Wrapper Classes for Primitives

Generally when you need to deal with numbers, you choose one of the primitive classes. However when you try to keep a group of numbers in a Collection object, then you cannot use primitives as Collection framework only supports objects. To overcome this limitation, Java provides us with equivalent wrapper classes for each primitive. A wrapper numbers class is nothing but a class which encloses a primitive and behaves as an object instead of primitive.

Here are some of the wrapper classes for primitives which all extend the top level abstract Number class:

• Integer
• Float
• Double
• Short
• Long

In addition to numbers, Java also provides a wrapper for boolean values:

• Boolean

Now you can create a collection of primitives using any of these wrapper classes.

While creating a wrapper object, you just assign the primitive value to the wrapper class's object reference. A wrapper object is created automatically. Converting primitive data types into object is called boxing. In a similar way, a primitive reference can get assigned with the primitive value when it is assigned a wrapper object. This process is called unboxing.

Here is an example

import java.util.ArrayList;
public class NumbersDemo {

    public static void main(String[] args) {
        Integer a = 2; // boxing
        int b = a; //unboxing
        System.out.println(b);

        ArrayList<Integer> c = new ArrayList<Integer>();
        c.add(3); //boxing
        c.add(4); //boxing

        System.out.println(c.get(1));
    }
}

Output:

2 4

Integer classes are also commonly used to get the primitive values from strings contains only primitive values. Here is an example:

public class ParseDemo {

    public static void main(String[] args) {
        String a = "123";
        int b = Integer.parseInt(a);
        System.out.println(b);

        String c = "true";
        Boolean d = Boolean.parseBoolean(c);
        System.out.println(d);
    }
}