Nullpointerexception when I perform depth first search

Question

Suppose area1 has three contents 2,3,4, area2 has three contents 1,2,3 and area3 has two contents 1,2. m is the map that keys are locationid and values are list of contentid. For example, area1 has the map (1,{2,3,4}). Each area choose 1 content and find all the combination. I use dfs (recursion) to solve this problem but there is a nullpointerexception in line1. The intermediate is a list of string and i iterate through the list and their types are both string, why there is a null pointer exception? This is a specific condition in nullpointerexception and it is not duplicate.

public static List<String> dfs(String digits,Map<String, List<String>> m) {
        List<String> result = new ArrayList<String>();
        if(digits.length() == 0){
            return result;
        }
        if(digits.length() == 1){
            return m.get(digits.charAt(0));
        }
        List<String> intermediate = dfs(digits.substring(1, digits.length()),m);
        for(String first : m.get(Character.toString(digits.charAt(0)))){
            for(String rest : intermediate){           // line1
                result.add(first + rest);
                }
            }
        return result;
    }

public static void main(String[] args) {
    // TODO Auto-generated method stub
    String digits="123";
    Map<String,List<String>> selected=new HashMap<>();
    selected.put("1", new ArrayList<>(Arrays.asList("4","2","3")));
    selected.put("2", new ArrayList<>(Arrays.asList("1","2","3")));
    selected.put("3", new ArrayList<>(Arrays.asList("1","2")));
    dfs(digits,selected);
}

I mean I can use stack but I want to find the error above. @elhefe — KKKK, Mar 27 '16 at 00:43
in simple term; post the nullpointerException block which you got after executing code. — Ashish Patil, Mar 27 '16 at 00:44
Dude. Read the wikipedia article. I'm not talking about using a stack data structure for your application. — elhefe, Mar 27 '16 at 00:45
This is the perfect time to use your debugger with a break point on the line pointed to by the stack trace. — adamdc78, Mar 27 '16 at 00:46

score 2 · Accepted Answer · answered Mar 27 '16 at 00:46

2

I guess the problem is here:

return m.get(digits.charAt(0));

it should return null, since digits.charAt(0) is not a String

you need to use substring or Character.toString( here to extract the number

answered Mar 27 '16 at 00:46

JohnnyAW

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and even without stack-trace:D but seriously, you need to get familiar with stack-trace and debugger, it will help you to find bugs and errors – JohnnyAW Mar 27 '16 at 00:52
@KKKK this is the exact thing I said in your other question – OneCricketeer Mar 27 '16 at 02:57

Nullpointerexception when I perform depth first search

1 Answers1