@Test
    public void testSetTransitionLength_negativeInput_exceptionThrown() {
        AnimClip animClip = new AnimClip("clip");
        ClipAction clipAction = new ClipAction(animClip);
        IllegalArgumentException thrown = assertThrows(IllegalArgumentException.class,
                () -> clipAction.setTransitionLength(-1));
        assertEquals("transitionLength must be greater than or equal to 0", thrown.getMessage());
    }
Test to verify setTransitionLength on BlendableAction does not accept negative values.
jme3-core/src/test/java/com/jme3/anim/tween/action/ClipActionTest.java