At the member level, you can also use the public or no modifier (package-private), as well as top-level classes with the same meaning. For members, there are two additional access modifiers: private and protected. The private modifier specifies that the member is accessible only in its own class. The protected modifier specifies that the member can only be accessed in its own package (as with package-private) and additionally through a subclass of its class in another package. Note: Often we are lazy in our English level description and say object when we mean class, or class when we mean object. The wise student will ask for clarification if it is not immediately clear from the context what this really means. Often, an “object” in a computer program attempts to model (i.e. approximate) a real entity. For example, a “car” in a computer could be modeled by information about the size of a car, the number of doors, the engine, color, etc. In addition, there would be small parts of the associated code (called methods) that manipulate the car, for example: start_car() would “simulate” engine start-up, station wagon() would simulate the slowing down of the car, etc. Once the data has been defined, what it means to be an object, the programmer needs ways to manipulate that data. These methods are called functions. Let`s look at a collection of classes and see how access levels affect visibility.
The following illustration shows the four classes in this example and their relationship. BUT: For any code where the variable “car” is a car, the compiler only allows “car functions” (although the variable refers to a sedan). Additional Note: The variable sedan would have access to all the features of the car and all the features of the sedan! The following explanations of OOP principles refer to a vehicle hierarchy: The following table shows the member access allowed by each modifier. The interface between the object of the car and the rest of the program (or world) would be the putlian methods allowed on the car. For example, turn the wheel to the left, turn the wheel to the right, accelerate. We don`t know (and we don`t care) how these functions work, as long as the car knows every function call and responds appropriately. Variables that contain the information that defines an object are usually called member variables, but sometimes data fields (or just fields) or “object-level” variables. Public – whoever creates our form can move it too, so we make this method public (accessible to everyone). The public keyword tells the class file (and other programmers) that this object can be created by anyone. Public means everyone. Alternatively, in some cases, you may see the keyword “Private” next to a function.
This, in turn, means that only the object itself can use the function. Creating a real (not abstract) class that uses Generic_Shape inheritance is the idea that a “child class” gets all the functionality of its parent class. This works because child classes are more specific examples of the parent class. For example, if we had a vehicle class, we could have a car class as a more specific example. Then we could have a “sedan” class as an even more concrete example. In most cases, you can read protected as “private”. The exception is when we use the object-oriented inheritance technique. Inheritance occurs when a class file is a child of another class file and thus “retrieves” all the code of the parent class for free. Now, let`s create a square class that generic_shape ISA.
Keep in mind that ISA means that the square class behaves like a generic form in many ways, but in some ways unique like a square. Access levels affect you in two ways. When you use classes from another source, such as Java platform classes, access levels determine which members of those classes can use their own classes. Second, when you write a class, you must decide how much access you want each member variable and method in your class to have. Object-oriented programming languages are defined by the following keywords: abstraction, encapsulation, inheritance, and polymorphism. An object is a container for data and functions that affect data. In OOP, a “child” object can “extend” (make it more specific) another object by inheriting from a “parent” object. Thus, the child receives all the data and functions of the parents “free of charge”. The idea of an “interface” is also the key to OOP. An interface is the permitted interaction between the outside world (e.g. other programs) and the object itself.
An object design pattern (in ActionScript) can be found in the following design pattern Ok, we`ve worked hard to integrate (most) of our functionality into class Generic_Shape, because we don`t need to repeat the work for all of our “sub-” classes (or child classes). In earlier computer languages, objects (then called structures) could be created “empty”. Leer meant, “Everything we remembered at the time.” The developers of these languages “knew” that programmers would always initialize these objects immediately after they were created. This is a great example of something that was “known” but was actually NOT TRUE.
