// Calculator.java

public class Calculator {

  private static final char NO_OP = '\0';

  private static final char PLUS = '+';

  private static final char SUBTRACT = '-';

  private static final char MULTIPLY = '*';

  private static final char DIVIDE = '/';

  private float  number1 = 0.0F;

  //private float  number2 = Float.NaN;

  private char   operator = NO_OP;

  //等于运算

  public String opEquals(String number) {

    float result;

    if ( operator == NO_OP ) {

      result = parseNumber(number);

    } else {

      result = performOperation(parseNumber(number));

    }

    operator = NO_OP;

    number1 = result;

    return Float.toString(result);

  }

  //加法运算

  public String opAdd(String number) {

    float result;

    if ( operator == NO_OP ) {

      result = parseNumber(number);

    } else {

      result = performOperation(parseNumber(number));

    }

    operator = PLUS;

    number1 = result;

    return Float.toString(result);

  }

  //减法运算

  public String opSubtract(String number) {

    float result;

    if ( operator == NO_OP ) {

      result = parseNumber(number);

    } else {

      result = performOperation(parseNumber(number));

    }

    operator = SUBTRACT;

    number1 = result;

    return Float.toString(result);

  }

  //乘法运算

  public String opMultiply(String number) {

    float result;

    if ( operator == NO_OP ) {

      result = parseNumber(number);

    } else {

      result = performOperation(parseNumber(number));

    }

    operator = MULTIPLY;

    number1 = result;

    return Float.toString(result);

  }

  //除法运算

  public String opDivide(String number) {

    float result;

    if ( operator == NO_OP ) {

      result = parseNumber(number);

    } else {

      result = performOperation(parseNumber(number));

    }

    operator = DIVIDE;

    number1 = result;

    return Float.toString(result);

  }

  //处理运算符

  private float performOperation(float number2) {

    float result = 0.0F;

    switch ( operator ) {

    case PLUS:

      result = number1 + number2;

      break;

    case SUBTRACT:

      result = number1 - number2;

      break;

    case MULTIPLY:

      result = number1 * number2;

      break;

    case DIVIDE:

      result = number1 / number2;

      break;

    }

    return result;

  }

  //将字符串类型转换为浮点型

  private static float parseNumber(String number) {

    float real_number;

    try {

      real_number = Float.parseFloat(number);

    } catch (NumberFormatException e) {

      real_number = Float.NaN;

    }

    return real_number;

  }

}

//July/整理/05.23/。