What is class and object in Java ?

<div dir="ltr" style="text-align: left;" trbidi="on"> <h2 style="text-align: left;"> <b><span style="font-family: Georgia, Times New Roman, serif; font-size: small; line-height: 115%;"><span style="color: #0b5394;">What is class and object?</span><o:p></o:p></span></b></h2> <div class="separator" style="clear: both; text-align: center;"> <a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjM7LRFt46JAk-xJizFxui1htbhKoQDx_aky9ACaYGZk1howlFo4t_wnHwAhzbp7t_0RSrHBSBwmdTiLdWxQFKCKmgGWccEfCI0e9MVwF29Tm1PJwh7UDj13e0iyWkzvgiBivVS9zd3jLc/s1600/java_object_class_difference.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: Georgia, Times New Roman, serif;"><img alt="What is class and object in Java" border="0" height="218" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjM7LRFt46JAk-xJizFxui1htbhKoQDx_aky9ACaYGZk1howlFo4t_wnHwAhzbp7t_0RSrHBSBwmdTiLdWxQFKCKmgGWccEfCI0e9MVwF29Tm1PJwh7UDj13e0iyWkzvgiBivVS9zd3jLc/s400/java_object_class_difference.png" title="What is class and object in Java" width="400" /></span></a></div> <div class="separator" style="clear: both; text-align: center;"> <span style="font-family: Georgia, Times New Roman, serif;"><br /></span></div> <div class="MsoNormal" style="margin-left: .5in; mso-list: l1 level1 lfo1; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <span style="font-family: Georgia, Times New Roman, serif;"><br /></span></div> <div class="MsoNormal" style="margin-left: .5in; mso-list: l1 level1 lfo1; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <span style="font-family: Georgia, Times New Roman, serif;"><span style="text-align: left; text-indent: -0.25in;"><ol style="text-align: left;"> <li><span style="text-indent: -0.25in;">  </span><span style="text-indent: -0.25in;">A class is a <b>template from which objects are created</b>. That is objects are instance of a class.</span></li> <li><span style="text-indent: -0.25in;"> </span><span style="text-indent: -0.25in;">When you create a class, you are creating a new data-type. You can use this type to declare objects of that type.</span></li> <li><span style="text-indent: -0.25in;"> </span><span style="text-indent: -0.25in;">Class defines structure and behavior (data & code) that will be shared by a set of objects</span></li> </ol> </span><span style="text-align: left; text-indent: -0.25in;">A class is declared by use of the </span><b style="text-align: left; text-indent: -0.25in;">class keyword</b><span style="text-align: left; text-indent: -0.25in;">. Classes may contain data and code both.</span><br /><span style="text-align: left; text-indent: -0.25in;">The general form of al class definition:</span></span><ul style="text-align: left;"> </ul> </div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">            class ClassName<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">            {<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                        type instance variable1;<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                        type instance variable2;<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                        <o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                        type methodname1 (parameter list)<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                        {<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                                    body of method;<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                        }<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                        type methodname2 (parameter list)<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                        {<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                                    body of method;<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">                        }<o:p></o:p></span></div> <div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> <span style="color: #1d1d1e; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial;">             }<o:p></o:p></span></div> <div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;"><br /></span></div> <div class="MsoNormal" style="margin-left: .5in; mso-list: l0 level1 lfo2; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <!--[if !supportLists]--><span style="font-family: Georgia, Times New Roman, serif;">v  <!--[endif]-->The data or variable are called <b>instance variable</b>. The code is contained within methods.<o:p></o:p></span></div> <div class="MsoNormal" style="margin-left: .5in; mso-list: l0 level1 lfo2; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <!--[if !supportLists]--><span style="font-family: Georgia, Times New Roman, serif;">v  <!--[endif]-->The method and variables defined within a class are called <b>member of the class</b>.<o:p></o:p></span></div> <div class="MsoNormal" style="margin-left: .5in; mso-list: l0 level1 lfo2; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <span style="font-family: Georgia, Times New Roman, serif;"><br /></span></div> <h3 style="text-align: left;"> <span style="color: #0b5394; font-family: Georgia, Times New Roman, serif; font-size: small;">Example of Class</span></h3> <div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;"><br /></span></div> <pre class="java" name="code"><span style="font-family: Georgia, Times New Roman, serif;">class Box { double width; double height; double depth; } class BoxDemo { public static void main (String args[]) { Box mybox = new Box (); double vol; mybox.width =10; mybox.height = 20; mybox.depth = 30; vol = mybox.width * mybox.height * mybox.depth; System.out.println (“Volume is: - “+vol); } } </span></pre> <pre class="java" name="code"></pre> <pre class="java" name="code"><div class="MsoNormal" style="mso-layout-grid-align: none; text-align: justify; text-autospace: none;"> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="margin-left: .5in; mso-list: l4 level1 lfo5; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <ul style="text-align: left;"><span style="font-family: Georgia, Times New Roman, serif;"> </span> <li><span style="font-family: Georgia, Times New Roman, serif;">Each object contains its own copy of each variable defined by the class. </span></li> <span style="font-family: Georgia, Times New Roman, serif;"> </span></ul> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="margin-left: .5in; mso-list: l4 level1 lfo5; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <ul style="text-align: left;"><span style="font-family: Georgia, Times New Roman, serif;"> </span> <li><span style="font-family: Georgia, Times New Roman, serif;"> So, every Box object contains its own copy of the instance variables width, height and depth. </span></li> <span style="font-family: Georgia, Times New Roman, serif;"> </span></ul> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="margin-left: .5in; mso-list: l4 level1 lfo5; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <ul style="text-align: left;"><span style="font-family: Georgia, Times New Roman, serif;"> </span> <li><span style="font-family: Georgia, Times New Roman, serif;">To access these variables, you will use the <b>dot (.) operator</b>. The dot operator links the name of the object with the name of an instance variable. </span></li> <span style="font-family: Georgia, Times New Roman, serif;"> </span></ul> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="margin-left: .5in; text-align: justify;"> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <h4> <span style="font-family: Georgia, Times New Roman, serif;"> <b><span style="color: #0b5394;">Declaring Object</span></b><span style="color: #0b5394;"> </span><o:p></o:p></span></h4> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="margin-left: .5in; mso-list: l0 level1 lfo1; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <!--[if !supportLists]--><span style="font-family: Georgia, Times New Roman, serif;">•        Box mybox;     // declare ref. to object which contains null value.<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="margin-left: .5in; mso-list: l0 level1 lfo1; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <!--[if !supportLists]--><span style="font-family: Georgia, Times New Roman, serif;">•        mybox = new Box ();   // allocate a Box object.<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> <span style="color: #0b5394;"> </span></span><div class="MsoNormal" style="text-align: justify;"> <h4> <span style="font-family: Georgia, Times New Roman, serif;"> <span style="color: #0b5394;"><b>General form of a new</b> <o:p></o:p></span></span></h4> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">            class var = new classname ();<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">            mybox = new Box ();<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">Where:<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">            class var =  variable of the class type.<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">            classname = name of the class.<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="margin-left: .5in; mso-list: l2 level1 lfo6; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <div style="text-align: left;"> <span style="font-family: Georgia, 'Times New Roman', serif;"> </span></div> <span style="font-family: Georgia, 'Times New Roman', serif; text-align: left; text-indent: -0.25in;"><div style="text-align: left;"> <span style="text-indent: -0.25in;"> </span><span style="text-indent: -0.25in;">The classname followed by parentheses specifies the constructor for the class.</span></div> </span><span style="font-family: Georgia, Times New Roman, serif;"><div style="text-align: left;"> </div> <div style="text-align: left;"> </div> </span></div> <div class="MsoNormal" style="margin-left: .5in; mso-list: l2 level1 lfo6; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <span style="font-family: Georgia, 'Times New Roman', serif; text-align: left; text-indent: -0.25in;"><div style="text-align: left;"> <span style="text-indent: -0.25in;">A constructor defines what occurs when an object of a class is created.</span></div> </span><div style="text-align: left;"> </div> <span style="font-family: Georgia, 'Times New Roman', serif; text-align: left; text-indent: -0.25in;"><div style="text-align: left;"> <span style="text-indent: -0.25in;"> </span><span style="text-indent: -0.25in;">Most classes explicitly define their own constructors within their class definition but if no explicit constructor is specified then java will automatically supply a default constructor.</span></div> </span></div> <div class="MsoNormal" style="margin-left: .5in; mso-list: l2 level1 lfo6; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <div style="text-align: left;"> <span style="font-family: Georgia, 'Times New Roman', serif;"> </span></div> <span style="font-family: Georgia, 'Times New Roman', serif; text-align: left; text-indent: -0.25in;"><div style="text-align: left;"> <span style="text-indent: -0.25in;"> </span><span style="text-indent: -0.25in;">This is the case with Box. This is default constructor. </span></div> </span><ul style="text-align: left;"><span style="font-family: Georgia, Times New Roman, serif;"> </span></ul> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <h4> <span style="font-family: Georgia, Times New Roman, serif;"> <b><span style="color: #0b5394;">Assigning Object Reference Variable</span></b><span style="color: #0b5394;"> </span></span></h4> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">Box b1 = new Box ();</span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">Box b2 = new b1;<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="margin-left: .5in; mso-list: l1 level1 lfo2; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <ul style="text-align: left;"><span style="font-family: Georgia, Times New Roman, serif;"> </span></ul> <span style="font-family: Georgia, 'Times New Roman', serif; text-align: left; text-indent: -0.25in;"> After this executes, b1 and b2 will both refer to the same object. </span><br /> <ul style="text-align: left;"> </ul> </div> <div class="MsoNormal" style="margin-left: .5in; mso-list: l1 level1 lfo2; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <span style="font-family: Georgia, 'Times New Roman', serif; text-align: left; text-indent: -0.25in;">The assignment of b1 to b2 did not allocate any memory or copy any part of the original object.</span> <ul style="text-align: left;"> </ul> </div> <div class="MsoNormal" style="margin-left: .5in; mso-list: l1 level1 lfo2; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <span style="font-family: Georgia, 'Times New Roman', serif; text-align: left; text-indent: -0.25in;"> </span><span style="font-family: Georgia, 'Times New Roman', serif; text-align: left; text-indent: -0.25in;">It simply makes b2 refer to the same object as does b1. Thus, any changes made to the object through b2 will affect the object to which b1 is referring,since they are the same object. </span> <ul style="text-align: left;"><span style="font-family: Georgia, Times New Roman, serif;"> </span></ul> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><h3 style="text-align: left;"> <span style="font-family: Georgia, Times New Roman, serif; font-size: small;"> <b><span style="color: #0b5394;">Introducing Methods</span></b><span style="color: #0b5394;"> </span></span></h3> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">type name (parameter-list)<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">{<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">            body of method<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">} <o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;"> </span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;">Where:</span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;"><b>Type : </b>Specifies the type of data returned by the method.If the method does not return a value its ret urn type must be void. <o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;"><b>Name: </b>Specifies the name of the method. <o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;"><b>Parameter-list: </b>It is a sequence of type & identifiers pairs separated by commas.<o:p></o:p></span></div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="margin-left: .5in; mso-list: l5 level1 lfo3; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <ul style="text-align: left;"><span style="font-family: Georgia, Times New Roman, serif;"> </span></ul> <span style="font-family: Georgia, 'Times New Roman', serif; text-align: left; text-indent: -0.25in;">Parameters are variables that receive the value of the argument passed to the method when it is called. If the method has no parameters, then the parameter list will be empty.</span> <ul style="text-align: left;"> </ul> </div> <div class="MsoNormal" style="margin-left: .5in; mso-list: l5 level1 lfo3; tab-stops: list .5in; text-align: justify; text-indent: -.25in;"> <span style="font-family: Georgia, Times New Roman, serif; text-align: left; text-indent: -0.25in;"> Methods that have a return type other than void return a value to the calling routine </span><span style="font-family: Georgia, 'Times New Roman', serif; text-align: left; text-indent: -0.25in;">using the following form of the return statement;</span> <ul style="text-align: left;"><span style="font-family: Georgia, Times New Roman, serif;"> </span></ul> </div> <span style="font-family: Georgia, Times New Roman, serif;"> </span><div class="MsoNormal" style="text-align: justify;"> <span style="font-family: Georgia, Times New Roman, serif;"><b>Return value;</b> </span></div> <h3 style="text-align: left;"> </h3> <h3 style="text-align: left;"> <span style="font-family: Georgia, Times New Roman, serif; font-size: small;"><b><span style="color: #0b5394;">Types of Methods</span></b><span style="color: #0b5394;"> </span> </span></h3> <span style="font-family: Georgia, Times New Roman, serif;"> </span><ol start="1" style="margin-top: 0in;" type="1"><span style="font-family: Georgia, Times New Roman, serif;"> </span> <li class="MsoNormal" style="mso-list: l3 level1 lfo4; tab-stops: list .5in; text-align: justify;"><span style="color: #444444; font-family: Georgia, Times New Roman, serif; mso-bidi-font-family: Arial; mso-bidi-font-weight: bold;">Does not return value – void</span></li> <span style="font-family: Georgia, Times New Roman, serif;"> </span> <li class="MsoNormal" style="mso-list: l3 level1 lfo4; tab-stops: list .5in; text-align: justify;"><span style="color: #444444; font-family: Georgia, Times New Roman, serif;">Returning a value <o:p></o:p></span></li> <span style="font-family: Georgia, Times New Roman, serif;"> </span> <li class="MsoNormal" style="mso-list: l3 level1 lfo4; tab-stops: list .5in; text-align: justify;"><span style="color: #444444; font-family: Georgia, Times New Roman, serif;">Method which takes parameter <o:p></o:p></span></li> <span style="font-family: Georgia, Times New Roman, serif;"> </span></ol> <span style="font-family: Georgia, Times New Roman, serif;"> </span></pre> <br /></div>

What is class and object in Java ?

What is class and object?    A class is a template from which objects are created . That is objects are instance of a class.   W...

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