Object Oriented programming Instructor: Dr. Essam H. Houssein
Immutable Objects and Classes Normally, you create an object and allow its contents to be changed later. Occasionally it is desirable to create an object whose contents cannot be changed, once the object is created. We call such an object an immutable object and its class an immutable class. The String class, for example, is immutable. If you deleted the set method in the Circle class in Listing 8.9, the class would be immutable, because radius is private and cannot be changed without a set method.
If a class is immutable, then all its data fields must be private and it cannot contain public set methods for any data fields.
For a class to be immutable, it must meet the following requirements: ■ all data fields private; ■ no mutator methods; ■ no accessor method that returns a reference to a data field that is mutable.
The Scope of Variables Instance and static variables in a class are referred to as the class’s variables or data fields. A variable defined inside a method is referred to as a local variable. The scope of a class’s variables is the entire class, regardless of where the variables are declared. A class’s variables and methods can appear in any order in the class. The exception is when a data field is initialized based on a reference to another data field.
You can declare a class’s variable only once, but you can declare the same variable name in a method many times in different non nesting blocks. If a local variable has the same name as a class’s variable, the local variable takes precedence and the class’s variable with the same name is hidden. For example, in the following program, x is defined as an instance variable and as a local variable in the method.
The this Reference The this keyword is the name of a reference that refers to a calling object itself. One of its common uses is to reference a class’s hidden data fields. For example, a data-field name is often used as the parameter name in set method for the data field. In this case, the data field is hidden in the set method. You need to reference the hidden data-field name in the method in order to set a new value to it.
A hidden static variable can be accessed simply by using the ClassName.StaticVariable reference. A hidden instance variable can be accessed by using the keyword this, as shown in Figure 10.2(a).
Another common use of the this keyword is to enable a constructor to invoke another constructor of the same class. For example, you can rewrite the Circle class as follows:
Class Abstraction and Encapsulation Java provides many levels of abstraction. Class abstraction is the separation of class implementation from the use of a class. The creator of a class describes it and lets the user know how it can be used. The collection of methods and fields that are accessible from outside the class, together with the description of how these members are expected to behave, serves as the class’s contract.
The details of implementation are encapsulated and hidden from the user. This is known as class encapsulation.
Object-Oriented Thinking Classes provide more flexibility and modularity for building reusable Software. The procedural paradigm focuses on designing methods. The object oriented paradigm couples data and methods together into objects. Software design using the object-oriented paradigm focuses on objects and operations on objects. The object-oriented approach combines the power of the procedural paradigm with an added dimension that integrates data with operations into objects.
In procedural programming, data and operations on the data are separate, and this methodology requires sending data to methods. Object-oriented programming places data and the operations that pertain to them in an object. This approach solves many of the problems inherent in procedural programming.
The object-oriented programming approach organizes programs in a way that mirrors the real world, in which all objects are associated with both attributes and activities. Using objects improves software reusability and makes programs easier to develop and easier to maintain. Programming in Java involves thinking in terms of objects; a Java program can be viewed as a collection of cooperating objects.
Class Design Guidelines 1- Cohesion A class should describe a single entity, and all the class operations should logically fit together to support a coherent purpose. You can use a class for students, for example, but you should not combine students and staff in the same class, because students and staff are different entities.
2- Consistency • Follow standard Java programming style and naming conventions. • Choose informative names for classes, data fields, and methods. • A popular style is to place the data declaration before the constructor and place constructors before methods. • - naming consistency • - no-arg constructor • - If you want to prevent users from creating an object for a class, you may declare a private constructor in the class,
3- Encapsulation A class should use the private modifier to hide its data from direct access by clients. Thismakes the class easy to maintain. Provide a get method only if you want the field to be readable, and provide a set method only if you want the field to be updateable.
4- Clarity Cohesion, consistency, and encapsulation are good guidelines for achieving design clarity. Additionally, a class should have a clear contract that is easy to explain and easy to understand.
5- Completeness Classes are designed for use by many different customers. In order to be useful in a wide range of applications, a class should provide a variety of ways for customization through properties and methods.
6- Instance vs. Static A variable or method that is dependent on a specific instance of the class should be an instance variable or method. A variable that is shared by all the instances of a class should be declared static.
Always reference static variables and methods from a class name (rather than a reference variable) to improve readability and avoid errors. Do not pass a parameter from a constructor to initialize a static data field. It is better to use a set method to change the static data field.
Instance and static are integral parts of object-oriented programming. A data field or method is either instance or static. Do not mistakenly overlook static data fields or methods. It is a common design error to define an instance method that should have been static. For example, the factorial(int n) method for computing the factorial of n should be defined static, because it is independent of any specific instance.