Why doesn't System.out.println() throw NullPointerException?

Question

This might be a very basic question, but I still don't know the answer.

String abc = null;    
System.out.println(abc);

Why does System.out.println print "null" and does not throw NullPointerException?

Answer 1

It's behaving as it's documented to. PrintStream.println(String) is documented as:

Prints a String and then terminate the line. This method behaves as though it invokes print(String) and then println().

PrintStream.print(String) is documented as:

Prints a string. If the argument is null then the string "null" is printed. Otherwise, the string's characters are converted into bytes according to the platform's default character encoding, and these bytes are written in exactly the manner of the write(int) method.

When in doubt, read the documentation :)

Answer 2

Because it eventually reaches the print method, which prints "null" for a null String :

public void println(String x) {
    synchronized (lock) {
        print(x);
        println();
    }
}

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

The same behavior exists for printing of any null reference (in this case the "null" String is returned by String.valueOf(null) :

public void println(Object x) {
    String s = String.valueOf(x);
    synchronized (lock) {
        print(s);
        println();
    }
}

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}