如何连接 java 中的字段？

Question

In my project I ran into a situation that I could not quickly google. I have several objects and write a value in the id field of each, when I change this id later, I need to update the value in the id fields of each object. Example b = null; a = b; => a = null; b = null; a = b; => a = null; after, for example, b = 3 , it is necessary to update the value of a = 3 without a = b again. I will try to explain in more detail.

class Student { private Long id; } 
class Class { private Long id; } 
Student s = new Student (); 
Class c = new Class (); 
c.getid = null; 
s.setId (c.getId); => s.getId == null;

after some operation c.getId == 1; but s.getId == null how i can do s.getId == 1 without s.setId (c.getId) ?

Is this possible?

Answer 1

b = 3, it is necessary to update the value of a = 3 without a = b again. Is this possible?

If fields a and b are of primitive data type then no. You would need to update both a and b .

If a and b are instance on an Object then yes, since a and b share the same memory reference. So if your fields are primitive data types you can wrap them inside a Custom Object with proper a set/get method and you are good to go.

If those fields can be read/write concurrently by multithreads you may need to also ensure mutual exclusion when accessing them.

Answer 2

As I suspected, your question is in fact an XY Problem where you are barking up the wrong tree. The solution is not to change the Student's ID, but rather to pass references where they are needed. Here, I am suspecting that id represents the id of a class, and so the best solution is to give the Class class an ArrayList<Student> field (since it will hold multiple Students), and to give your Student an ArrayList<Class> field (since a Student can attend multiple classes).

Then, when a Student registers for a class (via some method), add that Class to its classes list, and add the Student to the Class's students list.

eg,

public class Student {
    private int id;
    private List<Class> classes = new ArrayList<>();
    
    // constructor, getters, setters
    
    public void addClass(Class class) {
        classes.add(class);
    }
    
    // also removeClass method, a getClasses method...
}

public class Class {
    private int id;
    private List<Student> students = new ArrayList<>();
    
    // constructor, getters, setters
    
    public void addStudent(Student student) {
        students.add(student);
    }
    
    // also removeStudent method, a getStudents method...
}

Answer 3

thanks for your good explanation of question, but that would be better if you told us why. buy the way, I think this is a XY problem, the more important question is "why do you need this"?

let's speak about your question, there are few options that you should consider.

1. type of a and b is primitive since primitives have their own memory (not pointing to somewhere else), like C language, after the assignment nothing connects a and b and they can (and will) change separately. for example

int a, b;
a = 3;
b = a;

a = 5;
// b is still 3

2. a and b are references and you want them to point to same object. Objects are in the heap and since there is no assignment overload (like we have in cpp ), after the assignment a and b are pointing to exact same object so each change in the object that a is pointing to, will seen from b. BUT REMEMBER this connection is unless a and b is pointing to same object and manipulate object state with . ;

class Student {
    int id;
}

class Demo {
    public static void main(String[] args) {
        Student a, b;
        a = new Student();
        a.id = 3;

        b = a;
        // they are now pointing to same object
        a.id = 5;
        // b.id is 5 too, basically same int

        b = null;
        // now they are disconnect
        // or maybe this: b = new Student()

    }
}

I think this could help and answer your question, but let's discuss about your code snippet too.

3. if you want references themselves change. for example:

String a, b;
a = "some string"
b = a;

a = null;
// b  should be null?

as far as i know, there isn't something in java that provide this for you, no operator overloading or preprocessor that can help.

but if you insist I can think of some hacks .

• use C or any other customized preprocessor and define some macro to do this. for example rename all b variables with a before compilation with java compiler. check this
• define a singleton object (or just a dummy class) and a , b keep references to that object, this is actually combination of case 2 and 3;

with static class:

class Global {
    static Object reference;
}

class Demo {
    public static void main(String[] args) {
        Global a = null, b = null;
        a.reference  = "some string value";
        b.reference = null;

        // a.reference is null

    }
}

and with singleton class:

class Singleton {
    private static final Singleton instance = new Singleton();
    public static Singleton getInstance() {
        return Singleton.instance;
    }
    private Singleton() {}

    Object ref;
}

class Demo {
    public static void main(String[] args) {
        Singleton a =  Singleton.getInstance();
        Singleton b = a;

        a.ref = "some string value";
        b.ref = null;

        // a.ref is null
        System.out.println(a.ref);
    }
}

(yes I know that isn't same thing that you wanted but java is simple language in syntax level)