Why does a null value appear in string output?

Question

When I execute the following code the output is \"nullHelloWorld\". How does Java treat null?

import java.util.*;
import java.lang.*;
import java.io.*;

/* N         


        

Answer 1

my two cent:

    String str = null;
    str = str.concat("Hello World"); // Exception in thread "main" java.lang.NullPointerException

but

str += "Hello World";
System.out.println(str); // Hello World