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Java Forum / General / May 2006instanceOf operator new bie question
I am using Java 1.4.2.x I wrote following code: NumberFormat df; df = NumberFormat.getInstance(); if (df instanceof DecimalFormat) { ((DecimalFormat) df).setMinimumIntegerDigits(1); ((DecimalFormat) df).setDecimalSeparatorAlwaysShown(true); } What does "df instanceof DecimalFormat" really mean? Does it mean that NumberFormat can be converted to DecimalFormat? Because my NumberFormat.getInstance() is returning me a NumberFormat and not DecimalFormat what else could it mean. I did read about instanceOf opeartor on google, but it is still confusing to me. Thanks a lot. Prem Well, in class inheritance if one class is on the same family tree or connected in someway to another Object or Interface, then that one class is an instance of that relative. In other words, if I have this class structure..... Object ^ Fruit ________|___________ ^ ^ Apple Citrus implements Squeezable ^ Orange and lets say that I create an orange object. That means that instance of will return true for Orange, Citrus, Squeezable, Fruit, and Object Orange o = new Orange; System.out.println (o instanceof Orange); //true System.out.println (o instanceof Citrus); //true System.out.println (o instanceof Squeezable); //true System.out.println (o instanceof Fruit); //true System.out.println (o instanceof Object); //true This would be false: System.out.println(o instanceof Apple); //false Because that's like comparing apples to oranges...pun intended. Thanks. But, it still have question: If I wrote code like: Fruit f = new Fruit; Syetem.out.println(f instanceof Orange); // will that be true. In case of DecimalFormat, it extends NumberFormat and not the other way around. java.text Class DecimalFormat java.lang.Object java.text.Format java.text.NumberFormat java.text.DecimalFormat > Thanks. But, it still have question: > If I wrote code like: > Fruit f = new Fruit; > Syetem.out.println(f instanceof Orange); // will that be true. Does not take long to write the snippet of code would it ? - but with your next question the answer to this one is significant....the answer will be false. > In case of DecimalFormat, it extends NumberFormat and not the other way > around. [quoted text clipped - 5 lines] > java.text.NumberFormat > java.text.DecimalFormat Yes, DecimalForm is a concrete subclass of NumberFormat (http://java.sun.com/j2se/1.4.2/docs/api/java/text/DecimalFormat.html) If you read the description carefull, the getInstance() factory method can return *any* subclass of NumberFormat - which means, when u call the getInstance method, you can be returned a DecimalFormat, or any other subclass.....hence the need to check whether you are really dealing with a DecimalFormat class... to explain it with analogy given above... let us say, i told you, you can eat a fruit, smell a fruit, squeeze a fruit...for all these operations on the fruit, you would not need to know, what fruit it is - unless ofcourse, you end up trying to squeeze a jack fruit hurting your hands ;) But, operation like peel the fruit - would not be applicable for strawberry, apples - but applicable for oranges - in which case, you would need to know, whether the fruit at hand, is really an apple or an orange before you can 'peel' it..... hence, the use of instanceOf.... -- Arvind Just to add, you can use DecimalFormat df = new DecimalFormat(); or DecimalFormat df = new DecimalFormat(pattern); If you know you want DecimalFormat straight up. I know I can use that, but some Java Documentation says. you should not create an instance of DecimalFormat directly. Yeah, it is a weird sentence too. I never quite understood the meaning of it.... "In general, do not call the DecimalFormat constructors directly, since the NumberFormat factory methods may return subclasses other than DecimalFormat. " I think they need some better documentation on this because calling NumberFormat factory methods has nothing to do with calling DecimalFormat constructors directly. > Yeah, it is a weird sentence too. I never quite understood the meaning > of it.... [quoted text clipped - 6 lines] > NumberFormat factory methods has nothing to do with calling > DecimalFormat constructors directly. What this is saying is that the "DecimalFormat" class may itself have secret subclasses that you don't know about. E.g: DecimalFormatSecretSubclassThatSpecializesInValuesBetweenZeroAndOne, or DecimalFormatSecretSubclassThatSpecializesInValuesBeyond4294967296. [*] So don't use the DecimalFormat constructor directly. Instead, use the factory methods of NumberFormat, which will decide the best subclass to use for the given situation and use that one. [*] As of Java 1.5.0_06, DecimalFormat does not have any secret subclasses, but Sun may change that in the future. - Oliver > [*] As of Java 1.5.0_06, DecimalFormat does not have any secret > subclasses, or if it does, they're really, really secret. >> [*] As of Java 1.5.0_06, DecimalFormat does not have any secret >> subclasses, > > or if it does, they're really, really secret. Right ;) I was basing my claim above by opening the JARs that Sun bundles with the JRE, and using Eclipse to build the class hierarchy diagram, and Eclipse says DecimalFormat has no subclasses. I also took a peek at the getInstance() source code, and it seems to always return DecimalFormat for now. - Oliver Oliver Wong schrieb: >>> [*] As of Java 1.5.0_06, DecimalFormat does not have any secret >>> subclasses, [quoted text clipped - 6 lines] > peek at the getInstance() source code, and it seems to always return > DecimalFormat for now. You can find subclasses using the java docs. Just click on "Use" to find all dependencies, including subsclasses. Example: http://java.sun.com/j2se/1.5.0/docs/api/java/text/class-use/NumberFormat.html Timo > Oliver Wong schrieb: >> [quoted text clipped - 12 lines] > all dependencies, including subsclasses. Example: > http://java.sun.com/j2se/1.5.0/docs/api/java/text/class-use/NumberFormat.html That would only show the publicly visible subclasses though. - Oliver >> Oliver Wong schrieb: >>> [quoted text clipped - 14 lines] > > That would only show the publicly visible subclasses though. In fact, only *documented* publicly visible subclasses, which coulf be a smaller set still. Mike Schilling schrieb: >> "Timo Stamm" <timo.stamm@arcor.de> wrote in message >>> You can find subclasses using the java docs. Just click on "Use" to find >>> all dependencies, including subsclasses. Example: >>> http://java.sun.com/j2se/1.5.0/docs/api/java/text/class-use/NumberFormat.html >> That would only show the publicly visible subclasses though. Sorry, I didn't read the thread and missed the point. > In fact, only *documented* publicly visible subclasses, which coulf be a > smaller set still. Yes, it only works with completely documented APIs like the JSE. Don't try it on my code :) Timo > > > [*] As of Java 1.5.0_06, DecimalFormat does not have any secret > > > subclasses, [quoted text clipped - 3 lines] > Right ;) I was basing my claim above by opening the JARs that Sun > bundles with the JRE, Not a safe guide in general -- the platform makes use of bytecode generation in a couple of places already[*], and who knows how far that might go... -- chris [*] java.lang.reflect.InvocationHandler and in the implementation of reflective access, and possibly others I don't know about. 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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