使用在IF语句中声明的变量

Question

我刚刚在大学学习了java编程课程。我们被要求做一个项目。但是，我在这里面临困境，一些援助将受到高度赞赏。

 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IF中的totalGPA，semtaken，semcode和GPA。我如何使用它们而不会显示错误“找不到变量”？我不能在IF之前初始化数组，因为我不知道它的大小。用户输入尺寸，因此必须在IF内。

Answer 1

您不能分配，但你可以将其引入。

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

Answer 2

你可以将其定义外，如果：

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

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

Answer 3

只是声明（但不初始化），你需要的if外if之前的变量。例如

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