Java – How to impose constraints on Generics in Java?

In our previous article, we discussed about generics in Java. There are a few constraints that we can impose while working with Generics using extends and super keywords.

The ‘extends’ keyword with respect to class

Let’s create the Demo<T> class using extends keyword.

class Demo<T extends Number> {
       T value;
}

This implies that the class Demo will only accept Type arguments that extend the Number class. That is the T can only be replaced by classes that extend the Number class. So, while creating an instance for the class Demo, we should only use the classes that extend the Number class.

Demo<Integer> d = new Demo<Integer>(); // valid
Demo<Float> f = new Demo<Float>(); // valid

Since Float and Integer classes extend the Number class they are valid to be specified as Type arguments. But, below statement throws an error; as the String class does not extend the Number class.

Demo<String> s = new Demo<String>(); // this throws an error

The ‘extends’ keyword with respect to methods

Let’s consider adding one more method to the Demo class:

void show(ArrayList obj) { 
    // -- Method code goes here
}

This method says that it accepts the ArrayList of Integers as its parameter. How can we modify it, so that it can only accept values that extend the Number class?

This is how we do it:

 void show(ArrayList<? extends Number> obj) {}

Now the ArrayList here can accept values of all the Types that extend the Number class. This is the syntax it;

The above method can also be written like this:

// T can be anything that will replace it
void show(ArrayList<? extends T> obj) {
 // -- Method code goes here
}

Usage of the ‘super’ keyword

The same method discussed above can be modified to be written like this:

void show(ArrayList<? super Integer> obj) {
 // -- Method code goes here
}

This means the ArrayList will accept values of Type Integer or of Type Number, since Number is the super class to Integer.

Let’s see a complete working example here;

After running the above program; the output will be;

Output:

15
[10, 10.2]
[20, 20.2]

More topics will be discussed in upcoming articles.

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