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<a href="tutorial.html" class="part-prev">Contents</a>
<a href="tutorial.html" class="part-toc">BlueJ Tutorial: Part 1</a>
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<h2>Object Bench and Method Invocation</h2>
<p>This tutorial comes with two classes supplied: Cat and CatView. Classes are represented by the yellow-ish boxes in the main window: for example, <a href="guicss:.class-target-id-Cat">click on this link to highlight the Cat class</a>. (Clicking on any such link in this tutorial will highlight the item on screen that they refer to.)</p>
<p>We will return to compilation later, but to begin with, click <a href="guicss:.compile-button">the Compile button</a> in the main window to make sure our classes are compiled and ready to use.</li></p>
<h3>Creating an object from a class</h3>
<p>Let's create an instance of one of our classes: the Cat class. To create an instance, right-click on <a href="guicss:.class-target-id-Cat">the Cat class</a>. (If you are on Mac and don't have a right click, hold control and click whenever this tutorial says to right-click.) Then select the <span class="menu">new Cat()</span> option from the menu. A dialog will appear asking for a name for the object. This name is unrelated to the Java code; it is just a name that BlueJ will use to distinguish one object from another. Leave it as <span class="entry">cat1</span>, and click OK.</p>
<h3>Invoking an object's methods</h3>
<p>You should now have a red cat1 object in the bottom part of the BlueJ window that we call <a href="guicss:.object-bench">the object bench</a>. The cat gets a random name and color, which you can find out by right-clicking on the <a href="guicss:.object-wrapper">red cat1 object</a> and selecting <span class="menu">String getDescription()</span>. This will show a pop-up with details of the cat. (You may need to resize the window to be a bit wider to see it all.)</p>
<p>You can change the color of the cat using the setColor method. To invoke this, right-click on the <a href="guicss:.object-wrapper">cat1 object</a> and select <span class="menu">void setColor(String color)</span>. A dialog will appear asking for the method's parameter. You must use double-quotes around the value because it is a string: enter <span class="entry">"black and white"</span> for example, and click OK. If you then call getDescription() again, you will see that the description will change.</p>
<h3>View an object's state</h3>
<p>You can see the internal details of the cat object by right-clicking on <a href="guicss:.object-wrapper">the object</a> and selecting <span class="menu red">Inspect</span>. This shows a window with the fields of the object and their values. The cat object is fairly straightforward, it has a field containing the name, one containing the color, and another showing if it's been fed.</p>
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