Reference Type

[jnrohit Jain]

Hi,

I am reading book "Oracle Certified Associate Java SE 8 Programmer I Study Guide" and I came across with a problem when I was doing hands on practice of example given on page 25 related to "Reference Types".
I am using JDK 1.8.0_102 version. Below is the code and my question:-

Program 1:Program 1:

public class ReferenceType {
  public static void main(String[] args) {
    String reference = null;
      System.out.println("Value of reference: "+reference); 
  }
}

Result:
C:\>javac ReferenceType.java
C:\>java ReferenceType
Value of reference: null


Program 2:Program 2:

public class ReferenceType {
  public static void main(String[] args) {
    String reference = null;
      System.out.println("Value of reference: "+reference);
      System.out.println("Size of value in reference: "+reference.length());
  }
}

Result:
C:\>javac ReferenceType.java
C:\>java ReferenceType
Value of reference: null
Exception in thread "main" java.lang.NullPointerException
       at ReferenceType.main(ReferenceType.java:5)


Question: 1. Program 1 ran successfully. What value is stored in variable "reference"? String 'null' or Value null?
2. Why Program 2 is throwing runtime exception at line 5?

Thanks!

[Knute Snortum]

The variable reference has the special value of null.  println is smart enough to know this and print the String "null", but that does not mean that reference is a String.

[Tobias Bachert]

The null reference is stored in "reference".
String concatenation implicit performs String.valueOf, which itself does not invoke toString on null references but returns the char sequence "null" instead, thus program 1 runs successfully. In program 2 you try to invoke a method on the null reference, which leads to the NPE.

https://docs.oracle.com/javase/specs/jls/se8/html/jls-5.html#jls-5.1.11 wrote:Now only reference values need to be considered:
   If the reference is null, it is converted to the string "null" [...]
   Otherwise, the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments; but if the result of invoking the toString method is null, then the string "null" is used instead.  

[Dave Tolls]

It's not println() doing that.
It's the String concatenation.
It uses a StringBuilder, so the null Object is added using the append() method, which calls String.valueOf(), which does a special null check.

[Campbell Ritchie]

Welcome to the Ranch

I removed the line numbers from inside the code tags (thank you for using the tags) because line numbers are added automatically.
You can read about the behaviour of the + operator on Strings in the Java® Language Specification (=JLS). That contains a link to the same source that TB quoted. Beware: the JLS can be very difficult to read.is