PostfixEval.java. At first, limit the operations to add, subtract, multiply, and divide. If we require that every token in the input string is separated by spaces, e.g., "33 -43 + -55 65 + *", we can evaluate decimals and negatives. Assume that the postfix string is well-formed. Use as test data the postfix expressions below. public static final String operators = "+ - * / % ^ !"; public static double eval(String str) { } public static double eval(double a, double b, String op) { } public static boolean isOperator(String op) { }

That is the assignment.

import java.util.*; public class PostfixEval { public static final String operators = "+ - * / % ^ !"; public static void main(String[] args) { System.out.println("Postfix --> Evaluate"); ArrayList postfixExp = new ArrayList(); /* build your list of expressions here */ for( String pf : postfixExp ) { System.out.println(pf + "\t\t" + eval(pf)); } } public static double eval(String pf) { List postfixParts = new ArrayList(Arrays.asList(pf.split(" "))); /* enter your code here */ } public static double eval(double a, double b, String ch) { } public static boolean isOperator(String op) { } }

that is the shell code.

I am not understanding how to code the eval method and whever I try it always says nullPointerException,