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; void displayPostfix() output the ET in postfix format in one line, each token is separated by one space.

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(BinaryNodeInterface 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; // < To be defined. >

return result; } // end getValueOf private double compute(String operator, double firstOperand, double secondOperand) { double result = 0; // < To be defined. > return result; } // end compute } // end ExpressionTree