java使用在IF语句中声明的变量

Question

我刚在大学上过java编程课程。我们被要求做一个项目。然而，我在这里面临着一个困境，我们将非常感谢您的帮助

        if (opt == 1) {
            System.out.println("Enter your name:");
            name1 = keyboard.next();
            name2 = keyboard.next();
            System.out.println("Enter your ID:");
            ID = keyboard.nextInt();
            System.out.println("Enter semesters taken:");
            semtaken = keyboard.nextInt();
            int[] semcode = new int[semtaken];
            int[] subjectTaken = new int[semtaken];
            double[] GPA = new double[semtaken];
            double finalGPA;
            for (i = 0; i < semtaken; i++) {
                System.out.println("Enter semester code for semester no " + (i + 1));
                semcode[i] = keyboard.nextInt();
                System.out.println("Enter subjects taken:");
                subjectTaken[i] = keyboard.nextInt();
                String[][] subject = new String[subjectTaken[i]][4];
                int subjectsTakenNO = subjectTaken[i];

                System.out.println("Enter your subject code, hours, letter");
                for (j = 0; j < subjectTaken.length; j++) {
                    for (int k = 0; k < 3; k++) {
                        subject[j][k] = keyboard.next();
                    }

                }

                GPA[i] = getGPA(subject, subjectsTakenNO);
                totalGPA += GPA[i];
                finalGPA = totalGPA / semtaken;
                System.out.println("Your GPA for this term is" + getGPA(subject, subjectsTakenNO));
            }
            opt1 = true;
        } /* Here is the PROBLEM. I want to get I want to get semtaken and gpa but it shows an
             error (Variable not found) */ 
        else if (opt == 2 && opt1) {
            summary(name1, name2, ID, GPA, totalGPA, semtaken, semcode);

所以基本上，我想使用totalGPA、Semtake、semcode和GPA，它们都在IF中。如何使用它们而不显示错误“未找到变量”？我无法在IF之前初始化数组，因为我不知道它的大小。用户输入大小，因此它必须位于IF中

Answer 1

如果满足以下条件，则可以在外部对其进行定义：

GPA[] array = null; //or new GPA[0];

if (...) {
   array = new GPA[x];
}

Answer 2

只需在if之前的if外部声明（但不初始化）所需的变量。比如说

int semtaken;
if (opt == 1) {
    ...
    semtaken = ...;
    ...
}
foo(semtaken);

Answer 3

你不能分配它，但你可以引入它

int[] list = null;
if (...) {
    list = new int[10];
}
Do what so ever with list.