Java Object-Oriented Programming: Understanding Objects, Classes, and Methods, Slides of Object Oriented Programming

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Objects And Classes In Java

Java OO (Object Orientation)

Python and Matlab have objects and classes. Strong-typing nature of Java changes how OO is done and how useful it is. Put aside your previous experience with OO (if any). This lecture: First : describe objects, demoing their creation and use. Second : Show you a class definition and how it defines functions, and procedures that appear in each object of the class. Third (if there is time). Show you a Java application, a class with a “static” procedure with a certain parameter.

Drawing an object of class javax.swing.JFrame

Object is associated with a window on your computer monitor JFrame@25c7f37d hide() show() setTitle(String) getTitle() getX() getY() setLocation(int, int) getWidth() getHeight() setSize(int,int) … JFrame Name of object, giving class name and its memory location (hexadecimal). Java creates name when it creates object Function: returns a value; call is an expression Procedure: does not return a value; call is a statement to do something Object contains methods (functions and procedures), which can be called to operate on the object

Evaluation of new-expression creates an object

Evaluation of new javax.swing.JFrame() creates an object and gives as its value the name of the object JFrame@25c7f37d hide() show() setTitle(String) getTitle() getX() getY() setLocation(int, int) getWidth() getHeight() setSize(int,int) … JFrame If evaluation creates this object, value of expression is JFrame@25c7f37d JFrame@25c7f37d 2 + 3 + 4

Class variable contains the name of an object

If variable h contains the name of an object, call methods of the object using dot-notation: JFrame@25c7f37d hide() show() setTitle(String) getTitle() getX() getY() setLocation(int, int) getWidth() getHeight() setSize(int,int) … JFrame h (^)? JFrame Procedure calls: h.show(); h.setTitle(“this is a title”); Function calls: h.getX() h.getX() + h.getWidth() JFrame@25c7f37d

Class definition

Class definition : Describes format of an object (instance) of the class. /** description of what the class is for */ public class C { } This is a comment declarations of methods (in any order) Access modifier public means C can be used anywhere Class definition C goes in its own file named C.java On your hard drive, have separate directory for each Java program you write; put all class definitions for program in that directory. You’ll see this when we demo Eclipse

Class extends (is a subclass of) JFrame

/** An instance is a subclass of JFrame*/ public class C extends javax.swing.JFrame { } C: subclass of JFrame JFrame: superclass of C C inherits all methods that are in a JFrame C@6667f34e hide() show() setTitle(String) getTitle() getX() getY() setLocation(int, int) getWidth() getHeight() … JFrame C Object has 2 partitions: one for JFrame methods, one for C methods Easy re-use of program part!

Class definition with a function definition

/** An instance is a subclass of JFrame with an area function / public class C extends javax.swing.JFrame { /* Return area of window */ public int area() { return getWidth() * getHeight(); } } C@6667f34e … getWidth() getHeight() area() JFrame C Spec, as a comment You know it is a function because it has a return type Function calls automatically call functions that are in the object

Inside-out rule for finding declaration

/** An instance … / public class C extends …JFrame { /* Return area of window */ public int area() { return getWidth() * getHeight(); } } C@6667f34e getWidth() getHeight() … area() { return getWidth() * getHeight(); } JFrame C C@2abcde getWidth() getHeight() … area() { return getWidth() * getHeight(); } JFrame C Function area: in each object. getWidth() calls function getWidth in the object in which it appears.

Class definition with a procedure definition

/** An instance is a JFrame with more methods / public class C extends javax.swing.JFrame { public int area() { return getWidth() * getHeight(); } /* Set width of window to its height */ public void setWtoH() { setSize(getHeight(), getHeight()); } } C@6667f34e … setSize(int, int) getWidth() getHeight() area() setWtoH() JFrame C It is a procedure because it has void instead of return type Call on procedure setSize

About null

v1 C@ v2 null C@ getName() C null denotes the absence of a name. v2 .getName() is a mistake! Program stops with a NullPointerException You can write things like: v1= null ;

Hello World!

/** A simple program that prints Hello, world! / public class myClass { /* Called to start program. */ public static void main(String[ ] args) { System. out.println("Hello, world!"); } } args is an array of String elements We explain static next week. Briefly : there is only one copy of procedure main, and it is not in any object