Java-如何在通用类中调用我的类T的方法

Question

I've an issue with my class ListFromFile<T> which extends ArrayList<T> . In that class, i would like to make a method that finds an element by its id attribute and returns it.

The T could be an object from the classes Student, Teacher etc. All these classes have a equals method that tests the id.

My problem is that i can't use the equals method which test the id in ListFromFile .

Here is my code :

public class ListFromFile<T> extends ArrayList<T> implements Serializable {
    public T getElement(int id) {
        for ( T o : this ) {
             if ( o.equals((int)id) ) {
                  return o;
             }
        }

        return null;
    }
}

Even if i specify in the equals method i'm working with an id which is an int, getElement() doesn't find the element ...

After some searches, it seems that i have to extend T by Student, Teacher etc., but how to extends multiples classes ?

Thank you

Answer 1

You should use an interface or an abstract class with a getId() method. Bind T to be a subtype of either the interface or the abstract class. Using an interface, it might look like this.

public interface ObjectWithId {
    int getId();
}

public class Student implements ObjectWithId {
    // ...
}

public class Teacher implements ObjectWithId {
    // ...
}


public class ListFromFile<T extends ObjectWithId> extends ArrayList<T> {
    public T getElement(int id) {
        for ( T o : this ) {
            if ( o.getId() == id ) {
                return o;
            }
        }

        return null;
    }
}

Answer 2

Here is sample application which shows how you can solve this problem:

import java.io.Serializable;
import java.util.ArrayList;

public class test2 {
    // define basic interface for all objects
    interface ObjectWithID {
        public int getId();
    }

    public static class Teacher implements ObjectWithID {
        private final int id;

        public Teacher(final int id) {
            this.id = id;
        }

        @Override
        public int getId() {
            return this.id;
        }
    }

    public static class Student implements ObjectWithID {
        private final int id;

        public Student(final int id) {
            this.id = id;
        }

        @Override
        public int getId() {
            return this.id;
        }
    }

    // note T extends syntax
    public static class ListFromFile<T extends ObjectWithID> extends ArrayList<T> implements Serializable {
        public T getElement(final int id) {
            for (final T o : this)
                if (o.getId() == id)
                    return o;
            return null;
        }
    }

    public static void main(final String[] args) {
        final ListFromFile list = new ListFromFile<>();

        list.add(new Teacher(1));
        list.add(new Teacher(2));
        list.add(new Teacher(3));

        list.add(new Student(4));
        list.add(new Student(5));
        list.add(new Student(6));

        System.out.println(list.getElement(1)); // print teacher
        System.out.println(list.getElement(4)); // print student
    }
}