of overriding so that you can identify whether a method from Object is being overridden, overloaded, or illegally redeclared in a class. The equals() method in class Boo changes the argument from Object to Boo, so it becomes an overloaded method and won t be called unless it s from your own code that knows about this new, different method that happens to also be named equals.
So that takes care of equals(). Whoa not so fast. If you look at the Object class in the Java API documentation, you ll find what we call a contract specified in the equals() method. A Java contract is a set of rules that should be followed, or rather must be followed if you want to provide a correct implementation as others will expect it to be. Or to put it another way, if you
Overriding hashCode() and equals() (Exam Objective 9.2)
don t follow the contract, you may still compile and run, but your code (or someone else s) may break at runtime in some unexpected way.
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