Question

I have a simple recursive method written in both Java and C#

Java

public static String reverse(String letter) {
    if (letter == "")
        return "";
    else
        return letter.substring(letter.length() - 1)
                + reverse(letter.substring(0, letter.length() - 1));
}

C#

public static string reverse(string letter)
{
    if (letter == "")
        return "";
    else
        return letter[letter.Length - 1]
                + reverse(letter.Substring(0, letter.Length - 1));
}

But the Java version fails at letter == "", it returns false even if the letter is empty. Why does the C# version work while the Java version fails?

Answer 1

In both C# and Java, the == operator checks for reference equality by default.

However, in .NET, the == operator is overloaded for the string type to check for value equality instead:

public static bool operator ==(string a, string b)
{
    return string.Equals(a, b);
}

Another factor you need to take into account when using both languages is string interning, which can cause even reference equality to succeed for strings that might appear distinct. From MSDN (for C#):

The common language runtime conserves string storage by maintaining a table, called the intern pool, that contains a single reference to each unique literal string declared or created programmatically in your program. Consequently, an instance of a literal string with a particular value only exists once in the system.

For example, if you assign the same literal string to several variables, the runtime retrieves the same reference to the literal string from the intern pool and assigns it to each variable.

String interning is applied across both Java and C#, and typically gives consistent results. For example, both languages evaluate string concatenation at compile-time, meaning that "a" + "b" is stored as "ab":

"ab" == "a" + "b"                                      // Java: Gives true
object.ReferenceEquals("ab", "a" + "b")                // C#:   Gives true

However, when getting a zero-length substring, only C# returns the interned empty string:

"" == "abc".substring(0, 0)                            // Java: Gives false
object.ReferenceEquals("", "abc".Substring(0, 0))      // C#:   Gives true