Java – Extending base class containing abstract generic method in java

abstractgenericsjavajava-7

I am given a Base class like this:

 Class MyBase{

   public abstract <T> Student assign (T object);

 }

I am extending it like below:

 Class MyClass extends MyBase{

   @Override
   public abstract <T> Student assign (T object){

     Student stew = new Student();
     ......

   }
 }

My intended use is: the passed in object should be of type Teacher. Within the method I want to create a new Student which will get some values by invoking functions on Teacher. (For example, stew.setTeacherName = teacher.getName();) And then return Student to the caller.

Questions:
a) How do I get the intended behavior?
b) Whats the difference between declaring MyBase the current way vs. doing MyBase ?
c) I am interested in knowing the solution in case where I CAN change Base class and I CANNOT change the Base class.

Thanks in advance for your help. Also, if there are any useful resources/tutorials you can point me to that would be great. Thanks again.

Best Answer

You cannot guarantee that the object being passed in to MyClass's assign method is a Teacher, if MyBase is defined this way. If you are allowed to modify MyBase, do the following:

abstract class MyBase
{
   public abstract <T extends Teacher> Student assign (T object);
}

and extend it with your class:

class MyClass extends MyBase
{
   @Override
   public <T extends Teacher> Student assign (T object){

     Student stew = new Student();
     ......

   }
}

This will guarantee that the object being passed in is a Teacher, even if it's really an instance of a subclass of Teacher.

EDIT: Another way to modify MyBase. Make the generic parameter part of the class definition instead of the method definition.

abstract class MyBase<T extends Teacher>
{
   public abstract Student assign (T object);
}

class MyClass<T extends Teacher> extends MyBase<T>
{
   @Override
   public  Student  assign (T object)
   {
      Student stew = new Student();
      ......
   }
}

// Could also do this...
class MyClass2 extends MyBase<Teacher>
{
   @Override
   public  Student  assign (Teacher object)
   {
      Student stew = new Student();
      ......
   }
 }

Here's Oracle's Java Generics Tutorial.

Related Solutions

Java inner class and static nested class

From the Java Tutorial:

Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes.

Static nested classes are accessed using the enclosing class name:

OuterClass.StaticNestedClass

For example, to create an object for the static nested class, use this syntax:

OuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass();

Objects that are instances of an inner class exist within an instance of the outer class. Consider the following classes:

class OuterClass {
    ...
    class InnerClass {
        ...
    }
}

An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance.

To instantiate an inner class, you must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:

OuterClass outerObject = new OuterClass()
OuterClass.InnerClass innerObject = outerObject.new InnerClass();

see: Java Tutorial - Nested Classes

For completeness note that there is also such a thing as an inner class without an enclosing instance:

class A {
  int t() { return 1; }
  static A a =  new A() { int t() { return 2; } };
}

Here, new A() { ... } is an inner class defined in a static context and does not have an enclosing instance.

Java – Create instance of generic type in Java

You are correct. You can't do new E(). But you can change it to

private static class SomeContainer<E> {
    E createContents(Class<E> clazz) {
        return clazz.newInstance();
    }
}

It's a pain. But it works. Wrapping it in the factory pattern makes it a little more tolerable.

Best Answer

Related Solutions

Java inner class and static nested class

Java – Create instance of generic type in Java

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