Wrapper Class In Java

wrapper class in java

📘 Java Data Types Explained: A Complete Guide with Examples and Interview Questions

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🧩 1. Introduction

Understanding data types is essential for every Java programmer. Java is a statically typed language, meaning each variable must be declared with a data type, which defines the type of data it can hold.

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🔢 2. What Are Data Types in Java?

Data types in Java determine what kind of data a variable can store, such as numbers, characters, or objects.

🧪 Example:

java

int age = 30;
char grade = 'A';
boolean isJavaFun = true;

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🧱 3. Types of Data Types in Java

Java has two major categories of data types:

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✅ 3.1 Primitive Data Types

There are 8 primitive data types in Java:

Type	Size	Default	Example	Description
byte	1 byte	0	byte a = 100;	Small range integers
short	2 bytes	0	short s = 500;	Medium-range integers
int	4 bytes	0	int i = 1000;	Standard integer type
long	8 bytes	0L	long l = 100000L;	Large integers
float	4 bytes	0.0f	float f = 10.5f;	Single precision decimal
double	8 bytes	0.0d	double d = 99.99;	High precision decimal
char	2 bytes	'\u0000'	char c = 'A';	Single character using Unicode
boolean	1 bit	false	boolean b = true;	True or false

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🧾 3.2 Non-Primitive Data Types

These are reference types, including classes, arrays, and interfaces.

📌 Examples:

java

String name = "Alice";
int[] numbers = {1, 2, 3};

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🔄 4. Wrapper Classes in Java

Wrapper classes let primitive types be used as objects, useful in collections like ArrayList.

Primitive	Wrapper Class
int	Integer
char	Character
boolean	Boolean

🧪 Example:

java

int num = 10;
Integer obj = Integer.valueOf(num);  // Autoboxing

Integer wrapper = 20;
int unwrapped = wrapper;             // Unboxing

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🔍 5. Primitive vs Non-Primitive Comparison

Feature	Primitive Types	Non-Primitive Types
Stored In	Stack memory	Heap memory
Default Values	Yes	No (null if not initialized)
Can Call Methods	No	Yes
Nullable	No	Yes

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⚠️ 6. Common Mistakes to Avoid

1. Using == instead of .equals() for Strings

2. Forgetting to initialize local variables

3. Incorrect use of float literals

4. Assuming wrapper objects behave like primitives

🧪 Example:

java

String a = new String("Java");
String b = new String("Java");
System.out.println(a == b);        // false
System.out.println(a.equals(b));   // true

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❓ 7. Java Interview Questions on Data Types

🔹 7.1 Beginner Level

• What are the 8 primitive types in Java?

• What’s the default value of a boolean?

• Difference between int and Integer?

• Why isn’t String a primitive type?

🔸 7.2 Intermediate Level

• Explain autoboxing and unboxing with examples.

• What is type casting in Java?

• Can a char hold numbers?

🔺 7.3 Advanced Level

• What happens when an int overflows?

• Why are arrays reference types?

• How are primitive types stored in memory?

• Difference in memory between int and Integer?

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✅ 8. Summary

Java data types are essential to memory-efficient, safe, and performant programs. From int and boolean to complex objects like arrays and Strings, each type plays a unique role.

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🧠 9. Final Tips

• Use the most memory-efficient data type suitable for the job.

• Understand the default values before using uninitialized variables.

• Master wrapper classes for working with collections and generics.

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📌 10. Related Topics

• Java Type Casting

• Java Memory Management

• Wrapper Classes & Autoboxing

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Java Wrapper Classes Explained with Real-Life Examples and Interview Questions

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Learn everything about Java Wrapper Classes with real-life examples and top interview questions. Understand autoboxing, unboxing, and when to use wrappers.

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💡 Java Wrapper Classes Explained with Real-Life Examples and Interview Questions

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🧩 1. Introduction

In Java, wrapper classes are used to convert primitive data types into objects. They play a vital role when working with collections, generics, or frameworks like Hibernate and Spring, which require objects—not primitives.

✅ Why Wrapper Classes Matter:
They bridge the gap between Java’s primitive types and its object-oriented nature.

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🔢 2. What Are Wrapper Classes in Java?

Java has eight wrapper classes, one for each primitive type. These classes are part of the java.lang package and provide a way to use primitives as objects.

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

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🧪 3. Basic Example of Wrapper Class Usage

java

public class WrapperExample {
    public static void main(String[] args) {
        int num = 10;                     // Primitive
        Integer obj = Integer.valueOf(num); // Boxing

        System.out.println(obj);         // Output: 10
    }
}

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🔁 4. Autoboxing and Unboxing

• Autoboxing: Automatic conversion of a primitive to its wrapper.

• Unboxing: Automatic conversion of a wrapper object to its primitive.

java

public class AutoUnboxExample {
    public static void main(String[] args) {
        Integer obj = 100;  // Autoboxing
        int num = obj;      // Unboxing

        System.out.println(num);  // Output: 100
    }
}

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🌍 5. Real-Life Example of Wrapper Classes

🛒 Scenario: Shopping Cart with Total Price Calculation

Imagine you’re developing an e-commerce app. You want to store product prices in a List, which only accepts objects. Here, Double (wrapper for double) is necessary.

java

import java.util.ArrayList;
import java.util.List;

public class ShoppingCart {
    public static void main(String[] args) {
        List<Double> prices = new ArrayList<>();

        prices.add(19.99);  // Autoboxing
        prices.add(5.49);
        prices.add(12.50);

        double total = 0;
        for (Double price : prices) {
            total += price;  // Unboxing
        }

        System.out.println("Total: $" + total);
    }
}

🔍 Why Use Double Instead of double?
Java Collections like ArrayList only work with objects, not primitives.

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⚙️ 6. Key Features and Methods in Wrapper Classes

Wrapper	Useful Methods
Integer	parseInt(), toString()
Double	parseDouble(), isNaN()
Boolean	parseBoolean(), booleanValue()
Character	isDigit(), isLetter()

🧪 Example:

java

String s = "123";
int number = Integer.parseInt(s);  // Converts String to int
System.out.println(number + 10);   // Output: 133

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❓ 7. Interview Questions on Wrapper Classes

🔹 Beginner Level

1. What are wrapper classes in Java?

2. Why does Java need wrapper classes?

3. What is autoboxing and unboxing?

🔸 Intermediate Level

4. How are wrapper classes used in Java Collections?

5. Difference between == and .equals() for wrapper objects?

6. Can wrapper classes be null?

🔺 Advanced Level

7. How do wrapper classes affect memory and performance?

8. What’s the behavior of caching in Integer?

   java
   
   Integer a = 127;
   Integer b = 127;
   System.out.println(a == b); // true due to caching
   

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🚫 8. Common Pitfalls with Wrapper Classes

• Using == to compare wrapper objects instead of .equals().

• Forgetting that wrapper classes can be null, which can lead to NullPointerException.

• Assuming performance is the same as primitives.

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🧠 9. Summary

• Wrapper classes are essential for treating primitives as objects.

• They enable you to work with Java Collections, Streams, and Generics.

• Be mindful of autoboxing, performance, and nullability.

✅ Use wrapper classes when:

• Working with collections

• You need object features

• Interfacing with frameworks or libraries expecting objects

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📌 10. Related Topics

• Java Generics and Collections

• Java Type Conversion

• Java Memory Management