Noter til Java Programmering

Kapitel 1 Klasser og Objekter

Objekt Model

Object orientation is all about modelling real world objects or concepts in software. Java is an ideal language for doing this. In fact, unlike some languages like C and C++, it forces you to think in an object-oriented way, since every piece of Java code must be contained within a class definition. Most of the examples we will use are based around a car. In the real world, we create a design for a car, take some raw materials and follow the design to create our finished object (then drive away in it!). In our software world, we will create our design (the Java class), take some memory and form our object(s) out of it.

Of course, we ourselves don’t manufacture these objects ourselves; we define the features of an object in a class definition and then get Java (the "object factory") to create objects of this class for us.

Klasse begrebet

Klasse begrebet

A class is a template for building multiple objects with similar features. A class encapsulates all the features of an object, i.e. its data structure and its behaviour. So, when you write an object-oriented program, you don't write code for individual objects. Instead, you define the data structure and the behaviour (or operations) of a class of objects. Then, when you want to create objects of this class, you get Java to create instances of this class. All objects of the same class have the same data structure and behaviour (as defined in the class), but each object is independent of every object of its class. In other words, each object can store its own values in the data structure defined by the class.

Klasser og objekter

Klasser og objekter

A class is a template for building objects of that class. In object-oriented terminology, an object is an instance of a class. This simply means that each object of that class has the same data structure and operations. However, the values held in this data structure are unique to each object. Such data is therefore called instance data. Furthermore, the operations of an object act on that object's instance data.

Skabelse af objekter

Skabelse af objekter

Objects are created by using the new operator with the class of the object and any arguments needed by the class' constructor.