I found a lot of solutions for this but I am just trying to understand why my code doesn't work. Basically I've taken two arrays. In one I added all characters until I found repetition. If a character repeats I converted all elements in the array to string, found their length and send it to another arrays. Lastly I sorted the second array and printed the largest length.

    public int lengthOfLongestSubstring(String s) {
        char a[]=new char[s.length()];
        int b[]=new int[s.length()];
        StringBuilder sbf = new StringBuilder("");
        for(int x=0;x<s.length();x  ){
            char c=s.charAt(x);
            if (new String(a).indexOf(c) == -1) {
                sbf.append(a);
                b[x]=sbf.length();
                sbf=null;
            }
            else
            {
                a[x]=c;
            }
        }
        Arrays.sort(b);
        return b[(b.length)-1];
    }
}

This was my error message

WARNING: A command line option has enabled the Security Manager
WARNING: The Security Manager is deprecated and will be removed in a future release

java.lang.NullPointerException: 
Cannot invoke "java.lang.StringBuilder.append(char[])" 
because 
"<local4>" is null 
at line 10, Solution.lengthOfLongestSubstring 
at line 54, __DriverSolution__.__helper__ 
at line 84, __Driver__.main

The link for the problem

CodePudding user response：

I don't comment your logic but just point out the mistake.

Let's say this lines execute

if (new String(a).indexOf(c) == -1) {
            sbf.append(a);
            b[x]=sbf.length();
            sbf=null;
        }

After that, StringBuilder sbf is null and next time when it tries to append it fails because it is already null.

CodePudding user response：

You're setting sbf to null and then trying to call a method on it on the next iteration. You should set its length to 0 to reset it instead:

sbf.setLength(0);

You also want to change the condition of your loop to:

if (new String(a).indexOf(c) != -1)