Summary Absolute Java - Walter Savitch - CHAPTER 4 - DEFINING CLASSES I

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CHAPTER 4: DEFINING CLASSES I
Theory part



Object is a value of class type and is often
referred to as instance of the class. An object
differs from values of primitive types in a way that
it has both data and methods (actions).



Each object can have different data, but all
objects of a class have the same types of data
and all objects in a class have the same methods.



Each object has pieces of data that are instance
variables and the methods. Both data items and
methods are sometimes called members of the
object. The data items are also called fields.

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Objects are named by variables of class types.



DateFirstTry date1, date2



If the variables date1 and date2 are declared as
variables of class type (They belong to the class
DateFirstTry) then in line:
date1 = new DateFirstTry ();



The variable date1 is a name of the object that is
created in the expression. To obtain an object you
must use the new operator.
The declaration of a class variable and the
creation of the object are more typically
combined into one statement:

DateFirstTry date1 = new DateFirstTry ();
Instance variables can be named by giving the
object name followed by a dot and the name of

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the instance variable

date1.month
date1.day
date1.year



The statement date1.month = "Hello friend" in the
main (tester) class assigns the string "Hello
friend" to the instance variable month. Instance
variables can be used just like any other variable.
The instance variable date1.month can be used
just like any other variable of type String.



Practise part