visual basic barcode program Comparison of inheritance vs. dot operator for member access in Java

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Comparison of inheritance vs. dot operator for member access
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2: Declarations and Access Control
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You need to know the effect of different combinations of class and member access (such as a default class with a public variable). To figure this out, first look at the access level of the class. If the class itself will not be visible to another class, then none of the members will be either, even if the member is declared public. Once you ve confirmed that the class is visible, then it makes sense to look at access levels on individual members.
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Public Members When a method or variable member is declared public, it means all other classes, regardless of the package they belong to, can access the member (assuming the class itself is visible). Look at the following source file:
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package book; import cert.*; // Import all classes in the cert package class Goo { public static void main(String [] args) { Sludge o = new Sludge(); o.testIt(); } }
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Now look at the second file:
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package cert; public class Sludge { public void testIt() { System.out.println("sludge"); } }
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As you can see, Goo and Sludge are in different packages. However, Goo can invoke the method in Sludge without problems because both the Sludge class and its testIt() method are marked public. For a subclass, if a member of its superclass is declared public, the subclass inherits that member regardless of whether both classes are in the same package. Read the following code:
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package cert; public class Roo { public String doRooThings() { // imagine the fun code that goes here } }
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Declarations and Modifiers (Exam Objective 1.2)
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The Roo class declares the doRooThings() member as public. So if we make a subclass of Roo, any code in that Roo subclass can call its own inherited doRooThings() method.
package notcert; //Not the package Roo is in import cert.Roo; class Cloo extends Roo { public void testCloo() { System.out.println(doRooThings()); } }
Notice in the preceding code that the doRooThings() method is invoked without having to preface it with a reference. Remember, if you see a method invoked (or a variable accessed) without the dot operator (.), it means the method or variable belongs to the class where you see that code. It also means that the method or variable is implicitly being accessed using the this reference. So in the preceding code, the call to doRooThings() in the Cloo class could also have been written as this.doRooThings(). The reference this always refers to the currently executing object in other words, the object running the code where you see the this reference. Because the this reference is implicit, you don t need to preface your member access code with it, but it won t hurt. Some programmers include it to make the code easier to read for new (or non) java programmers. Besides being able to invoke the doRooThings() method on itself, code from some other class can call doRooThings() on a Cloo instance, as in the following:
class Toon { public static void main (String [] args) { Cloo c = new Cloo(); System.out.println(c.doRooThings()); //No problem; method is public } }
Private Members Members marked private can t be accessed by code in any class other than the class in which the private member was declared. Let s make a small change to the Roo class from an earlier example.
package cert; public class Roo { private String doRooThings() { // imagine the fun code that goes here, but only the Roo class knows } }
2: Declarations and Access Control
The doRooThings() method is now private, so no other class can use it. If we try to invoke the method from any other class, we ll run into trouble.
package notcert; import cert.Roo; class UseARoo { public void testIt() { Roo r = new Roo(); //So far so good; class Roo is still public System.out.println(r.doRooThings()); //Compiler error! } }
If we try to compile the UseARoo class, we get the following compiler error:
%javac Balloon.java Balloon.java:5: No method matching doRooThings() found in class cert.Roo. r.doRooThings(); 1 error
It s as if the method doRooThings() doesn t exist, and as far as any code outside of the Roo class is concerned, it s true. A private member is invisible to any code outside the member s own class. What about a subclass that tries to inherit a private member of its superclass When a member is declared private, a subclass can t inherit it. For the exam, you need to recognize that a subclass can t see, use, or even think about the private members of its superclass. You can, however, declare a matching method in the subclass. But regardless of how it looks, it is not an overriding method! It is simply a method that happens to have the same name as a private method (which you re not supposed to know about) in the superclass. The rules of overriding do not apply, so you can make this newly-declared-but-just-happens-to-match method declare new exceptions, or change the return type, or anything else you want to do with it.
package cert; public class Roo { private String doRooThings() { // imagine the fun code that goes here, but no other class will know } }
The doRooThings() method is now off limits to all subclasses, even those in the same package as the superclass.
Declarations and Modifiers (Exam Objective 1.2)
package cert; //Cloo and Roo are in the same package class Cloo extends Roo { //Still OK, superclass Roo is public public void testCloo() { System.out.println(doRooThings()); //Compiler error! } }
If we try to compile the subclass Cloo, the compiler is delighted to spit out the following error:
%javac Cloo.java Cloo.java:4: Undefined method: doRooThings() System.out.println(doRooThings()); 1 error
Although you re allowed to mark instance variables as public, in practice it s nearly always best to keep all variables private or protected. If variables need to be changed, set, or read, programmers should use public accessor methods, so that code in any other class has to ask to get or set a variable (by going through a method), rather than access it directly. Accessor methods should usually take the form get<propertyName> and set<propertyName>, and provide a place to check and/or validate before returning or modifying a value. Without this protection, the weight variable of a Cat object, for example, could be set to a negative number if the offending code goes straight to the public variable as in someCat.weight = -20. But an accessor method, setWeight(int wt), could check for an inappropriate number. (OK, wild speculation, but we re guessing a negative weight might be inappropriate for a cat. And no wisecracks from you cat haters.) 5 will discuss this data protection (encapsulation) in more detail.
Can a private method be overridden by a subclass That s an interesting question, but the answer is technically no. Since the subclass, as we ve seen, cannot inherit a private method, it therefore cannot override the method overriding depends on inheritance. We ll cover the implications of this in more detail a little later in this section as well as in 5, but for now just remember that a method marked private cannot be overridden. Figure 2-2 illustrates the effects of the public and private access modifiers on classes from the same or different packages. Protected and Default Members The protected and default access control levels are almost identical, but with one critical difference. A default member may
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