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Java Forum / General / November 2005[noob]Doubt regarding Interfaces
Can someone please explain the following code snippet I found on the Swing tutorial site. Thanks! Runnable doHelloWorld = new Runnable() { public void run() { System.out.println("Hello World on " + Thread.currentThread()); } }; Is doHelloWorld now an object or is it an abstract class or is it something altogether different? > Can someone please explain the following code snippet I found on the > Swing tutorial site. Thanks! [quoted text clipped - 8 lines] > Is doHelloWorld now an object or is it an abstract class or is it > something altogether different? All variables are either primitives or a reference to an Object. If they are a reference to an Object as in the case above then they can have a type of 1 or many. In the example above doHelloWorld is a reference to a concrete Object with an interface Type of Runnable. It has been created using an Annoymous class, since there is no class declaration. If the class was not annoymous then the Object would also have the Type of that class. Runnable // the Type you are assigning doHelloWorld // the variable you are assigning it to = // assignment new // instantiation Runnable() {...} // annoymous class definition ... // contents of class this bit : Runnable() {...} // annoymous class definition you can think of expanding to: private class AnnoymousClass implements Runnable { ... } HTH -- Mike W > If they are a reference to an Object as in the case above then they can have > a type of 1 or many. If they are a reference to an Object as in the case above then they can have 1 or many types. Yes, all at the same time. Google "run time type identification" and "polymorphism" -- Mike W. >>Can someone please explain the following code snippet I found on the >>Swing tutorial site. Thanks! [quoted text clipped - 12 lines] > If they are a reference to an Object as in the case above then they can have > a type of 1 or many. I'm not sure what you mean by that, but perhaps it's that the class of an object can be compatible with any positive number of types. It is necessary here to be clear about the difference between class and type. Every object has exactly one class. Every variable and expression has exactly one type. A non-primitive types is expressed in terms of class names, and it serves the purpose of describing to the compiler the possible classes of the object that its value (a reference) refers to. > In the example above doHelloWorld is a reference to a concrete Object with > an interface Type of Runnable. More precisely, doHelloWorld is a variable of type Runnable, which as the result of the assignment holds a reference to an object whose class implements Runnable and has Object as its direct superclass. > It has been created using an Annoymous class, since there is no class > declaration. Quibbling a bit here, but there *is* a class declaration -- an anonymous one. The syntax is a bit different from that for a named class declaration, but that doesn't make it any less a declaration. > If the class was not annoymous then the Object would also have > the Type of that class. Sorry, but that's nonsense. In the first place, Object is a superclass of the anonymous class, therefore references to its instances are assignment-compatible with type Object. *Every* class has Object as a superclass, and therefore *every* reference is assignment-compatible with type Object. In the second place, classes do not have types. They have superclasses, and they have interfaces implemented; these determine which types their instances are compatible with. They do not have types. > Runnable // the Type you are assigning Runnable // the type of ... > doHelloWorld // the variable you are assigning it to doHelloWorld // the variable you are assigning a value to > = // assignment > new // instantiation > Runnable() {...} // annoymous class definition Which one could also call an anonymous class declaration. [...] > this bit : > Runnable() {...} // annoymous class definition [quoted text clipped - 4 lines] > ... > } Yes, that's quite right. More right, in fact, than the idea that the class in question really has no name. The Java source code assigns no name to it, but the Java compiler invents one at compile time. The VM doesn't have any concept of a class that doesn't have a name; "anonymous class" really means something more like "class whose name I don't know and don't care about."  SignatureJohn Bollinger jobollin@indiana.edu Thanks everyone for your replies! I have a basic question - in the above instance Runnable is an interface and interfaces cannot be instantiated. So something like 'new Runnable()' seems puzzling to me. Is this syntax for instantiating an object of the Anonymous class? Also, is the above snippet similar to the following in the end result: class ac implements Runnable{ public void run() { System.out.println("Hello World on " + Thread.currentThread()); } }; ac doHelloWorld = new ac(); mmu2643@gmail.com wrote in news:1131466583.776932.254490 @g47g2000cwa.googlegroups.com: > Thanks everyone for your replies! > [quoted text clipped - 13 lines] > > ac doHelloWorld = new ac(); That's basically it yes. new Runnable() {...} creates a new anonymous class that implements interface Runnable and has whatever variables/methods you define between the brackets. This is often used to implement a simple event listener, for example for a button: Button b = new Button(); b.addActionListener(new ActionListener() { JOptionFrame.showMessageDialog("button clicked"); }); Note how the whole class definition is inside the parentheses of the addActionListener method, and it is ended - like any statement - with a semi-colon. > b.addActionListener(new ActionListener() > { public void actionPerformed(ActionEvent event) { > JOptionFrame.showMessageDialog("button clicked"); } > }); > > Note how the whole class definition is inside the parentheses of the > addActionListener method, and it is ended - like any statement - with a > semi-colon. Well, the class ends with the closing }. The enclosing expression carries on as normal. Tom Hawtin SignatureUnemployed English Java programmer http://jroller.com/page/tackline/ > Runnable doHelloWorld = new Runnable() { > public void run() { [quoted text clipped - 5 lines] >Is doHelloWorld now an object or is it an abstract class or is it >something altogether different? it is an instance of an inner anonymous class. see http://mindprod.com/jgloss/anonymousclasses.html SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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