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copied!<p>You need to split the string into tokens first:</p> <pre><code>/* Splits the expression up into several Strings, * all of which are either a number or and operator, * none of which have spaces in them. */ String [] expressionAsTokens = e.split(" "); </code></pre> <p>Then you need to make sure you compare Strings, not chars:</p> <pre><code>//compare strings instead of chars String token = expressionAsTokens[j]; if (!"+".equals(token) && !"*".equals(token) && !"-".equals(token) && !"/".equals(token)) { s.push(Integer.parseInt(token)); } else { //same code as you had before } </code></pre> <p>Also, is there any reason you are storing everything as a <code>char</code> array in your <code>Stacked</code> class? Your <code>pop()</code> method returns and integer, yet everything is stored as a <code>char</code>.</p> <p>For this application, everything should be stored as an integer:</p> <pre><code>public class Stacked { int stack[]; // array is of type integer int top; int maxLen; // constructor public void push() {/*...*/} public int pop() {/*...*/} //pop returns an int as before //... } </code></pre> <p>One final note: Be careful what order you add and subtract the numbers in. I don't remember if postfix operands are evaluated left first or right first, but make sure you get them in the right order. As you have it now, <code>2 3 - 4 *</code> would evaluate as <code>4 * (3 - 2)</code> and I think it should be <code>(2 - 3) * 4</code>. This won't matter with adding and multiplying, but it will with subtracting and dividing.</p>

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