当一个对象的jvm地址为0时，它是什么意思？

Question

I have an object, Mutation, that I have "new"ed. But when it prints out with toString(), the object says Mutation@0. That doesn't seem good to me. What might that mean?

Answer 1

That is Unsigned hexadecimal representation

As per Docs of toString() method in Object class

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

That zero in the sense the Hashcode not yet calculated.

Source Code:

   public String toString() {
237        return getClass().getName() + "@" + Integer.toHexString(hashCode());
238    }

Answer 2

If you look at the source code of Mutation , a good chance is you'll find this in there:

@Override public int hashCode() { 
   //TODO: implement this properly!
   return 0; 
}

The default implementation of toString() just reports this fact that hashCode returned 0.