char and int array difference

Question

When I try to print the uninitialized static char array it gives run time error (Null pointer exception) whereas the uninitialized static int array gives null value. Why?

public class abc {
    static int arr[];
    static char ch[];

    public static void main(String[] args) {
        System.out.println(ch); //it gives null pointer exception at run time
        System.out.println(arr); //it gives output as "null".
        }
    }

Answer 1

System.out is an instance of PrintStream and this class has a few overloaded println methods. In your case:

1. System.out.println(ch); is using public void println(char x[])
2. System.out.println(arr); is using public void println(Object x) (there is no public void println(int[] x) method so the closest available type for int[] which println can use is Object).

The second method is using

String.valueOf(x);

to get a string representation of the object we want to print and the code of the valueOf method looks like

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

so it is null-safe (will return the string "null" if the reference holds null).

The first method is at some level using

public void write(char cbuf[]) throws IOException {
    write(cbuf, 0, cbuf.length);
                 //    ^^^^^^^ this throws NPE
}

and because cbuf is null cbuf.length will throw NullPointerException because null doesn't have length (or any other) field.

Answer 2

The answer exists in the PrintWriter source code (of which System.out is an instance).

Start with the fact that the uninitialized arrays, as reference variables, are given the default of null.

The println(char[]) (eventually) attempts to call .length on the passed in array. It's null, resulting in the NullPointerException. println(char[]) (eventually) calls write(char[]):

public void write(char buf[]) {
    write(buf, 0, buf.length);
}

There is no overload of println matching int[], but there is a println(Object). There it (eventually) attempts String.valueOf, passing the null reference, so String.valueOf takes the null and returns the String "null". println(Object) calls print(Object):

public void print(Object obj) {
    write(String.valueOf(obj));
}