编程遇到问题啦！

题目在这里：
1.1 Learning objectives //学习使用项目
Using String methods //使用String方法
Using the matches method and regular expression //使用常用的表达试
Using exception //使用异常处理机制
1.2 Task //任务，作业
Write a program Calculator.java to implement a simple calculator of basic operations (addition, subtraction, multiplication and division) on integers.
//用java写一个计算器程序能进行简单的整数的加，减，乘，除运算

The user may enter a string comprising an operand, a binary operator, and a second operand, with any number of space characters between them. There may be leading spaces and trailing spaces in the input.
//用户可以输入一串字符表达试其中包括 操作数，操作符 和若干空格。

If an invalid input is encountered, a message "Error in expression" is displayed and the calculation is skipped. Otherwise, the respective operation is carried out, and the result displayed. The user then enters another string. To end the calculation, the user enters 'q' (without any leading or trailing spaces).
//如果输入的不是整数就抛出或显示“Error in expression"的信息，并跳过次此计算，否则就继续运行并显示结果。用户输入字符’q‘可以退出程序。
The operand is an integer. Examples of valid operands are 23, 0, -123, but not 23.5 (not an integer), 51.0 (not an integer), +123 (unary plus not accepted). You may assume that the operands and the result are within the range of value of the int type.
//操作数是整型的。例如：可以是23，0，-123但不能是23.5（不是整型），51.0（不是整型），+123（加号不被接收）。你要确保输入的操作数要在整型数据类型所能表示的范围之内。
You need to handle division-by-zero error so that your program does not crash when it happens.
A partial code is given:
//你需要在程序中写出代码捕捉除数为零的异常

import java.util.*;

class Calculator {

Scanner scanner = new Scanner(System.in);

//-----------------------------------------------------
// Data members
//-----------------------------------------------------
String expr;
int length;
int index;

//-----------------------------------------------------
// Constructor
//-----------------------------------------------------
public Calculator () {
}

//-----------------------------------------------------
// main method
//-----------------------------------------------------
public static void main (String [] args) {

Calculator cal = new Calculator();

// to be filled in

}

// You may add other methods here

}

The data members given in the partial code are just suggestions. You are free to use your own approach.

Please refer to the sample outputs below.

1.3 Sample runs
Sample run using interactive input：

$ javac Calculator.java
$ java Calculator
123 + 345
123 + 345 = 468
-20+13
-20 + 13 = -7
004 -10
4 - 10 = -6
12*-0011
12 * -11 = -132
30 / 4
30 / 4 = 7
123/-7
123 / -7 = -17
q

Another sample run that shows some invalid inputs:

//如果输入的不符合要求要显示提示信息例如：

$ java Calculator
xyz
Error in expression
a + b
Error in expression
12345
Error in expression
-10 *
Error in expression
12.3 + 32
Error in expression
1 + 2 + 3
Error in expression
+30 / 3
Error in expression
30 // 7
Error in expression
333 / 0
Division by zero error
333 / 4
333 / 4 = 83
q

1.4 Important notes //特别注意的是：

Ensure that the outputs of your program comform to the format of the sample outputs given.
//确保你程序的输出和给出的例子的输出格式保持一致。

很急啊！小女子先在这里谢谢各位了！