How correct to make (move) the class internal in Java?

Structure of my project is:

/src/
----/action/
--------/impl/
------------ActionCalculateQuadrangle.interface
--------CalculateQuadrangle.class
--------Line.class
----/other packages/

Code ActionCalculateQuadrangle.interface:

public interface ActionCalculateQuadrangle {
    double getArea(Quadrangle quadrangle);
    double getPerimeter(Quadrangle quadrangle);
    double getLengthLine(Line line);
    double getSlope(Line line);

    List<Point[]> getCombinationsElementsOver(Point[] points, int pointsOnOneLine);
    Point getCrossPoint(Line line1, Line line2);

    boolean isQuadrangle(Quadrangle quadrangle);
    boolean isPointsOnStraightLine(Point[] points);
    boolean isConvex(Quadrangle quadrangle);
    boolean isSquare(Quadrangle quadrangle);
    boolean isRhombus(Quadrangle quadrangle);
    boolean isTrapezoid(Quadrangle quadrangle);
    boolean isDeltoid(Quadrangle quadrangle);
    boolean isStraightPerpendicular(Line line1, Line line2);
    boolean isEqualLengthsDiagonals(Line line1, Line line2);
    boolean isCrossLines(Line line1, Line line2);
    boolean areSidesParallel(Line side1, Line side2);
}

Code Line.class:

public class Line {
    private final Point p1;
    private final Point p2;

    public Line(Point p1, Point p2) {
        this.p1 = p1;
        this.p2 = p2;
    }

    public Point getP1() {
        return this.p1;
    }

    public Point getP2() {
        return this.p2;
    }
}

Part of code unity-class 'CalculateQuadrangle.class` (I cite part of the code so as not to clutter up the place):

// ... some code

@Override
    public double getPerimeter(final Quadrangle quadrangle) {
        Line side1 = new Line(quadrangle.getP1(), quadrangle.getP2());
        Line side2 = new Line(quadrangle.getP2(), quadrangle.getP3());
        Line side3 = new Line(quadrangle.getP3(), quadrangle.getP4());
        Line side4 = new Line(quadrangle.getP1(), quadrangle.getP1());

        return getLengthLine(side1) + getLengthLine(side2) + getLengthLine(side3) + getLengthLine(side4);
    }

    @Override
    public double getLengthLine(final Line line) {
        Point p1 = line.getP1();
        Point p2 = line.getP2();

        return Math.sqrt(Math.pow(p2.getX() - p1.getX(), 2) + Math.pow(p2.getY() - p1.getY(), 2));
    }

    @Override
    public double getSlope(final Line line) {
        Point p1 = line.getP1();
        Point p2 = line.getP2();

        if ((p1.getY() == p2.getY()) || (p1.getX() == p2.getX())) {
            return 0;
        } else {
            return (p2.getY() - p1.getY()) / (p2.getX() - p1.getX());
        }
    }

// ... some code

That's ok. But A want to make class 'Line' internal.

How I do it? Like this:

// ... some code

// PUT IT HERE!
public class Line {
    private final Point p1;
    private final Point p2;

    public Line(Point p1, Point p2) {
        this.p1 = p1;
        this.p2 = p2;
    }

    public Point getP1() {
        return this.p1;
    }

    public Point getP2() {
        return this.p2;
    }
}

@Override
    public double getPerimeter(final Quadrangle quadrangle) {
        Line side1 = new Line(quadrangle.getP1(), quadrangle.getP2());
        Line side2 = new Line(quadrangle.getP2(), quadrangle.getP3());
        Line side3 = new Line(quadrangle.getP3(), quadrangle.getP4());
        Line side4 = new Line(quadrangle.getP1(), quadrangle.getP1());

        return getLengthLine(side1) + getLengthLine(side2) + getLengthLine(side3) + getLengthLine(side4);
    }

    @Override
    public double getLengthLine(final Line line) {
        Point p1 = line.getP1();
        Point p2 = line.getP2();

        return Math.sqrt(Math.pow(p2.getX() - p1.getX(), 2) + Math.pow(p2.getY() - p1.getY(), 2));
    }

    @Override
    public double getSlope(final Line line) {
        Point p1 = line.getP1();
        Point p2 = line.getP2();

        if ((p1.getY() == p2.getY()) || (p1.getX() == p2.getX())) {
            return 0;
        } else {
            return (p2.getY() - p1.getY()) / (p2.getX() - p1.getX());
        }
    }

// ... some code

But compiler java show me error like this - Class 'CalculateQuadrangle' must either be declared abstract or implement abstract method 'getLengthLine(Line)' in 'ActionCalculateQuadrangle'.

1. Declarate abstract - can't for me.
2. implement abstract method 'getLengthLine(Line)' in 'ActionCalculateQuadrangle'. That mean, what i must implement ALL abstract method, which have in the argument object class Line.

How correct to make (move) the class internal?

After update still have error:

The error you are seeing is because you now have two Line classes, and the method definition and interface definition are not referring to now the same class of CalculateQuadrangle.Line