Introduction

In Java, a String is used to store text data.

Example:

String name = "Jay";

Here "Jay" is a String.

Strings are one of the most commonly used data types in Java because almost every application works with text like usernames, emails, messages, passwords, etc.

What is String in Java?

A String is a sequence of characters.

Example:

String word = "Hello";

Characters:

H e l l o
Why String is Important?

Strings are used everywhere:

• Login systems
• Search functionality
• Form inputs
• APIs
• Database values
• Chat applications

Without Strings, handling text is impossible.

How to Create a String?

1. Using String Literal

String s1 = "Hello";

This is the most commonly used way.

Java stores it inside the String Pool.

2. Using new Keyword

String s2 = new String("Hello");

This creates a new object in heap memory.

Difference Between Literal and new Keyword

Example:

String a = "Java";
String b = "Java";


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

Output:

true

• Because both refer to the same object.

String a = new String("Java");
String b = new String("Java");

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

Output:

false

• Because two different objects are created.

String is Immutable

Immutable means:

Once a String is created, it cannot be changed.

Example:

String s = "Hello";

s.concat(" World");

System.out.println(s);

Output:

Hello

• Because concat() creates a new object.

Correct way:

s = s.concat(" World");

System.out.println(s);

Output:

Hello World

Important String Methods

length()

Returns total characters.

String s = "Java";

System.out.println(s.length());

Output:

4

charAt()

Gets character by index.

String s = "Java";

System.out.println(s.charAt(1));

Output:

a

toUpperCase()

String s = "java";

System.out.println(s.toUpperCase());

Output:

JAVA

toLowerCase()

String s = "JAVA";

System.out.println(s.toLowerCase());

Output:

java

equals()

Compares values.

String a = "Java";
String b = "Java";

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

Output:

true

equalsIgnoreCase()

Ignores uppercase/lowercase.

String a = "java";
String b = "JAVA";

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

Output:

true

contains()

Checks whether text exists.

String s = "Java Programming";

System.out.println(s.contains("Java"));

Output:

true

startsWith()

String s = "Java";

System.out.println(s.startsWith("Ja"));

Output:

true

endsWith()

String s = "Java";

System.out.println(s.endsWith("va"));

Output:

true

replace()

String s = "Java";

System.out.println(s.replace('a', 'o'));

Output:

Jovo

substring()

• Extracts part of String.

String s = "Programming";

System.out.println(s.substring(0, 6));

Output:

Progra

split()

• Splits String into parts.

String s = "Java Python React";

String arr[] = s.split(" ");

for(String word : arr)
{
    System.out.println(word);
}

Output:

Java
Python
React

== vs equals()

==

• Checks memory reference.

equals()

• Checks actual content.

Example:

String a = new String("Java");
String b = new String("Java");

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

Output:

false
true
String Pool

Java stores String literals in a special memory area called:

String Constant Pool

Example:

String a = "Java";
String b = "Java";

• Both point to the same object.
• This saves memory.

StringBuilder

Since String is immutable, repeated modifications create many objects.

To solve this, Java provides:

StringBuilder

Example:

StringBuilder sb = new StringBuilder("Hello");

sb.append(" World");

System.out.println(sb);

Output:

Hello World
StringBuffer

Similar to StringBuilder but thread-safe.

Common Interview Programs

Reverse String

String s = "Java";

for(int i = s.length()-1; i >= 0; i--)
{
    System.out.print(s.charAt(i));
}

Output:

avaJ

Palindrome String

String s = "madam";
String rev = "";

for(int i = s.length()-1; i >= 0; i--)
{
    rev += s.charAt(i);
}

if(s.equals(rev))
{
    System.out.println("Palindrome");
}
else
{
    System.out.println("Not Palindrome");
}

Output

Palindrome

And so on

Conclusion

String is one of the most important concepts in Java.

To become strong in Java:

• Understand immutability
• Learn String methods
• Practice String programs
• Know String Pool concept
• Learn difference between == and equals()

Mastering Strings will help a lot in:

• Interviews
• Backend Development
• Spring Boot
• Problem Solving
• Real-world Applications