Instantiating a generic class in Java

java generics
ripper234 picture ripper234 · Jul 7, 2009 · Viewed 186.7k times · Source

I know Java's generics are somewhat inferior to .Net's.

I have a generic class Foo<T>, and I really need to instantiate a T in Foo using a parameter-less constructor. How can one work around Java's limitation?

Answer

Jon Skeet picture Jon Skeet · Jul 7, 2009

One option is to pass in Bar.class (or whatever type you're interested in - any way of specifying the appropriate Class<T> reference) and keep that value as a field:

public class Test {
    public static void main(String[] args) throws IllegalAccessException,
            InstantiationException {
        Generic<Bar> x = new Generic<>(Bar.class);
        Bar y = x.buildOne();
    }
}

public class Generic<T> {
    private Class<T> clazz;

    public Generic(Class<T> clazz) {
        this.clazz = clazz;
    }

    public T buildOne() throws InstantiationException, IllegalAccessException {
        return clazz.newInstance();
    }
}

public class Bar {
    public Bar() {
        System.out.println("Constructing");
    }
}

Another option is to have a "factory" interface, and you pass a factory to the constructor of the generic class. That's more flexible, and you don't need to worry about the reflection exceptions.

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