Why does Boolean.class.newInstance() throw exception?

I think the sunject's pretty self-explanatory. I use JDK 1.6.0 update 26, and created a new project开发者_运维知识库 with just one line to confirm this:

Boolean.class.newInstance();

and it throws the following:

Exception in thread "main" java.lang.InstantiationException: java.lang.Boolean
    at java.lang.Class.newInstance0(Class.java:340)
    at java.lang.Class.newInstance(Class.java:308)

Does it suppose to fail? If so, why?

The Boolean class has two constructors, both take one argument. Calling Boolean.class.newInstance() is trying to call a zero-arg constructor that doesn't exist.

Given a class you can find out what it needs to be constructed:

Class cl = // initialize somehow
// get all constructors for class
Constructor[] constructors = cl.getConstructors(); 

 // for each constructor
for(Constructor c : constructors)
{
     // if there is a zero-parameter constructor
    if(c.getParameterTypes().length == 0)
    {
        // then we can safely create a constructor for it
        cl.newInstance(); 
    }
}

The Boolean class has two different constructors. Both of them take one argument. One of them takes a boolean itself but the other takes a String. If the string is NOT empty when you pass it to the Boolean class, it will be recognized as setting the Boolean to true. If it is null or contains nothing, then it is false.