help with how String.substring(int, int) works

To quote the substring Javadoc,

The substring begins at the specified beginIndex and extends to the character at index endIndex - 1.

In other words, endIndex is not included in the result.

As to your second question, it has to do with you reducing the length of name inside the loop. Your loop condition (i < stringLength) and the fact that you look at name.charAt(i) assume that the length of name remains constant (or least does not decrease). This gets violated the moment you start shortening the string.

Hi You get the Exception because you iteratate over the string but in the for loop you are actually changing the strings length name = name.substring(0, i) + c + name.substring(i+1, stringLength); So while you are still in the loop the size of the name is no longer equal to the old stringLength.

Boro

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 15 at java.lang.String.charAt(String.java:558) at RepairName.main(RepairName.java:17)

Occurs because you are altering name on line 21 and 29 and keeping the length of name the same in stringLength

ad 1) The javadocs are a very good reference, here java.lang.String.substring (...):

substring

public String substring(int beginIndex, int endIndex)

Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1. Thus the length of the substring is endIndex-beginIndex.

The benefit, in excluding the index of the end, is, that you can sequently iterate over values:

a [0] = x.substring (0, 5);
a [1] = x.substring (5, 10);
a [2] = x.substring (10, 15);

// or 

a [i] = x.substring (i*5, (i+1)*5);

that's correct substring works in this way:

public String substring(int beginIndex, int endIndex)

Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1. Thus the length of the substring is endIndex-beginIndex.

so for substring(0, 7) it takes 0..6 chars from string

Notice that this is really bad practice to modify iterate's value in for-loop (i++ inside for-loop-body). If after space you haven't any other chars, you'll encounter exception

ad 2)

You don't change i itself, but name gets truncated here:

name = name.substring (0, i-1) + c + name.substring(i+1, stringLength);

but the outer loop isn't informed about the new length.

ad 3) (Code-review):

You should declare and initialize your variables as late as possible, and mark as much as possible final. This tells you, that you don't have to worry about them, after they're initialized.

Given this code:

import javax.swing.*;

public class RepairName 
{
    public static void main(String[] args)
    {
        final String name = JOptionPane.showInputDialog(null, "Please enter your first and last name");
        final String saveOriginalName = name;
        final int stringLength = name.length ();
        System.out.println (name);
        for (int i = 0; i < stringLength; i++)
        {
                final char c = name.charAt (i);
                if (i == 0)
                {
                        char upper = Character.toUpperCase (c);
                        name = upper + name.substring (1, stringLength);
                }
                else
                    if (c == ' ')
                        {
                            i++;
                                final char c2 = name.charAt (i);
                                final char upper = Character.toUpperCase (c2);
                                name = name.substring (0, i-1) + upper + name.substring (i+1, stringLength);
                        }
        }
        JOptionPane.showMessageDialog (null, "Original name was " + saveOriginalName + "\nRepaired name is " + name);
    }
}

you would have get an error message, for name being final. Maybe this would have prevented you to accidentially get out of sync with the length.

modified, to have a mutable upName, but iterating over name:

    String upName = null;
    for (int i = 0; i < stringLength; i++)
    {
            final char c = name.charAt (i);
            if (i == 0)
            {
                    char upper = Character.toUpperCase (c);
                    upName = upper + name.substring (1, stringLength);
            }
            else
                if (c == ' ')
                    {
                        i++;
                            final char c2 = name.charAt (i);
                            final char upper = Character.toUpperCase (c2);
                            upName = upName.substring (0, i-1) + upper + name.substring (i+1, stringLength);
                    }
    }
    JOptionPane.showMessageDialog (null, "Original name was " + saveOriginalName + "\nRepaired name is " + upName);

gives you, for input "honky tonky" the output "HonkyTonky", which would have led you to the way, substring (from, to) works, maybe. :)