How can I create an instance of type dynamically in Java

Question

In my Java application I have method

public <T extends Transaction> boolean appendTransaction(T transaction) {
   ...
}

and inside of this method I need to create an instance of object T which extends Transaction

Is it correct to do it in this way

T newTransaction = (T) transaction.getClass().newInstance();

crunchdog · Accepted Answer · 2009-10-09 08:47:08Z

5

I think you should use a factory-interface of type T, that way you can force a create-instance interface on the method-user.

public <T extends Transaction> boolean appendTransaction(
        T transaction, 
        Factory<T> factory) {
    ...
    T newTransaction = factory.createTransaction();
    ...
}

answered Oct 9, 2009 at 8:47

crunchdog

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1 Comment

gustafc Over a year ago

I sorta agree with you here. It's definitely cleaner and less magical.

gustafc · Accepted Answer · 2009-10-09 08:37:13Z

2

More or less, except that Class.newInstance is evil:

Note that this method propagates any exception thrown by the nullary constructor, including a checked exception. Use of this method effectively bypasses the compile-time exception checking that would otherwise be performed by the compiler.

Use transaction.getClass().getConstructor().newInstance() instead, it wraps exceptions thrown in the constructor with an InvocationTargetException.

answered Oct 9, 2009 at 8:37

gustafc

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2 Comments

Bogdan Gusiev Over a year ago

Is more evil then just an alias for getConstructor().newInstance() ?

gustafc Over a year ago

I updated with a motivation. It magically rethrows any exceptions thrown by the default constructor, even checked exceptions which you then can't catch explicitly (because Class.newInstance doesn't declare them in its signature).

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