在 if 语句中创建的对象数组（范围问题）

Question

程序以以下提示开始：

1. 创建新的学生列表
2. 搜索学生。

问题：我创建了一个对象数组并在第一个 if 语句中填充它，然后尝试在第二个 if 语句中访问它，我知道我不能这样做。那么如何创建和填充对象数组并在以后访问它呢？有什么想法吗？

if(iUserSelection == 1) {

    System.out.println();
    System.out.println("How many students?");
    x = oScan.nextInt();
    System.out.println();

    // flush the buffer
    oScan.nextLine();

    Student[] oClassList = new Student[x];
    for(int i = 0; i < x; i++) {
        System.out.println("*********************");
        System.out.println("Student " + (i + 1) + " of " + x);
        System.out.println("*********************");

        oClassList[i] = new Student("","",0,0,0,0);

        System.out.print("First Name: ");
        oClassList[i].setFirstName(oScan.nextLine());

        System.out.print("Last Name: ");
        oClassList[i].setLastName(oScan.nextLine());

        System.out.print("Homework average: ");
        oClassList[i].setHWAve(oScan.nextInt());

        System.out.print("Quiz average: ");
        oClassList[i].setQuizAve(oScan.nextInt());

        System.out.print("Project average: ");
        oClassList[i].setProjectAve(oScan.nextInt());

        System.out.print("Test average: ");
        oClassList[i].setTestAve(oScan.nextInt());

        // flush the buffer
        oScan.nextLine();

        System.out.println();
        oClassList[i].printStudent();
    }
}

if(iUserSelection == 2) {
    // flush the buffer
    oScan.nextLine();

    if(oClassList[0] != null) { 
        System.out.println("Student search");

        System.out.print("Enter last name: ");
        sSearchLastName = oScan.nextLine();

        System.out.print("Enter first name: ");
        sSearchFirstName = oScan.nextLine();
    }

    for(int y = 0; y >= oClassList.length; y++) {
        if(sSearchLastName == oClassList[y].lastName) {
            System.out.println("found elements");
        }
        else
            System.out.println("Error - Student not found");
    }
}

Answer 1

为了防止数组在 if 语句退出时被删除，请在 if 语句之外声明它，使其范围更广。然后可以在 if 语句中填充数组，而不会在 if 语句退出时超出范围。例如，

int[] arr;
if (true) {
    arr = new int[1];
    arr[0] = 5;
}
System.out.println(arr[0]);

输出：

5

arr 将在退出 if 语句时保持其值，因为它是在 if 语句之外声明的，然后在内部实例化。

您更正的代码是：

Student[] oClassList;                //Declared outside of both if-statements

if(iUserSelection == 1) {

    System.out.println();

    System.out.println("How many students?");
    x = oScan.nextInt();

    System.out.println();


    // flush the buffer
    oScan.nextLine();

    oClassList = new Student[x];

    for(int i = 0; i < x; i++) {
        System.out.println("*********************");
        System.out.println("Student " + (i + 1) + " of " + x);
        System.out.println("*********************");


        oClassList[i] = new Student("","",0,0,0,0);


        System.out.print("First Name: ");
        oClassList[i].setFirstName(oScan.nextLine());

        System.out.print("Last Name: ");
        oClassList[i].setLastName(oScan.nextLine());

        System.out.print("Homework average: ");
        oClassList[i].setHWAve(oScan.nextInt());

        System.out.print("Quiz average: ");
        oClassList[i].setQuizAve(oScan.nextInt());

        System.out.print("Project average: ");
        oClassList[i].setProjectAve(oScan.nextInt());

        System.out.print("Test average: ");
        oClassList[i].setTestAve(oScan.nextInt());



        // flush the buffer
        oScan.nextLine();

        System.out.println();



        oClassList[i].printStudent();
    }
}

if(iUserSelection == 2) {
    // flush the buffer
    oScan.nextLine();

    if(oClassList[0] != null) { 
        System.out.println("Student search");

        System.out.print("Enter last name: ");
        sSearchLastName = oScan.nextLine();

        System.out.print("Enter first name: ");
        sSearchFirstName = oScan.nextLine();
    }


    for(int y = 0; y >= oClassList.length; y++) {
        if(sSearchLastName == oClassList[y].lastName) {
            System.out.println("found elements");
        }
        else
            System.out.println("Error - Student not found");
    }


}

Answer 2

你已经知道你的答案了。这是一个范围问题，因此解决方案是在“更广泛”的范围内定义您的数组，您的第二个 if 也可以看到。所以，基本上在 if 语句之前定义它。

Answer 3

范围由块定义，块由括号{} 分隔。如果您在 if 块内创建数组，您将无法在另一个 if 块中访问它。

如何解决这个问题？您可以在外部块中声明数组，这样您就可以在所有块中访问它。

Student[] oClassList = null;

if (iUserSelection == 1) {
    // ...
    oClassList = new Student[x];
    // ...
}

if (iUserSelection == 2) {
    if(oClassList != null)
        // ...
    // ...
}