Question

I know why there is an error and i have fixed it.. i need someone to explain it properly.. It compiles fine but at runTime it shows Null Pointer Exception at line appString[i][j] = s[j];..Some one know the reason…?

appString = new String[app.size()][];
for(int i = 0; i<app.size();i++)
{
    Vector appVec = (Vector) app.get(i);
    String[] s = new String[appVec.size()];
    appVec.toArray(s);
    for(int j = 0 ; j<s.length;j++)
    {
        //String s1 = s[j];
        appString[i][j] = s[j];
    }
}

Answer 1

You’ve created the top-level array, but each element of it is null. You need something like:

appString[i] = new String[s.length];

inside your loop. Or given that you don’t need the array again anyway, you don’t need to loop:

appString = new String[app.size()][];
for(int i = 0; i<app.size();i++)
{
    Vector appVec = (Vector) app.get(i);
    String[] s = new String[appVec.size()];
    appVec.toArray(s);
    appString[i] = s;
}