Why do Strings start with a “” in Java? [duplicate

Question

Possible Duplicate:
Why does “abcd”.StartsWith(“”) return true?

Whilst debugging through some code I found a particular piece of my validation was using the .startsWith() method on the String class to check if a String started with a blank character

Considering the following :

public static void main(String args[])
{

    String s = "Hello";
    if (s.startsWith(""))
    {
        System.out.println("It does");
    }

}

It prints out It does

My question is, why do Strings start off with a blank character? I'm presuming that under the hood Strings are essentially character arrays, but in this case I would have thought the first character would be H

Can anyone explain please?

Answer 1

An empty string is not a blank character. Assuming your question with empty string, I guess they decided to leave it that way but it does seem odd. They could have checked the length but they didn't.

Answer 2

The empty String ("") basically "satisfies" every string. In your example, java calls

s.startsWith(""); 

to

s.startsWith("", 0);

which essentially follows the principle that "an empty element(string) satisfies its constraint (your string sentence).".

From String.java

/**
     * Tests if the substring of this string beginning at the
     * specified index starts with the specified prefix.
     *
     * @param   prefix    the prefix.
     * @param   toffset   where to begin looking in this string.
     * @return  <code>true</code> if the character sequence represented by the
     *          argument is a prefix of the substring of this object starting
     *          at index <code>toffset</code>; <code>false</code> otherwise.
     *          The result is <code>false</code> if <code>toffset</code> is
     *          negative or greater than the length of this
     *          <code>String</code> object; otherwise the result is the same
     *          as the result of the expression
     *          <pre>
     *          this.substring(toffset).startsWith(prefix)
     *          </pre>
     */
    public boolean startsWith(String prefix, int toffset) {
    char ta[] = value;
    int to = offset + toffset;
    char pa[] = prefix.value;
    int po = prefix.offset;
    int pc = prefix.count;
    // Note: toffset might be near -1>>>1.
    if ((toffset < 0) || (toffset > count - pc)) {
        return false;
    }
    while (--pc >= 0) {
        if (ta[to++] != pa[po++]) {
            return false;
        }
    }
    return true;
    }