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javanullpointerexceptionstringbuilder

StringBuilder.append() causes java.lang.NullPointerException

I'm experiencing a strange issue using the append() method of StringBuilder class; here is the method:

public StringBuilder toStringBuilder(byte[] b)
{
    StringBuilder s = new StringBuilder();
    for (int i = 0; i < b.length; i++) 
        {
            s.append(Integer.toString((b[i] & 0xff) + 0x100,16).substring(1));
        }
    System.out.println(s);
    return s;
}

It takes a byte array and converts it into a StringBuilder (doing other stuff in the meanwhile).

Everything seems OK, when I run the program it prints the correct result, but shows also a NullPointerException that doesn't let me continue using the program; the exception is pointed to the fourth line 

(for (int i = 0; i < b.length; i++))

Here is a screenshot:

Screenshot

Solution

  • If this line:

    for (int i = 0; i < b.length; i++)

    throws an NPE, there is only one possible explanation. b is null.

    You can prove (or disprove) this by adding this statement immediately before the for statement.

    System.out.println("b is " + b);

    Once you've confirmed that, you need to work backwards to figure out why the b is null at that point. I can't offer any insights based on the code you've shown us.