手动实现 String.length 方法

Question

I have to implement the.length method from String class "by hand" and I have no idea and hope you can help somehow. No other methods or functions are allowed, than:

1. String.charAt()
2. String.substring()
3. String.isEmpty()
4. Bit-operations &,|, &&,||, <<, >>,>>>, ,=, ==
5. Arithmetic operations
6. for and while Loop
7. recursion
8. if else statement
9. self created methods (int,String,char,boolean etc.)
10. self-created Arrays. (no Methods of them)

static void manual_length2(String length) {
//example String length = "Hello" = 5 letters.
        
        int counter = 0;
        int i = 0;
        char g = ' ';

        while(i <= 4 ) { /*4 is the number i already know */

            g = length.charAt(i);
            counter += 1;
            length.substring(1);
            ++i;

        }

System.out.println(counter);

Console: 5

This was my approach, but I'm stuck in the while statement's condition to terminate. With the example "Hello" i already know that this word has 5 letters, but it needs to fit for all inputs. So i don't know how to express to border-value of the while statement.

Another approach is by recursion, but also, i ask myself how can i express the limit of the recursion. How can i express:

.... lengthMethod1(String length, int ???) {

  if(n == 0) {
     return length.charAt(0);
  }
  else {
     
     return ???? lengthMethod1(length, n - 1);

}

Answer 1

You can loop until the String is empty while removing the first character on each iteration.

static int manual_length(String str) {
    int len = 0;
    while(!str.isEmpty()){
       ++len;
       str = str.substring(1);
    }
    return len;
}

This can be converted to a tail-recursive method as well.

static int manual_length(String str) {
    return str.isEmpty() ? 0 : 1 + manual_length(str.substring(1));
}

Answer 2

Another approach is by recursion, but also, i ask myself how can i express the limit of the recursion. How can i express:

Yes, you can do recursively like this:

static int manual_length(String str, int len) {
        return str.isEmpty() ? len : manual_length(str.substring(1), len + 1);
    }

You use an accumulator variable ( ie, len ), that you increment, while removing a char from the string ( ie, str.substring(1) ). When you reach the end ( ie, str.isEmpty() ) you return the accumulator.