java - 对象数组困惑

Question

程序开始时会提示:

1. 创建新的学生列表
2. 搜索学生
3. 退出

问题:在第二个 if 语句中 oClassList 未初始化。如何在第一个 if 语句中实例化的对象数组中搜索第二个 if 语句中的姓氏？

    int z = 0;

    while(run) {

        if(z == 0) {
            System.out.println("Please choose an option.  (Enter 1, 2, or 3)\n");
        }
        else {
            System.out.println();
            System.out.println("Would you like to continue?  If so, please choose an option.\n");
        }

        System.out.println("1. New Class List");
        System.out.println("2. Search for a Student");
        System.out.println("3. Exit");

        iUserSelection = oScan.nextInt();


        Student[] oClassList;

        // creates new class list
        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();

                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");
                }



        }

        if(iUserSelection == 3) {
            run = false;

            System.out.println();
            System.out.println("Goodbye.");
        }





        z++;
    }

Answer 1

您的变量 Student[] oClassList; 是一个局部变量，嵌套在 while 循环内。这将导致每次程序离开 while 范围时重置 oClassList(因此每个 while 处理都会有自己的类列表)。

您只需将该变量声明移至 while 循环上方 - 该变量的范围将跨越整个程序。

因此，只需将 Student[] oClassList; 直接移到 int z = 0; 下面，一切都会正常工作。

<小时/>

编辑:

作用域并不是一切，你还必须初始化变量。空检查(如另一个答案中提到的)也不是一个坏主意。

int z = 0;

Student[] oClassList = null;


while(run) {

    if(z == 0) {
        System.out.println("Please choose an option.  (Enter 1, 2, or 3)\n");
    }
    else {
        System.out.println();
        System.out.println("Would you like to continue?  If so, please choose an option.\n");
    }

    System.out.println("1. New Class List");
    System.out.println("2. Search for a Student");
    System.out.println("3. Exit");

    iUserSelection = oScan.nextInt();


    // creates new class list
    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();

            System.out.println("Student search");

            if (oClassList == null) {
                System.out.println("No class list defined yet");
            } else {

                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");
                }
            }



    }

    if(iUserSelection == 3) {
        run = false;

        System.out.println();
        System.out.println("Goodbye.");
    }





    z++;
}