Converting JSON string to JSON Object having null value

The concatenation in your string is not alright. Is abc an object? or is it just the string abc?

String test = "{" + "abc" + ":null}";

but in the above case, you should simply do:

String test = "{abc:null}";

Or if abc is another defined String variable, say like this:

String abc = "awesomeText";
String test = "{" + abc + ":null}";

Or maybe you need those quotes from abc? use "\" to escape the quote character. Like this:

String test ="{\"abc\":null}";

I made this test and it worked fine for me:

public static void main(String[] args) {
    BufferedReader br;
    try {
        br = new BufferedReader(new FileReader("C:\\test.json"));

        String currentLine;
        while ((currentLine = br.readLine()) != null) {
            JSONObject testObj = new JSONObject(currentLine);
            System.out.println(testObj);
        }
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (JSONException e) {
        e.printStackTrace();
    }
}

I tried many successful tests while playing with the content of the test.json file:

1. {"abc":null}
2. {'abc':null}
3. {abc:null}