Converting from Integer wrapper class to int primitive class

Question

I've been trying to convert an Integer Wrapper class to int primitive class. I haven't yet found a proper way to make the code compile. I'm using Intellij IDEA, Java 11 Amazon Coretto, but I need to run it on a computer that runs java 8.

Here's the original code below:

static class Line<Integer> extends ArrayList<Integer> implements Comparable<Line<Integer>> {

        @Override
        public int compareTo(Line<Integer> other) {
            int len = Math.min(this.size(), other.size());
            for (int i = 0; i < len; i++) {;
                if ((int) this.get(i) != (int) other.get(i)) {
                    if ((int this.get(i) < (int) this.get(i)) {
                        return -1;
                    } else if ((int) this.get(i) > (int)this.get(i)) {
                        return 1;
                    } else {}
                }
            }
    ...

note that the Line is inserted to an ArrayList.

Originally I used forced casting on all the Integer objects so it'll be like (int) this.get(i). It worked on my local terminal and my Intellij wasn't bothered about it, but unfortunately not the other computer. It couldn't compile there

I thought it was because of the forced casting, since the other computer returned

Main.java:159: error: incompatible types: Integer cannot be converted to int
if ((int) this.get(i) != (int) other.get(i)) {
^
where Integer is a type-variable:
Integer extends Object declared in class Line

so I deleted them all and thought I could let the machine unbox the Integer wrapper on its own. It still didn't compile.

If the code is left like what's written above (no forced casting), it will return "Operator '<' not applicable for 'Integer', 'Integer'" So I used the .compareTo() method. Compile error.

Then I tried to assign them to an int variable. Intellij IDEA was screaming at me that it required int but found Integer instead. So I force-casted, like so

int thisLine = (int) this.get(i);
int otherLine = (int) other.get(i);
if (thisLine != otherLine) {
    if (thisLine < otherLine) {
        return -1;
    } else if (thisLine > otherLine) {
        return 1;
    } else {}

Nope, didn't work. Removing the cast also didn't work.

I looked up the Javadocs (https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html#intValue--) this time about the Integer class and found a promising little method called intValue(). Problem is? Intellij cannot resolve that method (oddly, VSCode does not consider this an error). I used it like this

int thisLine = this.get(i).intValue();
int otherLine = other.get(i).intValue();
if (this.get(i) != other.get(i)) {
    if (thisLine < otherLine) {
        return -1;
    } else if (thisLine > otherLine) {
        return 1;

and sure enough, another compile error on that stubborn computer.

I'm running out of options. I'm seriously considering creating a new custom class just so I can store int values in an ArrayList without having to deal with all this Java backwards incompatibility nonsense. Anyone here know a consistent method for converting an Integer wrapper object to an int primitive object in Java?

Answer 1

This is the clue in the error message that explains it:

where Integer is a type-variable:

Integer extends Object declared in class Line

Integer is not java.lang.Integer but a type variable with a confusing name...

You declared the type variable here:

static class Line<Integer> extends ArrayList<Integer> implements Comparable<Line<Integer>>

It's as if you declared it like this:

static class Line<T> extends ArrayList<T> implements Comparable<Line<T>>

but by naming the type variable Integer instead of T, and then you try to cast objects of the type T to int later on.

Fix it by not declaring a type parameter named Integer, you don't need that here:

static class Line extends ArrayList<Integer> implements Comparable<Line<Integer>>