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<br /><b>Lesson:</b> Classes and Objects
<br /><b>Section:</b> Classes
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<div class="linkLESSON"><a href="index.html">Classes and Objects</a></div>
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<div class="linkBHEAD"><a href="classdecl.html">Declaring Classes</a></div>
<div class="linkBHEAD"><a href="variables.html">Declaring Member Variables</a></div>
<div class="linkBHEAD"><a href="methods.html">Defining Methods</a></div>
<div class="nolinkBHEAD">Providing Constructors for Your Classes</div>
<div class="linkBHEAD"><a href="arguments.html">Passing Information to a Method or a Constructor</a></div>
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<div class="linkBHEAD"><a href="objectcreation.html">Creating Objects</a></div>
<div class="linkBHEAD"><a href="usingobject.html">Using Objects</a></div>
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<div class="linkBHEAD"><a href="returnvalue.html">Returning a Value from a Method</a></div>
<div class="linkBHEAD"><a href="thiskey.html">Using the this Keyword</a></div>
<div class="linkBHEAD"><a href="accesscontrol.html">Controlling Access to Members of a Class</a></div>
<div class="linkBHEAD"><a href="classvars.html">Understanding Class Members</a></div>
<div class="linkBHEAD"><a href="initial.html">Initializing Fields</a></div>
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<div id="PageTitle"><h1>Providing Constructors for Your Classes</h1></div>
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<!-- Providing Constructors for Your Classes -->
<p>A class contains constructors that are invoked to create objects from the class blueprint. Constructor declarations look like method declarations—except that they use the name of the class and have no return type. For example, <code>Bicycle</code> has one constructor:</p>
<div class="codeblock"><pre>
public Bicycle(int startCadence, int startSpeed, int startGear) {
gear = startGear;
cadence = startCadence;
speed = startSpeed;
}
</pre></div>
<p>To create a new <code>Bicycle</code> object called <code>myBike</code>, a constructor is called by the <code>new</code> operator:</p>
<div class="codeblock"><pre>
Bicycle myBike = new Bicycle(30, 0, 8);
</pre></div>
<p><code>new Bicycle(30, 0, 8)</code> creates space in memory for the object and initializes its fields.</p>
<p>Although <code>Bicycle</code> only has one constructor, it could have others, including a no-argument constructor:</p>
<div class="codeblock"><pre>
public Bicycle() {
gear = 1;
cadence = 10;
speed = 0;
}
</pre></div>
<p><code>Bicycle yourBike = new Bicycle();</code> invokes the no-argument constructor to create a new <code>Bicycle</code> object called <code>yourBike</code>.</p>
<p>Both constructors could have been declared in <code>Bicycle</code> because they have different argument lists. As with methods, the Java platform differentiates constructors on the basis of the number of arguments in the list and their types. You cannot write two constructors that have the same number and type of arguments for the same class, because the platform would not be able to tell them apart. Doing so causes a compile-time error.</p>
<p>You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of <code>Object</code>, which <i>does</i> have a no-argument constructor.</p>
<p>You can use a superclass constructor yourself. The <code>MountainBike</code> class at the beginning of this
lesson
did just that. This will be discussed later, in the
lesson
on interfaces and inheritance.</p>
<p>You can use access modifiers in a constructor's declaration to control which other classes can call the constructor.</p>
<div class="note"><hr /><strong>Note:</strong> If another class cannot call a <code>MyClass</code> constructor, it cannot directly create <code>MyClass</code> objects.
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