Zero2OneDev - Master Programming from Zero to One

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What are Wrapper Classes?

Wrapper classes provide a way to use primitive data types as objects. Java provides a wrapper class for each primitive type:

Primitive	Wrapper Class
byte	Byte
short	Short
int	Integer
long	Long
float	Float
double	Double
char	Character
boolean	Boolean

Autoboxing & Unboxing

Autoboxing: Automatic conversion of primitive to wrapper class

Unboxing: Automatic conversion of wrapper class to primitive

⚠️ Null Pointer Exception: Wrapper objects can be null. Unboxing null throws NPE! Always check for null.

Why Use Wrapper Classes?

Working with Collections (ArrayList, HashMap) which require objects
Passing by reference when needed
Using methods like parseInt(), parseDouble()
Handling null values
Type checking and conversion utilities

Code Examples

Manual boxing and unboxing of primitive types

java

1// Manual boxing (wrapping primitive in object)
2int primitiveInt = 25;
3Integer wrappedInt = Integer.valueOf(primitiveInt);  // Manual boxing
4
5double primitiveDouble = 3.14;
6Double wrappedDouble = Double.valueOf(primitiveDouble);
7
8// Manual unboxing (extracting primitive from object)
9int unwrapped = wrappedInt.intValue();
10double unwrappedDouble = wrappedDouble.doubleValue();

Automatic autoboxing and unboxing in Java

java

1// Autoboxing (automatic conversion)
2int value = 10;
3Integer autoBoxed = value;  // Automatically wrapped
4
5// Unboxing (automatic conversion)
6Integer wrapped = Integer.valueOf(20);
7int unboxed = wrapped;  // Automatically unwrapped
8
9// Works in collections
10ArrayList<Integer> numbers = new ArrayList<>();
11numbers.add(100);  // Autoboxed to Integer
12int first = numbers.get(0);  // Unboxed to int

Useful methods provided by wrapper classes

java

1// Useful wrapper class methods
2String str = "123";
3int number = Integer.parseInt(str);  // String to int
4
5int max = Integer.MAX_VALUE;  // Get maximum value
6int min = Integer.MIN_VALUE;  // Get minimum value
7
8String binaryStr = Integer.toBinaryString(10);  // "1010"
9String hexStr = Integer.toHexString(255);       // "ff"
10
11Integer parsed = Integer.parseInt("42");
12boolean result = parsed.equals(42);  // true

Handling null values with wrapper classes safely

java

1// Null handling with wrapper classes
2Integer nullable = null;  // Valid, primitives can't be null
3
4// NullPointerException risk!
5// int unwrapped = nullable;  // Would throw NPE!
6
7// Safe approach
8if (nullable != null) {
9    int value = nullable;  // Safe unboxing
10    System.out.println(value);
11}
12
13// Using Optional (Java 8+)
14Optional<Integer> optional = Optional.ofNullable(nullable);
15optional.ifPresent(System.out::println);

Use Cases

Working with Collections that require objects
Converting Strings to numeric types
Handling optional/nullable values
API methods that expect wrapper types
Accessing utility methods like parseInt(), MAX_VALUE, etc.
Type conversion and validation

Common Mistakes to Avoid

NullPointerException when unboxing null wrapper
Comparing wrapper objects with == instead of .equals()
Not understanding autoboxing performance implications
Assuming wrapper == primitive (they're different types)
Not caching wrapper objects (-128 to 127 for Integer)