Dynamic object creation in java using reflection

Question

What does Class[] null and Object[] null mean in the code below?

public Object selfFactory() {
    Class cl = this.getClass();
    Constructor cons;
    cons = cl.getConstructor((Class[]) null);
    return cons.newInstance((Object[]) null);
}

First statement casts it to type Class and second statement casts null to type Object - You can cast null to any reference type without getting any exception. — Infinite Recursion
– Infinite Recursion, Commented Oct 13, 2014 at 10:42

Community · Accepted Answer · 2017-05-23 12:05:50Z

4

cl.getConstructor((Class[]) null); takes the parameter for both its value and type as a type parameter <T>, so casting is necessary if we're supplying a null Class[] instead of a null Object for method invocation of .getConstructor().

Similarly, cons.newInstance((Object[]) null); creates a new instance of cl by invoking a constructor cons which takes an Object[], given as null in this case. Without the typecast, a null Object will be passed.

This is useful for when methods or constructors are overloaded. Invocation with null without cast will be of type Object and often ambiguous:

{
    eat(null); // ???
}

void eat(int[] arr);
void eat(Class arr);

With a cast, the correct constructor or method can be called.

{
    eat((Class) null);
}

Also as documented by polygenelubricants in How to work with varargs and reflection,

[...] invoke(Object obj, Object... args) makes it tricky if you need to pass an array of reference type as an only argument, because it is cast-able to Object[], even though it should be wrapped inside a new Object[1] instead.

You can do:
m.invoke(this, new Object[] {a}); // Bohzo's solution
This bypasses the vararg mechanism. More succinctly you can also do:
m.invoke(this, (Object) a);
The cast to Object makes the vararg mechanism do the work of creating the array for you.

The trick is also needed when passing a null as an argument to varargs, and has nothing to do with reflection.
public void foo(String... ss) {
    System.out.println(ss[0]);
}

    foo(null); // causes NullPointerException
    foo((String) null); // prints "null"

edited May 23, 2017 at 12:05

CommunityBot

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answered Oct 13, 2014 at 10:41

Unihedron

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5 Comments

mike_x_ Over a year ago

so if i have myType(), myType(int x) and myType(Object x) as constructors in myType class, how can i modify the code to call the 2nd one with the int parameter?

Vogel612 Over a year ago

@Unihedron 0 is not really a valid Class :( Instead cl.getConstructor(Integer.class) should do the trick. alternatively cl.getDeclaredConstructors()[1]

Unihedron Over a year ago

Thanks Vog I was looking at the wrong thing. @mike_x_ ^

mike_x_ Over a year ago

and what should i pass as a param in newinstance() if i have one int parameter?

Unihedron Over a year ago

@mike An int value equal to what you're calling it with.

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