1. Week 4 Object-based Programming (2) Classes and Objects: A Deeper Look
MSIS670: Object-Oriented Software Engineering
Week 4
Object-based Programming (2)
Classes and Objects: A Deeper Look
12/30/2018
4.1

2. Constructor Class Constructor Same name as class
Initializes instance variables of a class object
Called when program instantiates an object of that class
Class can have several constructors through overloading (next class).
When you want to use an object, which is defined by the programmer, a class (object) can call a class (actually class constructor) to instantiate an object of the class.
ClassName ref = new ClassName( arguments );
ClassName indicates type of object created, ref is the reference to that object (used as a variable), and new indicates that a new object is created. Arguments specifies constructor argument values. This argument should matches the constructor arguments (number of variables, and each type) – c.f. Overloaded Constructors.
e.g., Time1Test line 9,
Time1 time = new Time1();
time is a new object of the type “Time1”, with no argument required.
Then constructor initializes the object’s instance variables according to the specification of its constructor.
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3. Implementing a Time Abstract Data Type with a Class: Time2. java (ppp
Time2.java defines the class Time
Time2Test.java defines the class Time2Test, which is an application.
Public classes must be defined in separate files.
Every Java class must extend another class. If not extend a class, it implicitly extends Object.
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4. Using Overloaded Constructors
Methods in a class may have the same names.
Must have different arguments.
Different in number of parameters.
Different in data types.
e.g., (P ).
public time2(); public time2( int h ); …
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5. Overriding Methods Method Object.toString
Gives String representation of object
But we want it to be a standard time format
Time2 overrides method toString
Lines
Time2.toString is now different from Object.toString
If we remove lines of class Time1, Time1 still com[piles correctly. When a message calls for toString method, it uses method java.lang.Object.toString; giving String representation of object (not in a universal time format).
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6. toString() Method in Time2.java
Given a set of parameters (values of variables hour, minute, and second), this method toString gives back a String in a format of hh:mm:ss (in which hh is up to 12 format, not 24 hour format).
twoDigits is another object of a class called DecimalFormat, which has a method called format that changes the view of a number into a two digit (even it is a single digit).
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7. Using set and get methods
Mutator method – set
public method
Allow clients to modify private data
Accesor method – get
Allow clients to read private data
The set and /or get methods are not mandatory for each of the private data. Sometimes it is completely hidden from outside. Only if it makes sense to provide such methods, you should provide them.
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8. Lab Activities (Week 4) Coding Assignment 4 class MyRectangle
Two .java files. Both should be saved on a same project.
Similar to the Time2 example.
For more advanced: use GUI application to test multiple constructors
For this assignment, you have to create a class MyRectangle and another class to test this class. Place them together in a package (project) before you start coding them. Turn in the coding as usual for both files.
12/30/2018