2.3 The first object

In order to generate objects as part of a model in the computer, we must describe the objects in a language that can instruct the computer to generate the objects. As mentioned, such a language is called a programming language.

Below we show the description of an object representing the account of John Smith:

account_1010: obj
   owner: val "John Smith"
   balance: var float 

The keyword obj specifies that the program element is a description of an object. The name of the object is account_1010.

The object has two attributes characterizing the account: 

• The owner of the account is represented by the data-item owner holding the string value "John Smith". A string value (also called string literal) is a sequence of characters enclosed in double quotes ("). 
• The balance of the account is represented by the data-item balance that may hold the value of a real number. A real number may be a number with a decimal point as 350.56. The variable balance may hold this number. For this example we assume that the balance is in Euro. If balance holds the value 350.56, this means that balance is 350.56 Euro. 

The names String and float are examples of types. A type defines the set of values that a given data-item may hold. The data-item owner is said to be of type String since it may only hold String values. Similarly the data-item balance is said to be of type float since it may only hold real numbers.

The keyword val is a shorthand for value and specifies that the data-item holds a value that is constant as long as the object exists. The keyword var is a shorthand for variable and specifies that the data-time may hold different values during the life-time of the object. We return to this in chapter .

As said above, the code/program fragment above is a description of an object — it is not the object itself as represented in the computer. 

We may give this description to a compiler, which translates it into machine language. We may then execute the resulting machine language, which will generate the object in the computer. 

We use the term object-descriptor or simply descriptor for the text describing the attributes of the object. For account_1010, the attributes are the two data-items owner and balance.

As we shall see later, an object-descriptor may include other elements than data-items.

The description of John Smiths account is made in a programming language called qBeta. As mentioned, a programming language is a formal notation for instructing a computer. The elements of a program are phrases that may describe declarations, objects, data-items, statements to be executed and combinations of these. The description of account_1010 is an example of a declaration of a data-item – in this case an object.

A description written in a programming language is called a computer program or just a program, but is also referred to as source code or simply code. A collection of programs is often referred as software. The description of the above object is thus an example of a program although it is not very useful.