Using a constructor to put variables into an object throws java.lang.NullPointerException

Question

Using this code

System.out.println("New Mark Entry\n--------------" + "\n\nEnter the description (up to 40 characters will be displayed):");
name = TextIO.getlnString();
if (name.length() > 40) {
    name = name.substring(0,40);
}
System.out.println("What was the assignment out of?");
totalMark = TextIO.getlnDouble();
System.out.println("What was the students mark?");
mark = TextIO.getlnDouble();
System.out.println("What was the weight of this assignement?");
weight = TextIO.getlnDouble();
input = 1;
int openSpot = 0;
for(int i = 0; i < markbook.length; i++) {
    if(markbook[i].getAssignment(name) == null) {   // java.lang.NullPointerException is thrown here
        openSpot = i;
        break;
    }

}
markbook[openSpot] = new Mark(name, totalMark, mark, weight);
break;

Causes a java.lang.NullPointerException to be thrown. Im a tad confused at what to do to fix this. If anyone could help or point me in the right direction it would be greatly appreicated

Put breakpoint in this code..i.e. `name`,markbook[i] or like wise cases... — bNd, Apr 23 '14 at 16:43
possible duplicate of [What is a Null Pointer Exception?](http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception) — Makoto, Apr 23 '14 at 16:44
The program is incomplete. Provide the full description to link the code properly. — maali, Apr 23 '14 at 16:46

score 0 · Answer 1 · edited May 23 '17 at 12:20

0

If the NPE happens on that exact line, the most likely cause is that markbook[i] is null. You don't show how you've initialised it, but in addition to allocating the array you also need to create the elements.

For an example, see https://stackoverflow.com/a/10044418/367273

edited May 23 '17 at 12:20

Community

1
1

answered Apr 23 '14 at 16:43

NPE

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score 0 · Answer 2 · answered Apr 23 '14 at 16:44

I would look at the line where the NullPointerException has been throw.

Let say it is

markbook[i].getAssignment(name)

Then I would check that I actually set markbook[i] to something, because otherwise it will be null.

Note: this is not enough.

MyType[] markbook = new MyType[4];

as this is the same as

MyType[] markbook = { null, null, null, null };

If it still doesn't make sense, I would use a debugger to debug your code.

score 0 · Accepted Answer · answered Apr 23 '14 at 16:45

you are initializing markbook after and trying to use it before

for(int i = 0; i < markbook.length; i++) {
        if(markbook[i].getAssignment(name) == null) {   // java.lang.NullPointerException is thrown here
            openSpot = i;
            break;
        }

    }
    markbook[openSpot] = new Mark(name, totalMark, mark, weight);//you are initializing markbook after and trying to use it before

it should be like following

//initialize first
markbook[openSpot] = new Mark(name, totalMark, mark, weight);

//use later
for(int i = 0; i < markbook.length; i++) {
        if(markbook[i].getAssignment(name) == null) {   // java.lang.NullPointerException is thrown here
            openSpot = i;
            break;
        }

    }

Using a constructor to put variables into an object throws java.lang.NullPointerException

3 Answers3