郁闷，这段代码哪里有错？不能通过编译。

Question

在方法最后面加上一个 return null;试试

Answer 1

不是吧？这样不就成了这个method只return null？

Answer 2

while (flag) 循环外边必须有return语句，从编译的角度来说并不知道一定会进入循环！

Answer 3

给你改了一下，再编译一下看看吧，记住if else块中都要有返回值的，因为编译器不知道到底会执行到那一步，你的if里面有返回值，而else里面就没有，所以编译器报错，你照着这个思路好好找找看 private static User getPass() throws InvalidInputException
// returns three kinds of users depending on pass
{
boolean flag = true;
while (flag)
{
System.out.println();
System.out.print("Please input your password: ");
String pass = myIO.readStr(); // if pass is ENTER key, the use is an anonymous user
if (pass.equals(""))
return new Anonymous();
else
{
list.reset();
String p = ((University)(list.getCurrent().getElement())).getPass();
while (list.getCurrent() != null && !pass.equals(p))
list.advance();
// if pass matches one of the universities' pass, then the user is
// that university's administrator user
if (list.getCurrent() != null)
return new Admin();
else
{
list.reset();
HashTable table = ((University)(list.getCurrent().getElement())).getTable();
while (list.getCurrent() != null && !table.isStudentMatched(p))
list.advance();
// if pass matches one of the student's pass, then the user is a
// student user
if (list.getCurrent() != null)
return new StudentUser();
else
{
System.out.println("Oops! Have you typed a wrong password? Try again.");
return null;
}
return null;
}
return null;
}

}
}

Answer 4

private static User getPass() throws InvalidInputException
        // returns three kinds of users depending on pass
        {
                boolean flag = true;
                User user = null;
                while (flag)
                {
                        System.out.println();
                        System.out.print("Please input your password: ");
                        String pass = myIO.readStr();                        // if pass is ENTER key, the use is an anonymous user
                        if (pass.equals(""))
                                user = new Anonymous();
                        else
                        {
                                list.reset();
                                String p = ((University)(list.getCurrent().getElement())).getPass();
                                while (list.getCurrent() != null && !pass.equals(p)) 
                                        list.advance();
                                // if pass matches one of the universities' pass, then the user is 
                                // that university's administrator user
                                if (list.getCurrent() != null)
                                        user = new Admin();
                                else
                                {
                                        list.reset();
                                        HashTable table = ((University)(list.getCurrent().getElement())).getTable();
                                        while (list.getCurrent() != null && !table.isStudentMatched(p)) 
                                                list.advance();
                                        // if pass matches one of the student's pass, then the user is a
                                        // student user
                                        if (list.getCurrent() != null)
                                                user = StudentUser();
                                        else
                                        {
                                                System.out.println("Oops! Have you typed a wrong password? Try again.");                                                
                                        }
                                }
                        }
                }
                return user;
        }