java - 如何在Java中的一个类中调用另一个类的方法

Question

我正在尝试使用我在 class 中创建的方法Student，在另一个类中，Classroom. 我想知道是否甚至可以调用在另一个类中定义的方法，或者我是否以错误的方式接近这个。

类Student：

public class Student {
    String firstName;

    Student(String firstName){
        this.firstName=firstName;
    }

    public String getFirstName() {
        return firstName;
    }
}

类Classroom：

public class Classroom{

    public ArrayList<Student> students;

    public Classroom(ArrayList<Student> studentArrayList) {

    }

    Student getStudent(ArrayList students){
        this.students=students;
        System.out.println(students.get(0).getFirstName);
        return null;
    }
}

Main：

public class Main {

    public static void main(String[] args) {
        Student a = new Student("John");
        Student b = new Student("Jane");

        ArrayList<Student> studentArrayList = null;
        studentArrayList.add(a);
        studentArrayList.add(b);

        Classroom c = new Classroom(studentArrayList);

        System.out.println(c.getStudent(studentArrayList));
    }
}

我的问题似乎是当我尝试调用该方法getFirstName时，我想知道是否有一种方法可以getFirstName在Classroom类中调用。我对这段代码的意图是返回 object a。

标签： javamethods

Answer 1

问题是，你忘了getFirstName()在Classroom类后面加上括号，而且，你应该在构造函数中初始化students。

它应该是这样的：

public class Classroom {

    public ArrayList<Student> students;

    public Classroom(ArrayList<Student> studentArrayList) {
        students = studentArrayList
        // you can add other code here
    }

    Student getStudent(int index) {
        // get the first name of the student at the given index
        System.out.println(students.get(index).getFirstName());
        return students.get(index); // return the Student object instead of null
    }
}

然后，在Main课堂上：

public class Main {

    public static void main(String[] args) {
        Student a = new Student("John");
        Student b = new Student("Jane");

        ArrayList<Student> studentArrayList = new ArrayList<Student>();
        studentArrayList.add(a);
        studentArrayList.add(b);

        Classroom c = new Classroom(studentArrayList); // now the students ArrayList is initialized

        System.out.println(c.getStudent(0)); // replace 0 with the index you want
    }
}

您可以保持Student课程不变。