need help with this part of my java assignment. Comments appreciated.

This class extends class Expression and represents PostfixExpressions

Interface for class PostfixExpression

public PostfixExpression( String str) : This constructor receives a String containing the expression text. It will invoke the constructor for class Expression, passing in the expression text.

public boolean isLegal(): This method returns true if the last token in the tokenList is an operator, AND the first two tokens are operands, AND the number of operands in the expression is equal to the number of operators plus 1.

PostfixExpression

NO ADDITIONAL DATA MEMBERS

+ PostfixExpression( in expr: String): constructor + isLegal(): boolean

+ evaluate(): double

public double evaluate(): This method uses a stack of type Double to evaluate the postfix expression.

o Ensure the expression is legal, if not Stop and throw an exception, using class ArithmeticException

o Obtain the tokenList o if the # of tokens in the tokenList < 3, Stop and throw an exception,

using class ArithmeticException

o Iterate through the token list ? if the current token is an operand, convert it to type double and

push onto the stack. ? if the current token is an operator:

if the size of the stack is >=2, o pop a value from the stack into operand2, and

pop a second value from the stack into operand1 o Evaluate the subexpression.. calling

evaluateSubExpression o push the result back onto the stack

else o Stop and throw an exception, using class

ArithmeticException

o When done (you are at the end of the token list): o if ONE value remains on the stack, pop and return the value o else Stop and throw an exception, using class ArithmeticException

My expression class:

package project2;

import java.util.ArrayList;

public abstract class Expression {

// data member

protected String expression;

protected ArrayList tokens;

// constructor

public Expression(String expression){

this.expression = expression;

tokens = new ArrayList();

String[] tokenList = expression.split(" "); // convert the string to array of tokens

// insert the tokens in the list

for(int i=0;i

tokens.add(tokenList[i]);

}

// method to return the expression

public String getExpression() {

return expression;

}

// method to return the tokens

public ArrayList getTokens() {

return tokens;

}

// method to check if the token is operand or not(i.e can be converted to double or not)

protected static boolean isOperand(String token) {

try {

double val = Double.valueOf(token);

return true; // token can be converted to double value

}catch(NumberFormatException e)

{

return false; // token cannot be converted to double value

}

}

// method to check if the token is operator or not

protected static boolean isOperator(String token) {

if(token.equals("+") || token.equals("-")|| token.equals("*") || token.equals("/") || token.equals("%"))

return true;

return false;

}

// method to return the value of the token(operand value)

protected static double getOperandValue(String token) throws NumberFormatException {

return(Double.valueOf(token));

}

// method to evaluate the subexpression and return the result

protected static double evaluateSubExpression(double operand1, String operator, double operand2) {

double result;

switch(operator) {

case "+" : result = operand1 + operand2;

break;

case "-" : result = operand1 - operand2;

break;

case "*" : result = operand1 * operand2;

break;

case "/" : result = operand1 / operand2;

break;

case "%" : result = operand1 % operand2;

break;

default : result = -1;

}

return result;

}

// abstract methods to be implemented in subclass

public abstract boolean isLegal(); // See my comment on page 3 of instructions.

public abstract double evaluate();

}

Given postfix code:

package project2;

public class PostfixExpression extends Expression {

// Feel free to create other private/protected members, methods, or constructors.

public PostfixExpression(String expression) {

// Remember the super constructor

}

@Override

public boolean isLegal() {

/*

* See my comment on page 3 of instructions.

* You can decide to implement isLegal() in Expression class.

* In which case, you can get rid of this method here.

*/

}

@Override

public double evaluate() {

}

}

Test class:

package project2;

import static org.junit.Assert.*;

import java.util.Arrays;

import org.junit.AfterClass; import org.junit.Test;

public class ExpressionTest { private static final double DELTA = 0.00000000001; private static final double LEGAL_DRINKING_AGE = 21; private static int score = 0;

@AfterClass public static void printScore() { System.out.println(score + "/90 + ?/10 Documentation"); }

// Testing Getters @Test public void testGetPrefixExpression() { assertEquals("", new PrefixExpression("").getExpression()); assertEquals(" ", new PrefixExpression(" ").getExpression()); assertEquals("+", new PrefixExpression("+").getExpression()); assertEquals("1", new PrefixExpression("1").getExpression()); assertEquals("+ 1.0 .1", new PrefixExpression("+ 1.0 .1").getExpression()); assertEquals("- 0.5 -.1", new PrefixExpression("- 0.5 -.1").getExpression()); assertEquals("0.5 -.1 /", new PrefixExpression("0.5 -.1 /").getExpression()); assertEquals("0.5 -.1 * % abc", new PrefixExpression("0.5 -.1 * % abc").getExpression()); assertEquals("", new PostfixExpression("").getExpression()); assertEquals(" ", new PostfixExpression(" ").getExpression()); assertEquals("+", new PostfixExpression("+").getExpression()); assertEquals("1", new PostfixExpression("1").getExpression()); assertEquals("+ 1.0 .1", new PostfixExpression("+ 1.0 .1").getExpression()); assertEquals("- 0.5 -.1", new PostfixExpression("- 0.5 -.1").getExpression()); assertEquals("0.5 -.1 /", new PostfixExpression("0.5 -.1 /").getExpression()); assertEquals("0.5 -.1 * % abc", new PostfixExpression("0.5 -.1 * % abc").getExpression()); score += 5; }

@Test public void testGetTokens() { assertEquals(Arrays.asList("+"), new PrefixExpression("+").getTokens()); assertEquals(Arrays.asList("1"), new PrefixExpression("1").getTokens()); assertEquals(Arrays.asList("+", "1.0", "0.1"), new PrefixExpression("+ 1.0 0.1").getTokens()); assertEquals(Arrays.asList("-", "0.5", "-0.1"), new PrefixExpression("- 0.5 -0.1").getTokens()); assertEquals(Arrays.asList("0.5", "-0.1", "*", "%", "abc"), new PrefixExpression("0.5 -0.1 * % abc").getTokens()); assertEquals(Arrays.asList("+"), new PostfixExpression("+").getTokens()); assertEquals(Arrays.asList("1"), new PostfixExpression("1").getTokens()); assertEquals(Arrays.asList("+", "1.0", "0.1"), new PostfixExpression("+ 1.0 0.1").getTokens()); assertEquals(Arrays.asList("-", "0.5", "-0.1"), new PostfixExpression("- 0.5 -0.1").getTokens()); assertEquals(Arrays.asList("0.5", "-0.1", "*", "%", "abc"), new PostfixExpression("0.5 -0.1 * % abc").getTokens()); score += 10; } @Test public void testIsLegalPrefixExpression() { assertFalse(new PrefixExpression("").isLegal()); assertFalse(new PrefixExpression(" ").isLegal()); assertFalse(new PrefixExpression("-").isLegal()); assertFalse(new PrefixExpression("1 1 -").isLegal()); assertFalse(new PrefixExpression("+ 1- 1").isLegal()); assertFalse(new PrefixExpression("1 / 2").isLegal()); assertFalse(new PrefixExpression("+ 1 2 3 -").isLegal()); assertFalse(new PrefixExpression("* / 5. -2 1 - 2").isLegal()); assertTrue(new PrefixExpression("+ 1 2").isLegal()); assertTrue(new PrefixExpression("+ 1 - 2 3").isLegal()); assertTrue(new PrefixExpression("/ 2 1.5").isLegal()); assertTrue(new PrefixExpression("% .3 -47.99").isLegal()); assertTrue(new PrefixExpression("+ 1 2 3 4 + + 5 + + + 6 7").isLegal()); score += 10; } @Test public void testIsLegalPostfixExpression() { assertFalse(new PostfixExpression("").isLegal()); assertFalse(new PostfixExpression(" ").isLegal()); assertFalse(new PostfixExpression("-").isLegal()); assertFalse(new PostfixExpression("- 1 1").isLegal()); assertFalse(new PostfixExpression("1 1- +").isLegal()); assertFalse(new PostfixExpression("1 / 2").isLegal()); assertFalse(new PostfixExpression("+ 1 2 3 -").isLegal()); assertFalse(new PostfixExpression("5. -2 1 - 2 % * / ").isLegal()); assertTrue(new PostfixExpression("1 2 +").isLegal()); assertTrue(new PostfixExpression("1 2 + 3 -").isLegal()); assertTrue(new PostfixExpression("2 1.5 / ").isLegal()); assertTrue(new PostfixExpression(".3 -47.99 %").isLegal()); assertTrue(new PostfixExpression("1 2 3 4 + + 5 + + + 6 7 +").isLegal()); score += 10; } @Test public void evalSubExpression() { assertEquals(1.0, Expression.evaluateSubExpression(-1, "+", 2), DELTA); assertEquals(-1.0, Expression.evaluateSubExpression(1, "-", 2), DELTA); assertEquals(0.0, Expression.evaluateSubExpression(1, "*", 0), DELTA); assertEquals(0.5, Expression.evaluateSubExpression(-1, "/", -2), DELTA); assertEquals(0.0, Expression.evaluateSubExpression(4, "%", 2), DELTA); score += 15; } @Test public void evalPrefixExpression() { assertEquals(LEGAL_DRINKING_AGE, new PrefixExpression("* - 9 2 3").evaluate(), DELTA); assertEquals(10.5, new PrefixExpression("/ * + 5 2 3 2").evaluate(), DELTA); assertEquals(0.5, new PrefixExpression("% / * + 5 2 3 2 2").evaluate(), DELTA); assertEquals(0.5, new PrefixExpression("/ * - 9 11 -.5 2").evaluate(), DELTA); score += 20; } @Test public void evalPostfixExpression() { assertEquals(LEGAL_DRINKING_AGE, new PostfixExpression("3 9 2 - *").evaluate(), DELTA); assertEquals(10.5, new PostfixExpression("3 5 2 + * 2 /").evaluate(), DELTA); assertEquals(0.5, new PostfixExpression("3 5 2 + * 2 / 2 %").evaluate(), DELTA); assertEquals(0.5, new PostfixExpression("-.5 9 11 - * 2 /").evaluate(), DELTA); score += 20; } }