Printing Lists in Java

What the Hash Code?

This week, I've been working on implementing three different sorting algorithms: Merge Sort, Bubble Sort, and Quick Sort. In the spirit of learning the lower-level workings of Java, I'm implementing these algorithms to work with my custom implementations of an array list and a linked list.

While figuring out why an algorithm wasn't working as expected, I reached for the trusty System.out.print() to see what was happening. I was surprised to see, instead of a nice print out of all the numbers in the array surrounded by square brackets, something like this:

main.MyArrayList@80169cf

This is the package name, the class name followed by "@", and the System.identityHashCode() of the array list that I was trying to print. To put it in an overly-reductive nutshell, this is essentially the memory address of the array list that the JVM uses to locate the data in memory.

The Quick Fix

In order to view the contents of the array, I wrote a quick function to print each element in the array to the console on a single line.

public static void printArray(MyList<Integer> arr) {
  for (int i = 0; i < arr.size(); i++)
    System.out.print(arr.get(i) + " ");
  System.out.println();
}

Instead of calling System.out.print(arr), I could now do printArray(arr) in order to debug my algorithm.

The Real Fix

That, however, didn't answer the question I had: Why is it that putting a Java-implemented array list into System.out.print() yielded the expected result and mine didn't?

A little digging into the documentation revealed that the print function, when given an Object as its input, first converts the Object using String.valueOf(). String.valueOf(), in turn, calls the toString() method of the Object provided in order to get the string representation of the Object.

My implementation of my array list, however, didn't include a toString() method, so I added one:

public String toString() {
  if (size == 0) return "[ ]";
    
  String str = "[ ";
  for (Object o : array) {
    str = str.concat(o.toString().concat(" "));
  }
  str = str.concat("]");
  return str;
}

Now this works:

System.out.println(arr)

=> [ 1 2 3 4 5 6 7 8 9 ]

via GIPHY