Java Constructors and Null Pointer Exceptions

Question

I am playing around with java, and have run into an issue when I utilize custom constructors.

When I begin my program, the main file contains a line like this:

Word input = new Word(word);

The constructor for word looks like this:

public Word(String s){
        wordArray = s.toCharArray(); 
        protocol = new Protocol(wordArray.length);
    }

And the protocol looks like this:

public Protocol(int length)
    {
        letterList[0] = 'a';
        letterList[1] = 'b';
        letterList[2] = 'c';
        letterList[3] = 'd';
        letterList[4] = 'e';
        letterList[5] = 'f';
        letterList[6] = 'g';
        letterList[7] = 'h';
        letterList[8] = 'i';
        letterList[9] = 'j';
        letterList[10] = 'k';
        letterList[11] = 'l';
        letterList[12] = 'm';
        letterList[13] = 'n';
        letterList[14] = 'o';
        letterList[15] = 'p';
        letterList[16] = 'q';
        letterList[17] = 'r';
        letterList[18] = 's';
        letterList[19] = 't';
        letterList[20] = 'u';
        letterList[21] = 'v';
        letterList[22] = 'w';
        letterList[23] = 'x';
        letterList[24] = 'y';
        letterList[25] = 'z';
        wordLength = length;
        for(int i=0;i<wordLength*2-1;i++)
        {
            display[i] = '_';
            i++;
            display[i] = ' ';
        }
    }

I am getting a NullPointerException at the line within my main file where word is constructed, and then protocol .

I have experimented with the code, and noticed if I do not call the constructor for the protocol within word the build is successful past that point, but I need protocol to be built later as well, so it still has issues.

Am I not allowed to call a constructor within another constructor? Anyone have any ideas on what may be going on?

If you need more clarification, please let me know!

You dont need an array of letters, you can just use a + index. — Karthik T
– Karthik T, Commented Jan 28, 2013 at 5:03
Please post the stack trace and indicate or post the exact line of code referred to at the top of the stack trace. — user207421
– user207421, Commented Jan 28, 2013 at 5:15

Matt Ball · Accepted Answer · 2013-01-28 05:04:57Z

5

Am I not allowed to call a constructor within another constructor?

No, you are definitely allowed to do that.

Anyone have any ideas on what may be going on?

Clearly, word is null when passed to the Word constructor at this line:

Word input = new Word(word);

answered Jan 28, 2013 at 5:04

Matt Ball

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4 Comments

ZAX Over a year ago

I agree this is the simple answer. However, in lines above the word constructor, I call System.out.println(word), and it prints the word as expected. Thoughts?

Matt Ball Over a year ago

Then as Abu suggested, you may have failed to initialize letterList.

Matt Ball Over a year ago

Then step through your code with a debugger, and see what variable or field is null that your code is trying to dereference.

ZAX Over a year ago

Ah! I found it! Good call on the failure to initialize. I have another array which I failed to initialize after the letterList. After I fixed the first I didn't notice the line number change until I went back a few times! Thanks so much!

Abubakkar · Accepted Answer · 2013-01-28 05:06:50Z

3

The problem may be because you might not have initialized you letterList array, so try initializing it first before using it.

public Protocol(int length)
{
   letterList = new char[26];  
   letterList[0] = 'a';  
   letterList[1] = 'b';  
   letterList[2] = 'c';  
   ...  
   ...

}

answered Jan 28, 2013 at 5:06

Abubakkar

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1 Comment

Matt Ball Over a year ago

Indeed, this is another possible issue. +1

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