visual basic barcode printing Methods of Class Object Covered on the Exam in Java

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Methods of Class Object Covered on the Exam
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Method boolean equals(Object obj) void finalize() int hashCode()
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Description Decides whether two objects are meaningfully equivalent. Called by the garbage collector when the garbage collector sees that the object cannot be referenced. Returns a hashcode int value for an object, so that the object can be used in Collection classes that use hashing, including Hashtable, HashMap, and HashSet. Wakes up a thread that is waiting for this object s lock. Wakes up all threads that are waiting for this object s lock. Causes the current thread to wait until another thread calls notify or notifyAll on this object. Returns a text representation of the object.
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final void notify() final void notifyAll() final void wait() String toString()
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Overriding hashCode() and equals() (Exam Objective 9.2)
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The toString() Method Override toString() when you want a mere mortal to be able to read something meaningful about the objects of your class. Code can call toString() on your object when it wants to read useful details about your object. For example, when you pass an object reference to the System.out.println() method, the object s toString() method is called, and the return of toString() is what you see displayed as follows:
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public class HardToRead { public static void main (String [] args) { HardToRead h = new HardToRead(); System.out.println(h); } }
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Running the HardToRead class gives us the lovely and meaningful,
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% java HardToRead HardToRead@a47e0
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The preceding output is what you get when you don t override the toString() method of class Object. It gives you the class name (at least that s meaningful) followed by the @ symbol, followed by the unsigned hexadecimal representation of the object s hashcode. Seeing this perhaps motivates you to override the toString() method in your classes, for example,
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public class BobTest { public static void main (String[] args) { Bob f = new Bob("GoBobGo", 19); System.out.println(f); } } class Bob { int shoeSize; String nickName; Bob(String nickName, int shoeSize) { this.shoeSize = shoeSize; this.nickName = nickName; } public String toString() { return ("I am a Bob, but you can call me " + nickName + ". My shoe size is " + shoeSize); } }
7: Objects and Collections
This ought to be a bit more readable:
% java BobTest I am a Bob, but you can call me GoBobGo. My shoe size is 19
Some people affectionately refer to toString() as the spill-your-guts method, because the most common implementations of toString() simply spit out the object s state (in other words, the current values of the important instance variables). So that s it for toString(). Now we ll tackle equals() and hashCode().
Overriding equals()
You learned about the equals() method in 6, where we looked at the wrapper classes. We discussed how comparing two object references using the == operator evaluates true only when both references refer to the same object (because == simply looks at the bits in the variable, and they re either identical or they re not). You saw that the String class and the wrapper classes have overridden the equals() method (inherited from class Object), so that you could compare two different objects (of the same type) to see if their contents are meaningfully equivalent. If two different Integer instances both hold the int value 5, as far as you re concerned they are equal. The fact that the value 5 lives in two separate objects doesn t matter. When you really need to know if two references are identical, use ==. But when you need to know if the objects themselves (not the references) are equal, use the equals() method. For each class you write, you must decide if it makes sense to consider two different instances equal. For some classes, you might decide that two objects can never be equal. For example, imagine a class Car that has instance variables for things like make, model, year, configuration you certainly don t want your car suddenly to be treated as the very same car as someone with a car that has identical attributes. Your car is your car and you don t want your neighbor Billy driving off in it just because, hey, it s really the same car; the equals() method said so. So no two cars should ever be considered exactly equal. If two references refer to one car, then you know that both are talking about one car, not two cars that have the same attributes. So in the case of a Car you might not ever need, or want, to override the equals() method. Of course, you know that can t be the end of the story.
Overriding hashCode() and equals() (Exam Objective 9.2)
What It Means if You Don t Override equals()
There s a potential limitation lurking here: if you don t override the equals() method, you won t be able to use the object as a key in a hashtable. The equals() method in Object uses only the == operator for comparisons, so unless you override equals(), two objects are considered equal only if the two references refer to the same object. Let s look at what it means to not be able to use an object as a hashtable key. Imagine you have a car, a very specific car (say, John s red Subaru Outback as opposed to Moe and Mary s purple Mini) that you want to put in a HashMap (a type of hashtable we ll look at later in this chapter), so that you can search on a particular car and retrieve the corresponding Person object that represents the owner. So you add the car instance as the key to the HashMap (along with a corresponding Person object as the value). But now what happens when you want to do a search You want to say to the HashMap collection, Here s the car, now give me the Person object that goes with this car. But now you re in trouble unless you still have a reference to the exact object you used as the key when you added it to the Collection. In other words, you can t make an identical Car object and use it for the search. The bottom line is this: if you want objects of your class to be used as keys for a hashtable (or as elements in any data structure that uses equivalency for searching for and/or retrieving an object), then you must override equals() so that two different instances can be considered the same. So how would we fix the car You might override the equals() method so that it compares the unique VIN (Vehicle Identification Number) as the basis of comparison. That way, you can use one instance when you add it to a Collection, and essentially re-create an identical instance when you want to do a search based on that object as the key. Of course, overriding the equals() method for Car also allows the potential that more than one object representing a single unique car can exist, which might not be safe in your design. Fortunately, the String and wrapper classes work well as keys in hashtables they override the equals() method. So rather than using the actual car instance as the key into the car/owner pair, you could simply use a String that represents the unique identifier for the car. That way, you ll never have more than one instance representing a specific car, but you can still use the car or rather, one of the car s attributes as the search key.
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