Question

public  String starString(int n){               
    int m = (int)Math.pow(2,n);
    String str="";
    str = starString(m-1,str);                  
    return str;
}
private  String starString(int n, String str){          
    String temp ="";
    if (n<0) {
        try{
            throw new IllegalArgumentException();
        } 
        catch(IllegalArgumentException ex){               
        }          
    }
    else { 
        temp+=("*");              
        starString(n-1,str);        
    }                       
    return temp;       
}

Can someone please explain to me why this code returns a single asterisk even if its called by a value greater than n >= 0?

I debugged and noticed that after throwing the exception it recurses again and all the asterisks get chopped to “”. I’ve tried it many times. Its also required that you should throw the IllegalArgumentException if n < 0.

Answer 1

In Java strings are immuntable, hence you need to assign a new value to temp (and pass temp as the parameter):

temp = starString(n-1, temp);        

Additionally you’d need to assign str to temp, otherwise each recursion would just return a single asterisk:

String temp = str;

A much simpler, cleaner (and correct) version of your recursive method would look like this:

private  String starString(int n){          
  String temp = "*";

  //only recurse as long as n > 0, i.e. the last invocation would be made with n = 0
  if (n > 0){ 
    temp += starString(n-1);                     
  }                      
  return temp;       
}

Note that you don’t even need to pass the string as a parameter. Also note that recursion is overkill here, using a loop would make much nore sense. Also note that string concatenation is costly and gets slow quickly for higher values of n (due to immutable string instances being created over and over again). In that case you’d better use StringBuilder:

private  String starString(int n){          
  StringBuilder s = new StringBuilder();
  for( int i = 0; i <= n; i++ ) {
    s.append("*");
  }
  return s.toString();
}

On my machine a loop version using string concatenation takes around 12 seconds for n = 100000 whereas the StringBuilder version takes 0.007 seconds.