Java class and object relationship

Day Object-Oriented Programming and Java

java class and object relationship

Classes and Objects are basic concepts of Object Oriented Programming which A typical Java program creates many objects, which as you know, interact by. Inheritance defines relationships among classes in an object-oriented language. In the Java programming language, all classes descend from Java Object and Classes - Learn Java in simple and easy steps starting from basic to advanced concepts with examples including Java Syntax Object Oriented.

Nouns perform actions with verbs just as objects perform actions with functions, or methods. As you know, there are three base types, or classes, of nouns — people, places, and things — and from those, you can make infinitely more classes of nouns. Java has a similar idea. In this chapter, you will learn what a class is in Java and how you can make an object from a class.

The attributes of an object are variables that hold information, or data, about the object while its methods are ways the object can manipulate its data to create new data.

This is important because we can use this idea of structured data to write clean, flexible, and maintainable code. This should be read as "class A Has-A class B. In this case, any method of class B can be called from class A using the class B member variable.

If class A uses class B, it means that the methods in class A have a local instance of class B, which is used to call a method of class B. This should be used only for localized need. How Do we Relate Classes? Given two classes, how do I decide on which relationship to choose? One approach could be to relate classes based on the relationship of corresponding objects in the problem space. To further develop our understanding, here, we will take a problem statement and analyze its evolving design with a focus on maintenance and extensibility.

These evolving designs will also give us insight into the usage of these different types of class relationships. Problem Statement Design reader classes should be able to read from a file, socket, or pipe. It should be able to read byte-by-byte or a number of bytes together.

java class and object relationship

These requirements are just for the first version. We may have more requirements for the future, e. Based on what we decide to build in the future, these classes should accommodate the new changes as and when we want to build them.

Object Orientation: Maintaining Relationships Amongst Objects

The example pseudo code explains how to use these classes. Overall, this is a good design that uses the Is-A relationship for code reuse. Issues Applications using these libraries may need to read large data.

As the read method is reading it byte-by-byte, overall performance will be slow. The other issue is that it provides methods to read a byte or an array of bytes.

Java Object and Classes

Whereas, the application may need to read different data-types and not just bytes. If that facility is not available, then each application will have to do data conversion in their implementation, which would be an overhead. This also provides extensibility and maintainability. Design 2 The solution to any performance problem would be to implement buffering in methods. This buffer is maintained in the class. Each time the read method is called, it looks into the buffer and returns from there.

If the data is not available in the buffer, then it fills the entire buffer instead of reading just one byte. Changing the same read method in each class may not be advisable as some applications or customers may not want this overhead of extra memory getting utilized as an internal buffer for these classes. So, look at a design where new classes are introduced to get this new behavior.

This will ensure that the application will only need to take care of extra memory requirement, whereas all other application will be unaffected. Here, we notice that there are two different streams in the design. If import statements are present, then they must be written between the package statement and the class declaration.

If there are no package statements, then the import statement should be the first line in the source file. Import and package statements will imply to all the classes present in the source file. Classes have several access levels and there are different types of classes; abstract classes, final classes, etc.

We will be explaining about all these in the access modifiers chapter. Apart from the above mentioned types of classes, Java also has some special classes called Inner classes and Anonymous classes. Java Package In simple words, it is a way of categorizing the classes and interfaces. When developing applications in Java, hundreds of classes and interfaces will be written, therefore categorizing these classes is a must as well as makes life much easier.

java class and object relationship

Import Statements In Java if a fully qualified name, which includes the package and the class name is given, then the compiler can easily locate the source code or classes. Import statement is a way of giving the proper location for the compiler to find that particular class. They are Employee and EmployeeTest. First open notepad and add the following code.

Java Programming Tutorial - 43 - Composition

Remember this is the Employee class and the class is a public class. Now, save this source file with the name Employee. The Employee class has four instance variables - name, age, designation and salary. The class has one explicitly defined constructor, which takes a parameter. Therefore, in order for us to run this Employee class there should be a main method and objects should be created. We will be creating a separate class for these tasks.

Following is the EmployeeTest class, which creates two instances of the class Employee and invokes the methods for each object to assign values for each variable.