Tuesday, November 10, 2015

I tried to print my array but all I got was gobbledegook!

This is inspired by a post at /r/javahelp.

A user posted that they were trying to print a sorted array to the console with System.out,println, and that instead of anything that made sense they got [I@4969dd64...  So clearly something was wrong and the array was not being sorted.

But that's faulty troubleshooting, as one could tell by just trying to print the array before sorting.  You would see basically the same thing.

The reason for this is that when you do a System.out.println on any object, what you're actually doing is calling 'toString()'.  If you are printing your own objects for which you've created nice methods that override the default toString, you get nicely formatted output.  If you're using an ArrayList you get a nice, comma separated list enclosed in square brackets.

Arrays are more primitive than that.  Their default toString is just the one from Object, and it does nothing more than tell you where in the heap the object is located.  This is good for verifying that something isn't null, but useless for determining the content.

If you want to see the contents of the array, you've gotta do the work yourself, my friend.  And it's not difficult at all.  This, for instance, will work for any array of Objects that themselves support a reasonable version of 'toString':

public static String arrayToString(Object[] theArray) {
    StringBuilder output = new StringBuilder("[");
    for (Object o:theArray){
        if (output.toString().length() > 1) {
            output.append(",");
        }
        output.append(o);
    }
    output.append("]");
    return output.toString();
}

View code