@Test
 public void testMultiplicationNode() {
   TreeNode n1 = new LeafTreeNode(1.0);
   TreeNode n2 = new LeafTreeNode(2.0);
   TreeNode n3 = new MultiplicationTreeNode(n1, n2);
   TreeNode n4 = new MultiplicationTreeNode(n2, n1);
   EvaluateTreeVisitor v1 = new EvaluateTreeVisitor();
   n3.accept(v1);
   assertEquals(2.0, v1.getResult(), DELTA);
   EvaluateTreeVisitor v2 = new EvaluateTreeVisitor();
   n4.accept(v2);
   assertEquals(2.0, v2.getResult(), DELTA);
 }
 @Test
 public void testSingleLeafNode() {
   TreeNode n1 = new LeafTreeNode(1.0);
   EvaluateTreeVisitor v1 = new EvaluateTreeVisitor();
   n1.accept(v1);
   assertEquals(1.0, v1.getResult(), DELTA);
 }
 @Test
 public void nonNumberNodeForUnaryMinus() {
   TreeNode n1 = new LeafTreeNode(1.0);
   TreeNode n2 = new LeafTreeNode(2.0);
   TreeNode n3 = new AdditionTreeNode(n1, n2);
   TreeNode n4 = new UnaryMinusTreeNode(n3);
   EvaluateTreeVisitor v1 = new EvaluateTreeVisitor();
   n4.accept(v1);
   assertEquals(-3.0, v1.getResult(), DELTA);
 }
 @Test
 public void testUnaryMinusBinaryTree() {
   TreeNode n1 = new LeafTreeNode(4.0);
   TreeNode n2 = new LeafTreeNode(5.0);
   TreeNode n3 = new UnaryMinusTreeNode(n1);
   TreeNode n4 = new UnaryMinusTreeNode(n2);
   TreeNode n5 = new MultiplicationTreeNode(n3, n4);
   EvaluateTreeVisitor v1 = new EvaluateTreeVisitor();
   n5.accept(v1);
   assertEquals(20.0, v1.getResult(), DELTA);
 }
 @Test
 public void treeEvalBalancedTree() {
   TreeNode n1 = new LeafTreeNode(1.0);
   TreeNode n2 = new LeafTreeNode(2.0);
   TreeNode n3 = new LeafTreeNode(3.0);
   TreeNode n4 = new LeafTreeNode(4.0);
   TreeNode n5 = new AdditionTreeNode(n1, n2);
   TreeNode n6 = new MultiplicationTreeNode(n3, n4);
   TreeNode n7 = new DivisionTreeNode(n6, n5);
   EvaluateTreeVisitor v1 = new EvaluateTreeVisitor();
   n7.accept(v1);
   assertEquals(4.0, v1.getResult(), DELTA);
 }