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

问题:

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?

回答1:

"" is an empty string containing no characters. There is no "empty character", unless you mean a space or the null character, neither of which are empty strings.

You can think of a string as starting with an infinite number of empty strings, just like you can think of a number as starting with an infinite number of leading zeros without any change to the meaning.

1 = ...00001
"foo" = ... + "" + "" + "" + "foo"

Strings also end with an infinite number of empty strings (as do decimal numbers with zeros):

1 = 001.000000...
"foo" = "foo" + "" + "" + "" + ...

回答2:

Seems like there is a misunderstanding in your code. Your statement s.startsWith("") checks if string starts with an empty string (and not a blank character). It may be a weird implementation choice, anyway, it's as is : all strings will say you they start with an empty string.

Also notice a blank character will be the " " string, as opposed to your empty string "".

回答3:

"Hello" starts with "" and it also starts with "H" and it also starts with "He" and it also sharts with "Hel" ... do you see?

回答4:

That "" is not a blank it's an empty string. I guess that the API is asking the question is this a substring of that. And the zero-length empty string is a substring of everything.

回答5:

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;
    } 

回答6:

For folks who have taken automata theory, this makes sense because the empty string ε is a substring of any string and also is the concatenation identity element, ie:

for all strings x, ε + x = x, and x + ε = x

So yes, every string "startWith" the empty string. Also note (as many others said it), the empty string is different from a blank or null character.

回答7:

A blank is (" "), that's different from an empty string (""). A blank space is a character, the empty string is the absence of any character.

回答8:

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.