About substring testing in Java

Question

I'm writing some codes that test if there is "xx" in a string. For instance, doubleX("aaxxbb") should return true, and doubleX("axabb") should return false.

Here is my code:

private static boolean doubleX(String str) {
  for(int i=0;i<str.length()-1;i++){
      System.out.println(str.substring(i,i+2));
      if(str.substring(i,i+2) == "xx") return true;
  }
  return false;
}

Why does doubleX("aaxxbb") return false?

are you allowed to used `.indexOf()` [see http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/String.html#indexOf(java.lang.String) ] — blurfus, Nov 19 '13 at 02:12

score 2 · Answer 1 · answered Nov 19 '13 at 02:11

2

You have to use .equals instead of ==. For more information, follow the duplication message.

return str.contains("xx");

Is a lot clearer though.

answered Nov 19 '13 at 02:11

Jeroen Vannevel

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score 1 · Answer 2 · answered Nov 19 '13 at 02:11

You should understand the difference between == and equals: the first one compares references, the second compares actual values.

Your code is wildly inefficient.

I'd try something like this:

private static boolean doubleX(String str) {
    return (str.indexOf("xx") != -1);
}

score 0 · Answer 3 · answered Nov 19 '13 at 02:13

Use equals() for checking the content of a String to another rather than ==. == checks for reference equality.

private static boolean doubleX(String str) {
        for(int i=0;i<str.length()-1;i++){
            System.out.println(str.substring(i,i+2));
            if(str.substring(i,i+2).equals("xx")) return true;
        }
        return false;
      }

Even you can directly code like:

private static boolean doubleX(String str) {
        return str.contains("xx");
      }

About substring testing in Java

3 Answers3