Tree Implementations - Data Structures and Abstractions with Java - page 616

private double evaluate(BinaryNodeInterface<String> rootNode)
{
double result;
if (rootNode == null )
result = 0;
else if (rootNode.isLeaf())
{
String variable = rootNode.getData();
result = getValueOf(variable);
}
else
{
double firstOperand = evaluate(rootNode.getLeftChild());
double secondOperand = evaluate(rootNode.getRightChild());
String operator = rootNode.getData();
result = compute(operator, firstOperand, secondOperand);
} // end if
return result;
} // end evaluate
private double getValueOf(String variable)
{
. . .
} // end getValueOf
private double compute(String operator, double firstOperand,
double secondOperand)
{
. . .
} // end compute
} // end ExpressionTree
The public method evaluate calls a private method evaluate that is recursive. This private
method calls the private methods getValueOf and compute as well as methods declared in
BinaryNodeInterface . The method getValueOf returns the numeric value of a given variable in
the expression, and compute returns the result of a given arithmetic operation and two given
operands.
Notice how important the methods of the class BinaryNode are to the implementation of
evaluate . For this reason, we do not want BinaryNode to be hidden within BinaryTree . Rather, it
should be part of a package.
Question 6 Trace the method evaluate for the expression tree in Figure 23-14c of the pre-
vious chapter. What value is returned? Assume that a is 3, b is 4, and c is 5.