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Advanced Java Class Design

1. 2.1 Develop code that uses abstract classes and methods

Hello everyone. In this lesson, we will go over the first item of the Advanced Java Class Design topic in the Java SE Eight Programmer 2 exam syllabus, which is developing code that uses Abstract Classes and Methods. Let's start with abstract classes. Abstract classes are classes declared with the Abstract keyword. These classes may or may not include abstract methods. Here is an example of abstract classes. This class is marked "Abstract" using the Abstract keyword. Abstract classes cannot be instantiated, but they can be extended. Notice that when a subclass extends an abstract class, all abstract methods of the superclass must be overridden by concrete methods of the subclass, or the subclass itself is also declared as abstract. Let's move on to abstract methods. Abstract methods are methods declared with the abstract keyword. These methods must not include an implementation. This is an example of abstract methods. The method is marked "Abstract" using this abstract keyword. This method declaration ends with a semicolon as a normal Java statement. Abstract methods can only be declared within abstract classes and interfaces. Any other use of abstract methods is invalid. Now it's time for some practise questions to go over what we have learned. Here is the first question. As always, feel free to pause the video if you need more time. It is absolutely valid for an abstract class to contain non-abstract methods. Therefore, Option A is incorrect. Although an abstract class cannot be instantiated, it always has at least a constructor, which is the same as a normal concrete class. Such constructors are called by subclass constructors when instances of these subclasses need to be created. So option B is also incorrect. Since both options A and B are incorrect, the correct answer is C. The declaration is valid. Now for the second question. An abstract method must be declared in either an abstract class or an interface, which is not the case over here. Therefore, the correct answer is no. Now for the third question. When a subclass extends an abstract superclass, it must provide an implementation for all abstract methods, or the subclass itself is also abstract. In this scenario, the subclass does not define a concrete method to override Method A, an abstract method of the superclass, nor is it declared as abstract. As a result, the declaration of the subclass is invalid and option B is the correct answer. Now for the fourth question. The declaration of an abstract method is like a normal Java statement; it must always end with a semicolon. Therefore, Option A is incorrect. Except for static and default methods, all methods defined in an interface are implicitly abstract. Therefore, adding the abstract keyword to such methods is unnecessary. So options B and C are both incorrect. So the correct answer is D. The given declaration is already valid. If you want to explicitly mark a method abstract, you may add the abstract keyword to any place prior to the MethodsReturn type, which is void in this case. Therefore, this declaration has the same meaning as before. You can move the Abstract Keyword here like this. However, declaring any keywords in front of an access modifier is unusual. In this lesson, we have gone over abstract classes and abstract methods. In essence, abstract classes are those declared with the Abstract keyword. Abstract classes may or may not include abstract methods. Abstract methods are methods declared with the abstract keyword and include no implementation. Abstract methods can only be defined within abstract classes and interfaces.

2. 2.2 Develop code that uses the final keyword

Hello, everybody. In this lesson, we will go through the second item of the Advanced Java Class Design Topic. The syllabus for the Java SE 8 programmer two exam includes developing code that uses the final keyword. Let's look at the final variables first. When the final keyword is declared on a variable, it indicates that the variable cannot be changed once such a variable is initialised with a value. If a final variable is a static field, it must be initialised during declaration or inside a static initialization block. If a final variable is an instancefield, it must be initialised during declaration, inside an instance initialization block, or in all constructors of the containing class. So you can see that with final fields, whether they are static or instance, you cannot rely on the compiler to assign them a default value, as with nonfinal fields. Now we'll see how final instancefields can be initialised in action. Final static fields can be set in a similar way, except that initialization blocks are static and constructors are not involved. Let's get started with a primitive instance field. It is simple if the field is declared and initialised at the same time, like what is shown over here. However, if you declare this instancevariable without initialising it, you will end up with a compilation error. This issue can be resolved by assigning the final field a value in an initialization block. As you can see, the error has gone and the field declaration is valid again. You can also initialise the instance field in a constructor. Doing it this way has the same effect as using an initialization block, since the compiler copies such a block to all constructors during compilation. If you have more than a function Object() { [native code] } and do not initialise the final field in all of them, an error also occurs. This issue can be fixed by assigning a value to the field in the additional constructor. The behaviour of final primitive variables and final object-reference variables is a bit different due to the way that their values are stored in memory. In the primitive case, a variable stores the value itself, and a final variable does not allow such a value to be changed. On the other hand, an object-reference variable keeps the address of the object it is referencing. A final variable of this type does not permit the address to be modified, but the internal state of the reference object is not subject to such a constraint. When the final keyword is declared on a method, it prevents such a method from being overridden or hidden in subclasses of the containing class. This technique is normally used when the implementation of a method is critical and should never be changed. For example, methods that are called by constructors of subclasses should be final to avoid unexpected behaviour when those methods are accidentally overridden or hidden. When the final keyword is declared on a class, it keeps the class from being extended. The most common use case of final classes is to create immutable objects. Now it's time for some practise questions to go over what we have learned. Here is the first question. As always, feel free to pause the video if you need more time. A final variable is not required to be initialised in declaration. Instead, it just needs to be assigned a value before being used, and of course, it must not be reassigned. This is true for both local and class-level variables. As such, there is nothing wrong with this method. Declaration variable I is declared over here and initialised later before it is used. Therefore, the correct answer is E. It prints out the number one. Now for the second question. In this scenario, the data field is declared as final. This declaration prevents this field from being assigned to any other object, but does not have any impact on the internal state of the reference object itself. As a result, the mutation over here takes effect, and this statement prints out the number one. So the correct answer is D. Now for the third question. The final keyword over here indicates that this method cannot be overridden. This subclass extends the superclass by overriding the My Method method. This overwriting violates the restriction imposed by the final keyword when it is declared on the superclass's method. Therefore, the correct answer is "a compilation fails." Now for the fourth question. An abstract class cannot be instantiated. So in order to create an object of such a type, you must declare a subclass extending the Abstract class. The final keyword over here, however, prevents the MyClass class from being extended by any sub classes.As such, the abstract and final keywords contradict each other when used on the same class, resulting in a compilation error. So the correct answer is B. In this lesson, we have gone overboard with developing code using the final keyword. Essentially, the final keyword can be applied to variables, methods, and classes. It cannot be used anywhere else. To put it simply, final variables cannot be changed, final methods cannot be overridden or hidden, and final classes cannot be extended.

3. 2.3 Create inner classes including static inner class, local class, nested class

Hello everybody. In this lesson, we will go through the third item of the advanced Java Class Design topic and the Java SE Eight programmer two exam syllabus, which is creating inner classes, including static inner classes, equal classes, nested classes, and anonymous inner classes. Let's start with the definition of inner classes. They are basically classes defined inside the body of another class, also called an outer class. In this lesson, we will go over four different types of inner classes, including static inner classes, nonstatic inner classes, local classes, and anonymous classes. Static inner classes, also known as static nestedclasses, are static members of the enclosing class. This type of inner class has been covered in detail in the last lesson of the previous topic, Java Class Design. You can go back to that lesson to find more information. Instances of a non-static inner class can exist only within an instance of its enclosing class and have direct access to members of that instance. To instantiate a non-static inner class, you must first create an instance of the outer class, then instantiate the inner class within the outer object. Since non-static inner classes are associated with instances, they cannot define static members except for constant variables. You may wonder what constant variables are. A constant variable is a variable of a primitive type or type string that is declared as final and initialised with a compile-time constant expression. A compile-time constant expression is typically a string or an arithmetic expression that can be evaluated during compilation. This is an example of non-static inner classes. You can define this like a normal top-level class, except that instead of declaring it as an independent class, you put such a class into another class. If you want to instantiate the inner class from outside of the outer class, you must construct an instance of the outer class first, then create an instance of the inner class from that outer object. If you instantiate the inner class from within the outer class, the creation of an outer object is not required. Local classes are classes defined in a block of code, which is a group of zero or more statements between balanced curly brackets. You can define a local class in a methodbody, a conditional statement, a loop construct, etc. Unlike other types of classes, local classes must be declared before they can be used in the enclosing block. Before over here means the class declaration is at a higher position in the source file than where it is used. Local classes cannot be declared with any modifier other than abstract and final. The reason is simple. Local classes are part of the containing block and are only loaded when that block is executed. Therefore, all other modifiers do not make any sense if declared on local classes. Similar to non-static inner classes, local classes cannot define static members except for constant variables. Local classes are part of the enclosing block. Thus, they have the same access level to the enclosing class as that block. For instance, if the containing block is a static method, the enclosed local class can only access static members of the class of that method. When local classes access local variables or parameters of the enclosing block, they capture those variables in order not to break the code inside the local classes. Those captured variables must never be changed, or, in short, be effectively final. Here is an example of local classes. You can define it in a similar way to a nonstatic inner class. Notice that a local class must be defined before it can be used in the containing block. Anonymous classes are a special case of local classes, allowing you to declare and instantiate a class at the same time. The main difference between anonymous classes and local classes is that anonymous classes have no name. Since anonymous classes are a special case of local classes, they comply with all the restrictions local classes have. In addition, anonymous classes don't have a name, so they cannot define constructors. An anonymous class must extend a class or implement an interface. This requirement is obvious since we cannot specify an anonymous class without such a supertype. An anonymous class is an expression that can be declared and instantiated. Such an expression consists of the followingcomponents An anonymous class expression always starts with the new operator, similar to the invocation of a concrete class constructor. Next comes the name of an interface to implement or a class to extend. After that, you must declare arguments to a constructor, just like when you call a function Object() { [native code] } of a non-abstract class. Notice that the function Object() { [native code] } I'm talking about is a function Object() { [native code] } of the class that the anonymous class extends. If the anonymous class implements an interface, an empty pair of parentheses is used. The last component of the expression is the class body. This is where members of the anonymous class are defined. This is an example of anonymous classes. The expression starts with the new key word, then an interface name. After that, you can see a pair of empty parentheses representing an empty list of arguments to the constructor. Finally, there is a class body. Using inner classes, which comes with many restrictions, is a bit more complicated than working with top-level classes. However, they also bring in several advantages. If a class is useful to only one other class, it is reasonable to embed the helper class inside the class. Using it. Inner classes follow the encapsulation principle. Organizing classes this way enables private members of the outer class to be accessible to the inner class. And at the same time, this inner class is hidden from the outside world. Because it places the class declaration close to where it is used, using enter classes makes the code more readable and maintainable. Now it's time for some practise questions to go over what we have learned. Here is the first question. As always, feel free to pause the video if you need more time. A static inner class is a member of the enclosing class. It can be declared with any excess modifier. Therefore, there is nothing wrong with the inner class. On the other hand, the outer class is a toplevel class. Such a top-level class can only be public or packaged-private scoped, so the declaration of the outer class is invalid, leading to a compilation error. So the correct answer is that a compilation fails at line one, not line two. Now for the second question. Two public top-level classes are not allowed to be declared in the same source file. However, nothing stops you from declaring a public class inside of another public class. Therefore, there is no error at line one. A non-static inner class instance is associated with an object of the outer class inside that outer class. However, we do not need to instantiate it before creating an instance of the inner class. Therefore, there is nothing wrong with this statement. The inner class is a nonstatic class, so it cannot define any static members other than constant variables. A constant variable must be of a primitive type or type string, which is not the case over here. Consequently, this statement fails to be compiled, so the correct answer is B. Now for the third question. In this scenario, the data class is used before it is declared. This usage of a local class is forbidden. As a result, the given method fails to be compiled, and the correct answer is A. Now for the fourth question. Local classes and anonymous classes can only capture local variables in the enclosing block that are final or effectively final. The URL variable is initialised over here and then reassigned here. As such, this variable is not effectively final, so this statement fails to be compiled. As a result, the correct answer is a compilation failure. In this lesson, we have gone over different types of inner classes, including static and nonstatic inner classes, local classes, and anonymous classes. Here is a summary of what we have learned. Remember, a static inner class is associated with its enclosing class and does not have direct access to instance members of that outer class. A nonstatic inner class is associated with an instance of its enclosing class and has direct access to that object's members. A local class is a class that is defined in a block of code. You must declare a local class before being able to use it. An anonymous class is a local class with no name, allowing you to declare and instantiate a class at the same time.

4. 2.4 Use enumerated types including methods, and constructors in an enum type

Hello everyone. In this lesson, we will focus on the fourth item of the advanced Java class design topic in the Java SE 8 programmer's two exam syllabus, which is using enumerated types including methods, constructors, and an enum type. An enum type is a special datatype containing a set of predefined constants. An instance of this type must be equal to one of the values that have been defined. You must define enum types using the enum keyword. For example, you can declare a type representing geographic directions like this, with all the values being predefined. Enum instances are all constants. Thus, by convention, their name should be written in uppercase with the underscore character separating words. In our enum example, all constant names are in upper case. An enum type implicitly extends the enum class. Since a class can extend only oneparent, it cannot extend any other class. as there is only one instance of each enum constant. The equality operator can be used in place of the equals method when comparing two object references. if it is known that at least one of them refers to an enum constant. An enum type can contain various members like a normal class, including fields, methods, constructors, initialization blocks, and nested types. These members can be either static or non static.This direction enum type has constants, afield, a constructor, and a custom method. Notice that the only function Object() { [native code] } of this enumtype has a single parameter of type int. So all enum constants must be followed by an argument of the same type. In addition, enum constants must be specified before any other members. In the declaration of an enumtype or during compilation, an error occurs. Constructors of an enemy is always private scoped. It is a compilation error if a function Object() { [native code] } is declared with the public or protected access modifier. A function Object() { [native code] } with no access modifier is implicitly private. Constructors of an enum type cannot invoke a superclass function Object() { [native code] } like a normal class. The Enum class, which is the superclass of enum types, has a constructor. However, this function Object() { [native code] } is used by code produced by the compiler and cannot be directly invoked from an enum type function Object() { [native code] } like a normal class. If no constructors are defined for an enumtype, a default function Object() { [native code] } is implicitly declared. The default function Object() { [native code] } is private, has no formal parameters, and has no throws clause. This is a list of methods defined in the Java Lang enum class. Except for the value of method, the remaining methods are all instance methods. In addition, all the methods other than "value of and to string" are final and cannot be overridden. In EDOM types, the name method returns the name of this enum constant exactly as declared in its declaration. However, it is recommended to use the two-string method over this one. since the two-string method can return a more user-friendly value. The twostring method returns the name of the thisenam constant as contained in the declaration. This method can be overwritten to provide a user-friendly textual representation of the constant. It's worth noting that the only instance method of the enum class that can be overridden in enum types is twostring. The ordinal method returns the ordinal of this enum constant or its position in the declaration of its enum type. The first constant is assigned an ordinal of zero. The compare-to method compares this constant with another one for order. The natural order implemented by this method is the order in which the constants are declared. Enum constants can only be compared if they are of the same enum type. The get declaring class method returns the class object corresponding to the enum type of this constant. The value of the method returns the enum constant of the specified enum type with the specified name. The name must exactly match the identifier declaring the enum constant. This is the only static method of the enum class. In addition to methods inherited from the Java Lang enum class, an enum type also has two implicitly declared methods. Both methods are static. The values method returns an array containing all constants of this enum type in the order that they are declared. This method may be used to iterate over constants of the enum type. The value of the method returns the enum constant of this type with the specified name. The string argument must match the identifier declaring the enum constant. Enum constants can be declared with a class body. This body defines an anonymous class that extends the immediately enclosing enum type. An enum constant class body is governed by the normal rules of anonymous classes, including the fact that it cannot define constructors. Instance methods declared in those class bodies can be invoked outside of the enclosing enum type only if they override accessible methods in the enum type. This is an example of enom constants with a class body. After each constant, male and female, you can declare a class body that represents an anonymous class. In this case, such class bodies contain a single method named print, as shown here and here. Thanks to these anonymous classes, you can even declare abstract methods in the enclosing enum type like this. Since the print method is declared in the genderenum type and overwritten in anonymous classes associated with enum constants, you can call such a method on a constant, as you can see over here. Now it's time for some practise questions to go over what we have learned. Here is the first question. As always, feel free to pause the video if you need more time. Constructors of an enum type are always in private scope. Notice that unlike instructors of a normal class, where the absence of an access modifier implies a private package's scope, such an absence means private for constructors of an enum type. Therefore, this statement fails to be compiled, and the correct answer is A. Now for the second question. All instance methods of the enum class except for two strings are final. Thus, this method overriding is invalid, and the given code fails to be compiled. As a result, the correct answer is A. Now for the third question. This two-string method overrides the method of the same name in the enum class. So instead of returning the name of enum constants like black or white, it returns code values one or zero. The ordinal method returns the position of an enum constant and the declaration of its enum type. Starting with zero, the black constant is declared as the first instance of the colour enumtype, so its ordinal is zero. When this instance is created, integer one is passed in this constructor, and the instance's textual representation will be one as returned from the two string method. Based on the values returned from the ordinal and two string methods, it is clear that the numbers one and zero will be printed out, and the correct answer is D. Now for the fourth question, the value of "methodover here" is an implicitly declared method of the colour enum type returning the enum constant of this type with the specified name, which is black. In this case, since there is only one instance of each enum constant, both variables colorone and colortwo reference the same object. As such, this equality comparison evaluates to true, and the correct answer is B. In this lesson, we have gone over a number of types, especially their constructors and methods. In summary, an enum type is a special data type containing a set of predefined constants. You must define an enum type using the enum keyword. Constructors of the enum type are always privately scoped. Public and protected access modifiers are forbidden. An enum type contains implicitly declared methods that are inherited from the enum class, whereas the absence of such a modifier on a function Object() { [native code] } means private scoped. and you can also define custom methods for it.

5. 2.5 Develop code that declares, implements and/or extends interfaces and use the

Hello everyone. In this lesson, we will go over the fifth item of the advanced Java class design topic and the Java SE 8 programmer two exam syllabus, which is developing code that declares. Interfaces can be thought of as contracts that govern how software components interact, and in the Java programming language, interfaces are reference types that can be implemented by classes and extended by other interfaces but cannot be instantiated. An interface declaration consists of modifiers: the interface keyword, interface name, a comma-separated list of parent interfaces if there are one or more, and a body. Note that unlike classes where a subclass can extend only one superclass, a subinterface can extend more than one superinterface. Here is an example of interface declarations. My interface is extended to superinterfaces one and two. My interface inherits all fields and nonstatic methods from those superinterfaces. In addition to inherited members, this subinterface may define members of its own, including methods, static, final fields, and nested types. Remember that all members of an interface are public-scoped, so you cannot declare any access modifiers other than public on them. This public modifier is redundant and may be left out. There are three types of methods that can be declared in an interface. They are abstract methods, default methods, and static methods. We'll go over these methods in detail now. Abstract methods are methods with no body. These methods define the behaviour of the containing interface without enforcing specific implementations. Using abstract methods for interfaces allows software components to be loosely coupled, facilitating the development process. Methods of an interface that are not declaredas default or static or implicitly abstract, sothe abstract keyword may be left off. Here is an example of abstract methods. Unlike abstract methods defined in an abstract class,abstract methods of an interface do not requirethe abstract keyword to be present. Default methods enable you to enhance an interface without breaking the code written against older versions of that interface. For example, you may update an interface with new functionality so that the old client code can continue to use the interface without any problem, whereas new code can be created to take advantage of the new version. Another example is when implementing an abstract method is complicated and a default implementation is provided to save the development effort required to implement the interface. Default methods of an interface are inherited by its subinterfaces and classes that implement the interface. These subinterfaces and classes can override default methods with their own implementations. Default methods are declared using the default keyword and contain a body like normal concrete methods. Here is an example of default methods. Except for the default keyword, it is no different from a normal nonstatic concrete method. Static methods allow you to organise helper methods in a more manageable way by keeping methods associated with an interface in the same location. Static methods are integral parts of the interface where they are declared. These methods are not inherited by subinterfaces or implementation classes of that interface. Static methods are declared using the static keyword and contain a body like normal concrete methods. Here is an example of static methods on an interface. Its declaration is the same as a static method defined in a class. In order to specify that a class implements an interface, an Implements clause must be included in the class declaration. A class can implement more than one interface if an interface contains abstract methods, and it likely does, since abstract methods are the key feature of interfaces. The interfaces implementation classes must provideimplementations for all those methods, orthe classes are declared as abstract. Implementation classes can reuse or inherit default methods declared in the implemented interface as they are, without the need for redefining them. Implementation classes may override default methods and hide static methods of the implemented interface. Static methods are not inherited, but nothing stops implementation classes from defining their own methods that have the same name as the interface's static methods. In that case, methods of implementation classes hide methods of the interface. This is an example illustrating how a non-abstract class implements an interface. This default method already has an implementation, so my class can reuse it. The abstract method over here does not have such a body, so my class must provide an implementation. As shown here, this static method is associated with My Interface. The My class does not inherit that method and can only access it via the reference type Myinterface. Extending an interface enables you to add new functionality to that interface without breaking code written against it. The extended interface is called superinterface, and the extending interface is subinterface. A sub-interface may declare methods overriding abstract and default methods of its super-interfaces. An abstract method can not only override a method of the same type, but it can also override a default method, and a default method can override an abstract method. A sub-interface may define methods for hiding the static methods of its super-interfaces by using methods of all kinds, including abstract, default, and static methods. This is an example demonstrating how an interface extends other interfaces. Even though the subinterface is declared with an empty body, it inherits method B from Interface Super One and method C from Interface Super Two. Method B is abstract, so the class that implements the subinterface must provide an implementation method A in both the superinterfaces here and here that are associated with these interfaces. They have nothing to do with interface inheritance. The at override annotation indicates that the annotated method is intended to override a method of a supertype. If the containing type of such an annotated method is an interface, that supertype is an interface that the containing interface extends. If the containing type is a class, then the supertype is a class that the containing class extends, or an interface that the containing class implements. Notice that the at override annotation can only be used for method overriding. It cannot apply to the method of hiding. Here is an example illustrating the atoverride annotation. This instance method has the same signature as the method of a superinterface and overrides it. So you can declare that override annotation over here. This abstract method is in turn overridden by a method of the implementation class. Hence, the at override annotation can also be specified here. This static method hides this one, which has the same name and does not override it. As such, the static method in my class cannot be annotated with an override comment. This declaration will lead to a compilation error. Now it's time for some practise questions to go over what we have learned. Here is the first question. As always, feel free to pause the video if you need more time. All fields of an interface are static and final, even if those modifiers are not specified. Both the increase VAR and decrease VAR methods try to modify the value of such a final field. These attempts result in compilation errors. Therefore, the correct answer is D. The compilation fails at lines two and three. Now for the second question. This print URL method provides an implementation for an abstract method of the WizLabs interface and overrides that method. In the Java programming language, an overriding method cannot be more restrictive than the overridden one. All methods of an interface are public-scoped, so overriding methods cannot be declared with any scope other than public. This is not the case over here, where the overriding method is package private. As a result, the correct answer is Compilation fails at line two. Now for the third question. Static methods of an interface are integral parts of the interface itself. It is not inherited by subinterfaces and implementation classes. As such, the existence of two static methods of the same name, method A, with interfaces super one and super two, does not cause any problem. On the other hand, method B of super one, an abstract method, and method B of super two, a default method, conflict with each other, leading to compilation failure in the sub interface. So the correct answer is B. It fails due to the conflict with Method B. To avoid such an error, the subinterface may define a method with the same name, methodB, to override both conflicting methods. Now for the fourth question. A static method cannot override a nonstatic method. So method A fails to be compiled whether or not it is decorated with the override annotation. A nonstatic method can hide a static method that's not overridden. So method B would have been valid only if it were not declared with the at override annotation. The superinterface does not define any method named Method C. Therefore, the usage of the at-override annotation over here is invalid. All methods of the sub-interface, including method A, method B, and method C, are invalid. So the correct answer is D. None of the given methods in this lesson. We have gone over interfaces, how to declare, implement, and extend interfaces, as well as how to use the @Override annotation. Here is a quick summary. An interface declaration consists of modifiers: the interface keyword, the interface name, a comma-separated list of parent interfaces, if any, and a body. Classes that implement an interface must provide implementations for all methods that are not default or static, or the classes are declared as abstract. Extending an interface enables you to add new functionality to that interface without breaking code written against it. The app Override annotation indicates that the annotated method is intended to override a method of a super type.

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