Implement the following expression tree in JAVA that works with an expression that contains only variables or double values and supports operations +, -, *, and / by providing two additional methods: void setVariable(String name, double value) set the variable name with a value with exception handling;package TreePackage;

public class ExpressionTree extends BinaryTree implements ExpressionTreeInterface { public ExpressionTree() { } // end default constructor

public double evaluate() { return evaluate(getRootNode()); } // end evaluate

private double evaluate(BinaryNode 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) { // Strings allow multicharacter variables

double result = 0; if (variable.equals("a")) result = 2; else if (variable.equals("b")) result = 3; else if (variable.equals("c")) result = 4; else if (variable.equals("d")) result = 5; else if (variable.equals("e")) result = 2;

return result; } // end getValueOf

private double compute(String operator, double firstOperand, double secondOperand) { double result = 0; if (operator.equals("+")) result = firstOperand + secondOperand; else if (operator.equals("-")) result = firstOperand - secondOperand; else if (operator.equals("*")) result = firstOperand * secondOperand; else if (operator.equals("/")) result = firstOperand / secondOperand;

return result; } // end compute

// For testing purposes public void displayTree() { BinaryNode rootNode = (BinaryNode)getRootNode(); System.out.println("preorder:"); preorder(rootNode); System.out.println(); System.out.println("postorder:"); rootNode = (BinaryNode)getRootNode(); postorder(rootNode); System.out.println(); } // end displayTree

private void postorder(BinaryNode rootNode) { if (rootNode != null) { postorder(rootNode.getLeftChild()); postorder(rootNode.getRightChild()); System.out.print(rootNode.getData() + " "); } // end if } // end postorder

private void preorder(BinaryNode rootNode) { if (rootNode != null) { System.out.print(rootNode.getData() + " "); preorder(rootNode.getLeftChild()); preorder(rootNode.getRightChild()); } // end if } // end preorder } // end ExpressionTree