Why isn't this basic java program working?

Question

The goal is to have it count the number of "xx" in the given string. We'll say that overlapping is allowed, so "xxx" contains 2 "xx".

See http://codingbat.com/prob/p194667 I can't seem to figure out why its not working

    int countXX(String str) {
        int f = 0;
        for (int i = 0; i < str.length(); i++){
            if (!str.substring(i+1).isEmpty()){
                if (str.substring(i) == "x" && str.substring (i+1) == "x") {
                    f++;
                }
            }
        }
        return f;  
    }

While it's wrong to compare string values with `==`, this question is more about the misuse of `substring` to extract a character. — rgettman, May 03 '13 at 21:52
I have voted to reopen this question; it's not a duplicate of any common "==" vs. "equals" string comparison problem. — rgettman, May 03 '13 at 21:59

score 4 · Answer 1 · answered May 03 '13 at 21:48

4

Don't use == to compare strings; use equals().

int countXX(String str) {
    int f = 0;
    for (int i = 0; i < str.length(); i++) {
        if (!str.substring(i+1).isEmpty()) {
            if (str.substring(i).equals("x") && str.substring(i+1).equals("x")) {
                f++;
            }
        }
    }
    return f;  
}

I think this code will still have a problem when i equals the length of the string minus 1 and you try to access the character at (i+1).

answered May 03 '13 at 21:48

duffymo

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.equals() still doesn't work – user1399888 May 03 '13 at 21:52
There's still the issue of misusing `substring` to extract a character from the string. – rgettman May 03 '13 at 21:53
And i < str.length() - 1 – Michal Borek May 03 '13 at 21:54
equals works. your code is borked. – duffymo May 03 '13 at 22:11

score 4 · Answer 2 · answered May 03 '13 at 21:51

The substring method with one argument doesn't get just that character at that position, it gets everything from that position through to the end of the string. Use charAt to get the character at that position, and compare it with == with the character literal 'x', not with the String "x".

score 4 · Answer 3 · answered May 03 '13 at 21:53

4

Don't use substring for indexing chars.

int countXX(String str) {
   int f = 0;
   for (int i = 0; i < str.length()-1; i++) {
      if( str.charAt(i) == 'x' && str.charAt(i+1) == 'x' ) {
         f++;
      }
   }
   return f;  
}

answered May 03 '13 at 21:53

Mirco Ellmann

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7
8

score 2 · Answer 4 · answered May 03 '13 at 21:52

2

Three reasons:

substring() returns the whole string (so starting from "i" to the end of the string, and not only one letter).
Compare with .equals() and not ==
"i" should end at str.length() - 1

answered May 03 '13 at 21:52

Michal Borek

4,584
2
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score 1 · Answer 5 · answered May 03 '13 at 21:49

1

Use the string's 'equals' method in stead of == :

int countXX(String str) {
    int f = 0;
    for (int i = 0; i < str.length(); i++){
    if (!str.substring(i+1).isEmpty()){
    if (str.substring(i).equals("x") && str.substring (i+1).equals("x")) {
    f++;}}}
    return f;  
    }

answered May 03 '13 at 21:49

Toverbal

256
3
14

1

This only partly solves problem. – Michal Borek May 03 '13 at 21:53

score 0 · Answer 6 · answered May 03 '13 at 21:53

0

int countXX(String str) {
    int f = 0;
    for (int i = 0; i < str.length() - 1; i++){
        if (str.substring(i, i + 2).equals("xx")) {
            f++;
        }
    }
    return f;  
}

answered May 03 '13 at 21:53

Joop Eggen

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7
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Why isn't this basic java program working?

6 Answers6