Java Generics Warning

[aanal jethalia]

Hi

I have a class which has a static method that takes a String classname and returns the instance of type class. My method looks like:

public static <T extends Object> T getClassInstance(String className) throws Exception{
T instanceToReturn = null;

try {
Class c = Class.forName(className);
instanceToReturn = (T)c.newInstance();
} catch (InstantiationException e) {
throw e;
} catch (IllegalAccessException e) {
throw e;
} catch (ClassNotFoundException e) {
throw e;
}
return instanceToReturn;
}

I get the following warning:

Type safety: The cast from Object to T is actually checking against the erased type Object. Should I be worried about this warning? How can I avoid it?

I am very new to Generics, so am not sure if this is the correct way to use it. Any help would be great.

Thanks
Aanal

[Ilja Preuss]

Sadly, the only way to get rid of that warning is by adding a @SuppressWarnings annotation to the method.

[Jim Yingst]

I don't think there's anything sad about it. Ultimately, you get a warning because within that method, there's really no way to know that the String argument passed in will be compatible with the type T which must be inferred from the return type. If someone calls this method with

String x = getClassInstance("java.lang.Integer");

it will fail, period. Whereas

String x = getClassInstance("java.lang.String");

works fine. Only the calling code can determine if this will work; thus, it's in the calling code that the decision should be made as to whether to cast or not. Best to just drop the generic type T, and declare the method to return an Object. That's the only thing you can guarantee from within the method. Let the caller decide whether a cast is appropriate.

[Jesper de Jong]

I have a different remark on your code:

try {
    // ...
} catch (InstantiationException e) {
throw e;
} catch (IllegalAccessException e) {
throw e;
} catch (ClassNotFoundException e) {
throw e;
}

Why are you catching all those exceptions if you're immediately re-throwing them? You don't need the try-catch block, your code looks cleaner and shorter if you leave it off:

public static <T extends Object> T getClassInstance(String className) throws Exception {
    T instanceToReturn = null;
 
    Class c = Class.forName(className);
    instanceToReturn = (T)c.newInstance();
 
     return instanceToReturn;
}

Or you could make it even shorter:

public static <T extends Object> T getClassInstance(String className) throws Exception {
    return (T) Class.forName(className).newInstance();
}

[aanal jethalia]

I wrap the Exception with my application specific Exception and throw it again. Thats why I catch and throw.

Thanks for your help.