Wrapper Classes | Engineering Interview Kit

TL;DR

Wrapper Classes are objects that encapsulate (wrap) a primitive data type.
Example: int is a primitive, Integer is its wrapper class.
They allow primitives to be used in contexts that strictly require objects, such as the Java Collections Framework (e.g., ArrayList).

Concept

Java is an object-oriented language, but it retains the 8 primitive types for performance. However, many of Java’s built-in utilities and data structures can only work with objects.

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

Utility Methods

Wrapper classes also provide very useful static methods for data conversion (e.g., parsing a String into an integer).

Examples

import java.util.ArrayList;

public class WrapperExample {
    public static void main(String[] args) {
        // 1. Collections require Wrapper Classes
        // ArrayList<int> list = new ArrayList<>(); // Compile error!
        ArrayList<Integer> list = new ArrayList<>();
        list.add(10); // Autoboxing takes over here
        
        // 2. Utility Methods
        String numberStr = "123";
        int parsedNumber = Integer.parseInt(numberStr);
        
        System.out.println(parsedNumber + 10); // Outputs: 133
        
        // 3. Constants
        System.out.println("Max Int: " + Integer.MAX_VALUE);
    }
}

Interview Questions

Q: Can wrapper classes be null?
A: Yes. Unlike primitives which always have a default value (like 0 or false), Wrapper classes are Objects. Like any object reference in Java, they can be null.

Q: Are Wrapper classes immutable?
A: Yes. All wrapper classes (Integer, Double, etc.) and String are immutable. Once an Integer object is created with a value, that internal value cannot be changed. If you reassign the variable, a brand new Integer object is created.