I build an expression into a binary tree each time it rolls through the loop, creating a new tree at each ending of ")" and pushing those operators/operands into a stack to be popped back into one complete binary tree.
My Build Method:
package lab5;
import net.datastructures.*;
public class Expression<T> {
/** Contain Linked Tree and Linked Stack instance variables **/
LinkedBinaryTree<T> tree;
LinkedStack<LinkedBinaryTree<T>> stack;
public Expression () {
tree = new LinkedBinaryTree<T> ();
stack = new LinkedStack<LinkedBinaryTree<T>>();
} // end constructor
public LinkedBinaryTree<T> buildExpression (String expression) {// LinkedBinaryTree<T> is a type of LinkedBinaryTree
// major TODO to implement the algorithm]
LinkedBinaryTree<T> operand, op1, op2;
LinkedStack<LinkedBinaryTree<T>> newStack = new LinkedStack<LinkedBinaryTree<T>>();
String symbol;
int i = 0;
int len = expression.length();
for (i = 0; i < len; i++) {
symbol = expression.substring(i, i+1);
if ((!symbol.equals ("(")) && (!symbol.equals (")"))) {
operand = new LinkedBinaryTree<T> ();
operand.addRoot((T)symbol);
newStack.push(operand);
} else if (symbol.equals ("(")){
continue;
} else {
op2 = newStack.pop();
operand = newStack.pop();
op1 = newStack.pop();
tree.attach(operand.root(), op1, op2);
newStack.push(tree);
}
}
tree = newStack.pop();
return tree;
} // end method buildExpression
}
My Test:
package lab5;
import net.datastructures.*;
public class ExpressionTest {
/**
* @param args
* @throws EmptyTreeException
*/
/** Paranthesize is code fragment 8.26 pg. 346 **/
/** evaluateExpression method apart of LinkedBinaryTree class in net.datastructures **/
public static void main(String[] args) {
// declare local variables/objects
String s = new String ();
String exp = "((((3+1)x3)/((9-5)+2))-((3x(7-4))+6))"; //-13
Expression<String> expression = new Expression<String> ();
// print the expression string
System.out.printf ("The original Expression String generated via printf: %s", exp);
// create the tree using the 'stub' method
// i.e. it does nothing
LinkedBinaryTree<String> tree = expression.buildExpression (exp);
// use Object.toString simply to print its reference
System.out.printf ("\n\nThe Tree: %s\n", tree);
}
}
I am getting a NullPointerException and I do not know why. Do I need to 'Try' and then let the error roll through?
