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copied!<p>A few suggestions.</p> <p>First, I would suggest when using a <code>boolean</code> as a conditional for an <code>if</code> statement, avoid comparison with <code>true</code> and <code>false</code> -- there are only two states for <code>boolean</code> anyway. Also, since there are only two states, rather than using <code>else if (false)</code>, an <code>else</code> will suffice:</p> <pre><code>if (condition == true) { // when condition is true } else if (condition == false) { // when condition is false } </code></pre> <p>can be rewritten as:</p> <pre><code>if (condition) { // when condition is true } else { // when condition is false } </code></pre> <p>Second, rather than comparing the string literals <code>"add"</code>, <code>"subtract"</code> and such, try to use constants (<code>final</code> variables), or <a href="http://java.sun.com/j2se/1.5.0/docs/guide/language/enums.html" rel="nofollow noreferrer"><code>enum</code></a>s. Doing a <code>String</code> comparison such as <code>(operator == "add")</code> is performing a check to see whether the string literal <code>"add"</code> and the <code>operator</code> variable are both <em>refering</em> to the same object, not whether the <em>values</em> are the same. So under certain circumstances, you may have the <code>operator</code> set to <code>"add"</code> but the comparison may not be <code>true</code> because the string literal is refering to a separate object. A simple workaround would be:</p> <pre><code>final String operatorAdd = "add"; // ... if (input.equals("+")) operator = operatorAdd; // ... if (operator == operatorAdd) // ... </code></pre> <p>Now, both the assignment of <code>operator</code> and the comparison of <code>operator</code> both are referecing the constant <code>operatorAdd</code>, so the comparison can use a <code>==</code> rather than a <code>equals()</code> method.</p> <p>Third, as this seems like the type of calculator which doesn't really require two operands (i.e. <code>operand1 + operand2</code>), but rather a single operand which is acting upon a stored value (i.e. <code>operand + currentValue</code>), it probably would be easier to have some variable that holds the current value, and another variable that holds the operator, and a method which will act according to the current operator and operand. (More or less an idea of an <a href="http://en.wikipedia.org/wiki/Accumulator_(computing)" rel="nofollow noreferrer">accumulator machine</a>, or 1-operand computer.)</p> <p>The basic method of operation will be:</p> <ol> <li>Set the <code>currentValue</code>.</li> <li>Set the operator.</li> <li>Set the <code>operand</code>.</li> <li>Perform the calculation.</li> <li>Set the <code>currentValue</code> to the result of the calculation.</li> <li>Set the operator to blank state.</li> </ol> <p>Each step should check that the previous step took place -- be sure that an operation is specified (<code>operator</code> is set to a valid operator), then the next value entered becomes the <code>operand</code>. A calculator is like a <a href="http://en.wikipedia.org/wiki/Finite_state_machine" rel="nofollow noreferrer">state machine</a>, where going from one step to another must be performed in a certain order, or else it will not proceed to the next step.</p> <p>So, the calculator may be like this (pseudocode!):</p> <pre><code>// Initialize calculator (Step 1) currentValue = 0; operand = 0; operator = operatorNone; loop { operand = getOperand(); // Step 2 operator = getOperator(); // Step 3 // Step 4 and 5 if (operator == operatorAdd) currentValue += operand; if (operator == operatorSubtract) currentValue -= operand; // ... // Step 6 operator = operatorNone; } </code></pre> <p>Although the above code uses a single loop and doesn't work like a event-based GUI model, but it should outline the steps that it takes to run a calculator.</p>

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