Logic Error: Return true if the first instance of "x" in the string is immediately followed by another "x"

I'm doing a CodingBat Java problem and I have tried some of my solutions:

boolean doubleX(String str) {
  if (str.length() >= 2){
    for (int i = 0; str.charAt(i) == 'x' && str.charAt(i+1) == 'x'; i++){
      return true;
    }
  }
  return false;
}

boolean doubleX(String str) {
  if (str.length() >= 2){
    for (int i = 0; i < str.length()-1; i++){
      if (str.charAt(i) == 'x' && str.charAt(i+1) == 'x') {
        return true;
      } else {
        return false;
      }
    }
  }
  return false;
}

These two produced the same error with this test

doubleX("axxbb") → true

But my code turned out to be false

I also tried the below solution and it worked but I don't understand why. What is the difference between && and nested if statements?

boolean doubleX(String str) {
    for (int i = 0; i < str.length()-1; i++){
      if (str.charAt(i) == 'x') {
        if (str.charAt(i+1) == 'x') {
          return true;
        } else {
          return false;
        }
      }
    }
  return false;
}

Please help me explain. Thank you very much in advance

In your first example - the second statement in your for loop defines the condition for executing the code block - since char 0 and char 1 are not 'x', it exits the loop and returns false.

In your second example - your if statement is in the loop so the first execution again sees that char 0 and char 1 are not 'x' so it returns false before the next loop iteration.

3rd example - you are are correctly returning false Only after you confirmed the first x found was followed by a second x.

When you send "axxbb" as an argument in this function:

boolean doubleX(String str) {
  if (str.length() >= 2){
    for (int i = 0; str.charAt(i) == 'x' && str.charAt(i+1) == 'x'; i++){
      return true;
    }
  }
  return false;
}

This is what's happening in the for loop:

1. you're initializing i=0
2. it checks the condition str.charAt(i) == 'x' && str.charAt(i+1) == 'x' - the condition is false
3. condition is false so it exits the loop

The condition fails on first check so the loop ends there.

In this function:

boolean doubleX(String str) {
  if (str.length() >= 2){
    for (int i = 0; i < str.length()-1; i++){
      if (str.charAt(i) == 'x' && str.charAt(i+1) == 'x') {
        return true;
      } else {
        return false;
      }
    }
  }
  return false;
}

This is what the first iteration of the for loop will do:

1. check if 'a'=='x' and 'x'=='x' - the condition is not true so it goes straight to the else statement to execute the code there
2. the else statement returns false and your function ends there

The problem is that the for loop doesn't execute more than once because once the first condition is false, it will always return false and won't do more iterations.

Even the third third function you provided won't give you correct behavior when you send a string like "axaxx" because once the first condition if (str.charAt(i) == 'x') succeeds the next thing that it's going to do is check if the next character is also x. Because the next character is not x, it will go to the else statement and return false.

What you need to do for a correct behavior is have a condition in the for loop that checks if 2 subsequent characters are x and return true if they are. Don't add an else statement in the loop. If the loop finishes and the condition was never met, then you can return false.