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Java Forum / First Aid / January 2007How do I get an instance of a class given its name?
If I know the name of a class, is it possible to get an instance using just the name? Thanks,  SignatureKnute Johnson email s/nospam/knute/ And thus spoke Knute Johnson... > If I know the name of a class, is it possible to get an instance using > just the name? Example: Your class is called "Test" in the package "test": Test t = (Test)Class.forName("test.Test").newInstance(); ...of course, you'll need some try/catch for this. Flo > And thus spoke Knute Johnson... > [quoted text clipped - 8 lines] > > Flo So how then do I pass a parameter to the constructor? Thanks, SignatureKnute Johnson email s/nospam/knute/ And thus spoke Knute Johnson... > So how then do I pass a parameter to the constructor? Class[] c = { Integer.class, String.class }; Object[] o = { new Integer(4), "Hello" }; Test t = (Test)Class. forName("test.Test"). getConstructor(c). newInstance(o); For... public Test(Integer i, String s) {...} Flo > And thus spoke Knute Johnson... > [quoted text clipped - 12 lines] > > Flo Thanks Flo, that worked like a charm! SignatureKnute Johnson email s/nospam/knute/ >> And thus spoke Knute Johnson... >> [quoted text clipped - 14 lines] > > Thanks Flo, that worked like a charm! This seems to be a bit silly. In the code above, you say: Test t = (Test)Class.forName(...)..... Why not just Test t = new Test(...); That is, since you declare 't' to be of type Test, you already must have information about that class from some import. However, if 't' is known to be of type Mytype or some possibly unknown subclass of MyType, then it might make sense to use: Class c = Class.forName( name ); MyType t = (MyType)c.newInstance(); or MyType t = (MyType )c.getConstructor(...).newInstance(...); where MyType is known from some import. But I would hope that you would check 'c' to ensure it is indeed a MyType before you try creating an instance of it. SignatureFred L. Kleinschmidt Boeing Associate Technical Fellow Technical Architect, Software Reuse Project And thus spoke Fred Kleinschmidt... > This seems to be a bit silly. In the code above, you say: Nope. That was me. Not him. > Test t = (Test)Class.forName(...)..... > > Why not just > Test t = new Test(...); Because sometimes you just have the name of the class as a String. > That is, since you declare 't' to be of type Test, you already > must have information about that class from some import. Of course, it's just a simplified example, nothing more. :-) Flo > And thus spoke Fred Kleinschmidt... > [quoted text clipped - 8 lines] > > Because sometimes you just have the name of the class as a String. But then you do not know beforehand that the string is "Test", so how can you know that it is legal to cast it to a Test? And if you do klnow beforehand that it is guaranteed to be a Test, why not just use test t = new Test(); and dispense with parsing a name that you already know will be "Test". My comment was to show that you can create a Test instance only if you know that the argument to forName() is the name of the Test class or one of its subclasses. Thus you use Class c = Class.forName() and check the results to see if it really is a Test. Otherwise your code may throw an illegal cast exception. >> That is, since you declare 't' to be of type Test, you already >> must have information about that class from some import. > > Of course, it's just a simplified example, nothing more. :-) SignatureFred L. Kleinschmidt Boeing Associate Technical Fellow Technical Architect, Software Reuse Project And thus spoke Fred Kleinschmidt... > But then you do not know beforehand that the string is "Test", > so how can you know that it is legal to cast it to a Test? [quoted text clipped - 3 lines] > and dispense with parsing a name that you already know will > be "Test". Normaly you will not randomly create instances of classes :-) So it's more likely, that you get the name of a class as a string, for example, that is supposed to implement, for example, a specific interface. > My comment was to show that you can create a Test instance > only if you know that the argument to forName() is the name of > the Test class or one of its subclasses. Of course you have to test many details. But again: It's just a simplified example, I didn't write the exception and I didn't think it was needed to specify all tests. I am rather sure, that Knute didn't want to do it like Test t = ... :-) Flo 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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