public void setLength(double length) {
        if (length < 0.0) {
            throw new IllegalArgumentException("length must be greater than or equal to 0");
        }
        this.length = length;
    }
Base implementation of the Tween interface that provides
default implementations of the getLength() and interpolate()
methods that provide common tween clamping and bounds checking.
Subclasses need only override the doInterpolate() method and
the rest is handled for them.

@author Paul Speed
jme3-core/src/main/java/com/jme3/anim/tween/AbstractTween.java