CIS283: Introduction to Application Programming with Java

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CIS283 Introduction to Application Programming
with Java

• Jose perez-carballo
• Day 5

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• Creating classes and methods

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• How is a class defined
• class Person
• //body of the class
• //includes instance variables
• //class variables
• //constructors
• //methods
• //nothing else

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• A class inherits (properties and methods) from
  its superclasses
• The Object class is the superclass of all Java
  classes

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• You can create your own class hierarchy using the
  keyword extends
• Class Student extends Person

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• Define the variables of class Person
• Instance variables space for data each object of
  the class will have
• E.g. firstName, lastName
• Class variables space for data that all objects
  of the class will share
• E.g. nrOfPersonsCreated
• Use the word static to declare class variables

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• Since Student inherits from Person
• All the variables Person has are also variables
  of Student (I.e. Student has firstName,
  lastName).

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• A class has variables (data) and methods
  (behavior)
• To define a method we need
• The name of the method
• A list of parameters
• The type of the value returned by the method
• The body of the method

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• Examples of methods of Person
• void setFirst(String ff)
• // body of method
• void setLast(String ll)
• // body of method
• String getFullName()
• // body of method
• // this one must return a String

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• If the type returned is void then the method does
  not return a value
• If the type returned is no void then the method
  must return a value of that type

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• In order to use the classes we are defining we
  need a main method
• Create a class with a main method
• Declare in method main variables of type Student
  and Person
• Test that Student inherits the properties of
  Person

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• Scope of a name
• is the part of the program where that name exists
• Look at the different scopes of names of
  variables and methods in our current example
• See the code of class ScopeTest
• Avoid using the same name of different variables

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• Passing arguments to methods
• See class Passer

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• Class methods
• (beware couple of typos in book, p 123
• int now )
• Class methods dont need an instance of the class
  to be called
• Person pp0 new Person()
• System.out.println("nr of persons so far "
  Person.howManyPersons())

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• Java applications
• Need a main method
• There should be only one method called main

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• Passing arguments to an application
• Using the array of Strings
• main(String args)
• Write an application that counts how many
  arguments are passed to it
• And prints the arguments back

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• All arguments passed to an application are
  Strings
• If you want to use some as ints you must convert
  String to int
• See Averager class

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• constructors
• It is a method of the class
• When a new object of a class is created
• Java allocates memory for the object
• Initializes instance variables
• automatically executes one of the constructors of
  the class

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• A constructor
• Must have the same name as the class
• Doesnt have a return type (not even void)
• Write a constructor for Person class

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• Overloading methods
• Same method name, different method
• The compiler knows they are different because
• Different nr of arguments
• Data types of the arguments
• See example with Person

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• Constructors can be overloaded
• Write more constructors for Person

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• Overriding methods
• Given classes Person and Student
• And object st1
• When st.foo() is used
• Look for a method called foo in Student
• If not found look for it in Person
• Continue looking in all super classes of Person
• If not found report error
• Otherwise use the first definition of foo found
• Create methods called whatAmI() for Person and
  Student

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• You can still use the method of the super class
• Write method whatIsMySuperclass() for Student

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• x
• x