Why Java Collections Cannot Directly Store Primitives Types?
Primitive types are the most basic data types available within the Java language. Such types serve only one purpose — containing pure, simple values of a kind. Since java is a Statically typed language where each variable and expression type is already known at compile-time, thus you can not define a new operation for such primitive types.
Illustration:
Invalid : vector.addElement(3) ;
Valid : vector.addElelment("3") ;Conclusion:
- Java primitive types are not referenced types. For example, int is not an Object.
- Java does generics using type-erasure of reference types. For example, A List<?> is really a List<Object> at run-time.
Collections are the framework used to store and manipulate a group of objects. Java Collection means a single unit of objects. Since the above two statements are true, generic Java collections can not store primitive types directly.
Wrapper Class provides a way to use primitive data types (int, boolean, etc..) as objects or a Wrapper class is a class whose object wraps or contains primitive data types. It gives birth to two concepts as follows:
- Autoboxing
- Unboxing
| Primitive Data Type | Wrapper Class |
|---|---|
| byte | Byte |
| short | Short |
| int | Integer |
| long | Long |
| float | Float |
| double | Double |
| boolean | Boolean |
| char | Character |
Autoboxing is the automatic conversion of primitive types to the object of their corresponding wrapper classes is known as autoboxing. For instance:
- Conversion of int to Integer
- Conversion of long to Long
- Conversion of double to Double, etc.
Unboxing 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.
Illustration: Autoboxing
Java
// Importing input output classesimport java.io.*;class GFG { // Main driver method public static void main(String args[]) { // Custom input Integer i = new Integer(21); // Boxing Integer j = 5; System.out.println("i=" + i + "\n j=" + j); }} |
Output:
i=21 j=5
Illustration 2: Unboxing
Java
// Import input output classesimport java.io.*;// Classpublic class GFG { // MAin driver method public static void main(String args[]) { // Custom input Integer i = new Integer(50); // Unboxing int a = i; // Unboxing int b = i.intValue(); // Print and display System.out.println("a=" + a + "\nb=" + b); }} |
Output:
a=50 b=50
Implementation: While using the collection java compiler create a wrapper Object from the primitive type and adds it to the collection using generics.
Example 1:
Java
// Java Program to illustrate Collections// are not directly storing primitives types// Importing input output classesimport java.io.*;// Importing all classes from// java.util packageimport java.util.*;// Classclass GFG { // Main driver method public static void main(String[] args) { // Creating a list of elements of Integer type. List<Integer> list = new ArrayList<Integer>(); // Iterating over elements of List object for (int i = 0; i < 10; i++) { // Adding the int primitives type values // If elements are added using add() method // then compiler automatically treats as // add(Integer.valueOf(i)) list.add(i); // This is what compiler does and // hence the goal achieved. // Print the primitive values System.out.println(i); } }} |
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Example 2: Collections to store Primitive datatype
Java
// Java Program to illustrate Collections// are not directly storing primitives types// Importing Map and HashMap classes// from java.util packageimport java.util.HashMap;import java.util.Map;// Classpublic class GFG { // Main driver method public static void main(String[] args) throws Exception { // Creating an object of Map type Map map = new HashMap(); // Creating int wrapper object // Custom input Integer var = new Integer(21); // Storing int to map map.put("key", var); // Getting int value from map Integer refVar = (Integer)map.get("key"); // Get the integer value from wrapper object int i = refVar.intValue(); // Display message for successful compilation System.out.print("Successfully compiled and executed"); }} |
Output:
Successfully compiled and executed
