This expression sets the instance variable age of the newly created object (i.e., "this" object's age) to 0. Objects are always created using a constructor.Ī few things to note: the constructor contains the expression this.age = 0. In between these brackets is the source code that the program executes when the constructor is called (e.g., new Person ("Ada")). The parentheses that contain optional parameters are followed by curly brackets. The parameter is enclosed in parentheses and follows the constructor's name. The constructor is also provided, as a parameter, the name of the person object to be created. The class in the example above is named Person, so the constructor will also have to be named Person. The constructor's name is always the same as the class name. Below, an ArrayList object named integers is created and some integers are added to it. A method is always part of some class and is often used to modify the internal state of an object instantiated from a class.Īs an example, ArrayList is a class offered by Java, and we've made use of objects instantiated from it in our programs. The values of instance (i.e., object) variables define the internal state of an individual object, whereas methods define the functionality it offers.Ī Method is a piece of source code written inside a class that's been named and has the ability to be called. A class defines the attributes of objects, i.e., the information related to them (instance variables), and their commands, i.e., their methods. We've already used some of the classes and objects provided by Java. However, it will appear more sensible as we progress through the course, perhaps even self-evident. The statement "programs are built from small, clear, and cooperative objects" may not make much sense yet. These constructs, and the individual instances that are formed from them, i.e., objects, are used in solving the problem. Once concepts related to a given problem have been identified, we can also begin to build constructs that represent them into programs. In other words, we can form abstractions from problems that make those problems easier to approach. Concepts related to a problem can only be considered once they've been identified. Object-oriented programming is concerned with isolating concepts of a problem domain into separate entities and then using those entities to solve problems. From there on, we'll learn how to add functionality, i.e., methods to our program. We'll start with focusing on describing concepts and data using objects. We'll now begin our journey into the world of object-oriented programming. You know how to create classes and objects, and know how to use objects in your programs. You understand that a class defines an object's methods and that the values of instance (object) variables are object-specific. You're familiar with the concepts of class, object, constructor, object methods, and object variables.
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