The following method of the class Shape is implemented by Line2D.Float and Line2D.Double.
The spec is inherited however the assertion quoted below is not always satisfied for a line.

    /**
     * ...
     * Note that there is no guarantee that the returned
     * {@link Rectangle2D} is the smallest bounding box that encloses
     * the <code>Shape</code>, only that the <code>Shape</code> lies
     * entirely within the indicated <code>Rectangle2D</code>.
     * ...
     */
    public Rectangle2D getBounds2D();



Please see the following code sample for Line2D.Double, extracted from more complicated testcase with particular values (Line2D.Float has the same problems):


import java.awt.geom.Line2D;

public class GetBounds2D {
    public static void main(String[] args) {

        Line2D.Double vertLine = new Line2D.Double(0.0, 0.0, 0.0, 1.0);
        System.out.println(vertLine.getBounds2D().contains(0.0, 0.0));
        System.out.println(vertLine.getBounds2D().contains(0.0, 1.0));

        Line2D.Double horLine = new Line2D.Double(0.0, 0.0, 1.0, 0.0);
        System.out.println(horLine.getBounds2D().contains(0.0, 0.0));
        System.out.println(horLine.getBounds2D().contains(0.0, 1.0));

        // start point
        Line2D.Double line1 = new Line2D.Double(12345.4352, 12345.4352,
                -0.56, -1.34);
        System.out.println(line1.getBounds2D().contains(12345.4352, 12345.4352));

        // end point
        Line2D.Double line2 = new Line2D.Double(12345.4352, 0.0, -0.56, 1.234);
        System.out.println(line2.getBounds2D().contains(-0.56, 1.234));

    }

}

The output will be

false
false
false
false
false
false