HANDLING EXCEPTIONS AND EVENTS - Java Programming: From Problem Analysis to Program Design - page 770

The definition of the method actionPerformed is:

public void actionPerformed(ActionEvent e)
{

String resultStr; //Step 1

String str
= String.valueOf(e.getActionCommand()); //Steps 1 and 2

char ch = str.charAt(0); //Steps 1 and 3

switch (ch) //Step 4

{
case '0': //Step 4a

case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
if (firstInput)
{

numStr1 = numStr1 + ch;
displayText.setText(numStr1);

}
else
{

numStr2 = numStr2 + ch;
displayText.setText(numStr2);

}
break ;

case '+': //Step 4b

case '-':
case '*':
case '/':
op = ch;
firstInput = false ;
break ;
case '=':

//Step 4c

resultStr = evaluate();
displayText.setText(resultStr);
numStr1 = resultStr;
numStr2 = "";
firstInput = false ;
break ;