Wrapper Classes & Autoboxing

#Introduction

Java provides Wrapper Classes to represent primitive data types as objects. They are heavily used in:

• Collections (ArrayList, HashMap, HashSet)
• Generics
• Streams API
• Reflection
• Frameworks like Spring Boot and Hibernate

Without wrapper classes, many Java APIs would not work with primitive values.

#What are Wrapper Classes?

A Wrapper Class is an object representation of a primitive data type.

int age = 25;
Integer wrapperAge = Integer.valueOf(age);

Now wrapperAge is an object.

#Why Do We Need Wrapper Classes?

Collections store objects, not primitives.

Wrong:

ArrayList<int> list = new ArrayList<>();   // Compilation Error

Correct:

ArrayList<Integer> list = new ArrayList<>();

Collections require object references.

#Wrapper Class Hierarchy

#Creating Wrapper Objects

Method 1 — valueOf() (Recommended)

Integer number = Integer.valueOf(100);

Method 2 — Autoboxing

Integer number = 100;

The compiler converts it internally to:

Integer number = Integer.valueOf(100);

Deprecated Constructor

Integer number = new Integer(100);

#Unboxing

Converting Wrapper → Primitive.

Integer number = 100;
int value = number;

The compiler automatically converts this to:

int value = number.intValue();

#Autoboxing

Introduced in Java 5. Primitive → Wrapper.

Integer num = 50;

Internally:

Integer num = Integer.valueOf(50);

No explicit conversion required.

#Auto-Unboxing

Wrapper → Primitive.

Integer number = 10;
int value = number;

Internally:

int value = number.intValue();

#Complete Example

Integer age = 25;
int x = age;
Integer y = x;

#Integer Caching

This is one of the most common interview questions.

Integer a = 100;
Integer b = 100;
System.out.println(a == b);

true

Why? Java caches Integer objects from -128 to 127. Both variables refer to the same cached object.

Integer a = 200;
Integer b = 200;
System.out.println(a == b);

false

Because 200 is outside the cache range, two separate objects are created.

Integer Cache Memory

#equals() vs ==

Integer a = 200;
Integer b = 200;
System.out.println(a == b);

false

System.out.println(a.equals(b));

true

#NullPointerException During Unboxing

Integer number = null;
int value = number;

NullPointerException

Because Java tries to execute number.intValue() on null.

#Useful Wrapper Methods

#Parsing Numbers

Double.parseDouble("10.25");
Long.parseLong("500");
Float.parseFloat("20.5");
Boolean.parseBoolean("true");

Used frequently while reading configuration files, JSON, CSV and properties files.

#Wrapper Classes in Collections

List<Integer> numbers = List.of(10, 20, 30);

#Wrapper Classes in Streams

List<Integer> list = List.of(1, 2, 3);

list.stream()
    .map(i -> i * 2)
    .forEach(System.out::println);

Wrappers are required because collections cannot store primitives.

Primitive Streams

Instead of Stream<Integer>, Java provides:

• IntStream
• LongStream
• DoubleStream

These avoid boxing overhead and improve performance.

#Real Project Examples

Database

Integer employeeId;

Allows null values from databases. A primitive int cannot represent SQL NULL.

DTO

private Integer age;

A missing JSON field becomes null.

Collections

List<Integer>
Set<Long>
Map<Integer, String>

Collections require wrapper classes.

#Wrapper Classes are Immutable

Integer x = 100;
x++;