visual basic barcode printing Overloaded Constructors in Java

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Overloaded Constructors
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Overloading a constructor means typing in multiple versions of the constructor, each having a different argument lists, like the following examples:
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class Foo { Foo() { } Foo(String s) { } }
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The preceding Foo class has two overloaded constructors, one that takes a string, and one with no arguments. Because there s no code in the no-arg version, it s actually identical to the default constructor the compiler supplies, but remember since there s already a constructor in this class (the one that takes a string), the compiler won t supply a default constructor. If you want a no-arg constructor to overload the with-args version you already have, you re going to have to type it yourself, just as in the Foo example. Overloading a constructor is used typically to provide alternate ways for clients to instantiate objects of your class. For example, if a client knows the animal name, they can pass that to an Animal constructor that takes a string. But if they don t know the name, the client can call the no-arg constructor and that constructor can supply a default name. Here s what it looks like:
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1. public class Animal { 2. String name; 3. Animal(String name) { 4. this.name = name; 5. } 6. 7. Animal() { 8. this(makeRandomName()); 9. } 10. 11. static String makeRandomName() {
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5: Object Orientation, Overloading and Overriding, Constructors, and Return Types
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14. 15. } 16. 17. public static void main (String [] args) { 18. Animal a = new Animal(); 19. System.out.println(a.name); 20. Animal b = new Animal("Zeus"); 21. System.out.println(b.name); 22. } 23. }
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int x = (int) (Math.random() * 5); String name = new String[] {"Fluffy", "Fido", "Rover", "Spike", "Gigi"}[x]; return name;
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Running this code four times produces the output:
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% java Animal Gigi Zeus % java Animal Fluffy Zeus % java Animal Rover Zeus % java Animal Fluffy Zeus
There s a lot going on in the preceding code. Figure 5-6 shows the call stack for constructor invocations when a constructor is overloaded. Take a look at the call stack, and then let s walk through the code straight from the top.
Line 2
Declare a String instance variable name.
Lines 3 5 Line 7
Constructor that takes a String, and assigns it to instance variable name. Here s where it gets fun. Assume every animal needs a name, but the client (calling code) might not always know what the name should be, so you ll assign a random name. The no-arg constructor generates a name by invoking the makeRandomName() method.
Constructors and Instantiation (Exam Objectives 1.3, 6.3, 6.2)
FIGURE 5-6
Overloaded constructors on the call stack
Line 8
The no-arg constructor invokes its own overloaded constructor that takes a string, in effect calling it the same way it would be called if client code were doing a new to instantiate an object, passing it a string for the name. The overloaded invocation uses the keyword this, but uses it as though it were a method name, this(). So line 8 is simply calling the constructor on line 3, passing it a randomly selected string rather than a client-code chosen name. Notice that the makeRandomName() method is marked static! That s because you cannot invoke an instance (in other words, nonstatic) method (or access an instance variable) until after the super constructor has run. And since the super constructor will be invoked from the constructor on line 3, rather than from the one on line 7, line 8 can use only a static method to generate the name. If we wanted all animals not specifically named by the caller to have the same default name, say, Fred, then line 8 could have read
Line 11
this("Fred");
rather than calling a method that returns a string with the randomly chosen name.
Line 12 Line 13
Line 12 doesn t have anything to do with constructors, but since we re all here to learn it generates a random number between 0 and 5. Weird syntax, we know. We re creating a new String object (just a single String instance), but we want the string to be selected randomly from a list. Except we don t have the list, so we need to make it. So in that one line of code we
1. Declare a String variable, name. 2. Create a String array (anonymously we don t assign the array itself to anything). 3. Retrieve the string at index [x] (x being the random number generated on line 12) of the newly created String array.
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