Why toString() on an Object instance (which is null) is not throwing NPE?

Question

Consider below one :

Object nothingToHold = null;

System.out.println(nothingToHold);  //  Safely prints 'null'

Here, Sysout must be expecting String. So toString() must be getting invoked on instance.

So why null.toString() works awesome? Is Sysout taking care of this?

EDIT : Actually I saw this weird thing with the append() of StringBuilder. So tried with Sysout. Both behave in the same way. So is that method also taking care?

Answer 1

PrintWriter's println(Object) (which is the method called when you write System.out.println(nothingToHold)) calls String.valueOf(x) as explained in the Javadoc:

/**
 * Prints an Object and then terminates the line.  This method calls
 * at first String.valueOf(x) to get the printed object's string value,
 * then behaves as
 * though it invokes <code>{@link #print(String)}</code> and then
 * <code>{@link #println()}</code>.
 *
 * @param x  The <code>Object</code> to be printed.
 */
public void println(Object x)

String.valueOf(Object) converts the null to "null":

/**
 * Returns the string representation of the <code>Object</code> argument.
 *
 * @param   obj   an <code>Object</code>.
 * @return  if the argument is <code>null</code>, then a string equal to
 *          <code>"null"</code>; otherwise, the value of
 *          <code>obj.toString()</code> is returned.
 * @see     java.lang.Object#toString()
 */
public static String valueOf(Object obj)

Answer 2

The PrintStream#println(Object s) method invokes the PrintStream#print(String s) method, which first checks is the if the argument is null and if it is, then just sets "null" to be printed as a plain String.

However, what is passed to the .print() method is "null" as String, because the String.valueOf(String s) returns "null" before the .print() method being invoked.

public void print(String s) {
    if (s == null) {
        s = "null";
    }
    write(s);
}