A Wrapper class in Java is one whose object wraps or contains primitive data types. This leads to two key features: Autoboxing and Unboxing.
1. Autoboxing
The automatic conversion of primitive types to the object of their corresponding wrapper classes is known as autoboxing. For example: conversion of int to Integer, long to Long, double to Double, etc.
2. Unboxing
It is just the reverse process of autoboxing. Automatically converting an object of a wrapper class to its corresponding primitive type is known as unboxing. For example, conversion of Integer to int, Long to long, Double to double, etc.
Example 1: Java program to illustrate the Concept of Autoboxing and Unboxing
import java.io.*;
// Main class
class GFG {
// Main driver method
public static void main(String[] args)
{
// Creating an Integer Object with custom value say it be 10
Integer i = new Integer(10);
// Unboxing the Object
int i1 = i;
// Print statements
System.out.println("Value of i:" + i);
System.out.println("Value of i1: " + i1);
// Autoboxing of character
Character gfg = 'a';
// Auto-unboxing of Character
char ch = gfg;
// Print statements
System.out.println("Value of ch: " + ch);
System.out.println(" Value of gfg: " + gfg);
}
}
Output:
Let's understand how the compiler did autoboxing and unboxing in the example of Collections in Java using generics.
Example 2: Java Program to Illustrate Autoboxing
import java.io.*;
import java.util.*;
// Main class
class GFG {
// Main driver method
public static void main(String[] args)
{
// Creating an empty Arraylist of integer type
ArrayList<Integer> al = new ArrayList<Integer>();
// Adding the int primitives type values using add() method Autoboxing
al.add(1);
al.add(2);
al.add(24);
// Printing the ArrayList elements
System.out.println("ArrayList: " + al);
}
}
Output
ArrayList: [1, 2, 24]
Explanation:
In the above example, we have created a list of elements of the Integer type. We are adding int primitive type values instead of Integer Object and the code is successfully compiled. It does not generate a compile-time error as the Java compiler creates an Integer wrapper Object from primitive int i and adds it to the list.
Example 3: Java Program to Illustrate Autoboxing
import java.io.*;
import java.util.*;
// Main class
class GFG {
// Main driver method
public static void main(String[] args)
{
// Creating an empty ArrayList of integer type
List<Integer> list = new ArrayList<Integer>();
// Adding the int primitives type values by converting them into Integer wrapper object
for (int i = 0; i < 10; i++)
System.out.println(
list.add(Integer.valueOf(i)));
}
}
Output
true true true true true true true true true true
Another example of auto and unboxing is to find the sum of odd numbers in a list. An important point in the program is that the operators remainder (%) and unary plus (+=) operators do not apply to Integer objects. But still, code compiles successfully because the unboxing of Integer Object to primitive int value is taking place by invoking intValue() method at runtime.
Example 4: Java Program to Illustrate Find Sum of Odd Numbers using Autoboxing and Unboxing
import java.io.*;
import java.util.*;
// Main class
class GFG {
// Method 1 To sum odd numbers
public static int sumOfOddNumber(List<Integer> list)
{
// Initially setting sum to zero
int sum = 0;
for (Integer i : list) {
// Unboxing of i automatically
if (i % 2 != 0)
sum += i;
// Unboxing of i is done automatically using intvalue implicitly
if (i.intValue() % 2 != 0)
sum += i.intValue();
}
// Returning the odd sum
return sum;
}
// Main driver method
public static void main(String[] args)
{
// Creating an empty ArrayList of integer type
List<Integer> list = new ArrayList<Integer>();
// Adding the int primitives type values to List
for (int i = 0; i < 10; i++)
list.add(i);
// Getting sum of all odd numbers in List
int sumOdd = sumOfOddNumber(list);
// Printing sum of odd numbers
System.out.println("Sum of odd numbers = "
+ sumOdd);
}
}
Output
Sum of odd numbers = 50
Java Primitive Types and Their Wrapper Classes
| Primitive Type | Wrapper Class |
|---|---|
| boolean | Boolean |
| byte | Byte |
| char | Character |
| float | Float |
| int | Integer |
| long | Long |
| short | Short |
| double | Double |