Testing toString but it's failing?

Question

Using this code:

 public String toString()
{

    StringBuilder s = new StringBuilder();
    for (int i = 0; i < length; i++)
    {                     
        s.append(i).append(":").append(arr[i]).append(", ");                       
    }
    s.delete(s.length()-2, s.length()-1);
    return s.toString();
}

is not passing this test:

@Test
public void testToString()
{
    BetterArray<String> b = new BetterArray<String>();
    assertEquals("", b.toString());
    b.add("hello");
    assertEquals("0:hello", b.toString());
    b.add("bye");
    assertEquals("0:hello, 1:bye", b.toString());
    b.add("adios");
    assertEquals("0:hello, 1:bye, 2:adios", b.toString());
}

It needs to start out as an empty string (""), am i doing that correctly? I'm new to StringBuilder. I'm using the s.delete to remove the trailing comma and space

Answer 1

This will cause a java.lang.StringIndexOutOfBoundsException in the case of empty string since you will not have comma and trailing space.

s.delete(s.length()-2, s.length()-1);

Answer 2

You're only deleting the comma, the delete line needs to be s.delete(s.length()-2, s.length());.

(By convention, the end index of a range points 1 past the last element of the range. This is the same for String.substring() for example.)

Answer 3

Here's one way to do what you want that doesn't rely on any math tricks. If the length is zero, the method returns an empty string.

public String toString() {
    StringBuilder s = new StringBuilder();

    for (int i = 0; i < length; i++) {
        s.append(i);
        s.append(":");
        s.append(arr[i]);
        if (i < (length - 1)) s.append(", ");
    }

    return s.toString();
}