Question

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Which constructor is chosen when passing null?

I recently came across this curiosity while coding a few days back and can’t seem to figure out why the following happens:

Given the class below

public class RandomObject{
    public RandomObject(Object o){
        System.out.println(1);
    }
    public RandomObject(String[] s){ 
        System.out.println(2);
    }
}

When the call new RandomObject(null); is made the output is always 2 regardless of the order in which the constructors were created. Why does null refer to the string array rather than the object?

Answer 1

The key here is that Object is the super type of String[]

Java uses the most specific available method to resolve such cases. Null can be passed to both methods without compilation errors so Java has to find the most specific method here. The version with String[] is more specific – therefore it will be chosen for execution.