Question

I have a problem when setting a variable inside a Java class

Here is my code

This is where I create the instances (IdeaInfo is a class that acts similar to a Struct):

IdeaInfo[] IDEAS = new IdeaInfo[100];
String[] TITLES = new String[100];

This is the function that will use those instances:

    try {
        while ((line = reader.readLine()) != null) {
            sb.append(line + "\n");
            // This is adding title to array Ideas and Titles
            if(mode % 3 == 0)   {
                IDEAS[ideas_pos].setTitle(sb.toString());
                TITLES[titles_pos] = sb.toString();
                titles_pos++;
                mode++;
            }
            // This is adding the content to array Ideas
            else if(mode % 3 == 1)  {
                IDEAS[ideas_pos].mContent = sb.toString();
                mode++;
            }
            // This is adding the rating to array Ideas
            else if(mode % 3 == 2)  {
                IDEAS[ideas_pos].mRating = Float.valueOf(sb.toString().trim()).floatValue();
                ideas_pos++;
                mode++;
            }
        }
    }

This is what I have inside IdeaInfo class:

public class IdeaInfo {

    public String mTitle = new String();        // Store the Idea's title
    public String mContent = new String();  // Store the Idea's title
    public float mRating;       // Store the Idea's Rating

    /*
     * Function that set the Idea's title
     */
    public void setTitle(String temp){
      mTitle = temp;
    }
}

Apparently, the error occurred inside the try, exactly at IDEAS[ideas_pos].setTitle(sb.toString());
The debugger indicated that I am accessing a NullPointerException, this does not really make any sense to me since I already initialize those variables in the class.

By the way, I initialized ideas_pos to 0.

Answer 1

Arrays in java are initialized “clean”, that is, with all elements set to null or “zero” (whatever is appropriate for the type of array). When you write

IdeaInfo[] IDEAS = new IdeaInfo[100];

the JVM will treat it as if you wrote

IdeaInfo[] IDEAS = new IdeaInfo[100];
for (int i = 0; i < 100; i++) {
    IDEAS[i] = null;
}

This takes some getting used to if you are coming to Java from a language like C or C++ which has different conventions for initializing arrays.