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String Class

The String class in Java represents a sequence of characters and is one of the most important and frequently used classes in Core Java. It is heavily tested in interviews and widely used in real-time applications.

What Is a String in Java?

A String is an object that represents a sequence of characters
Belongs to the java.lang package
Immutable in nature
Stored in String Constant Pool (SCP) or Heap

String s = "Java";

Why Is String So Important?

Used for user input, messages, logs, data exchange
Frequently used in APIs and frameworks
Optimized for memory using String Constant Pool
Core interview topic (immutability, == vs equals())

String Creation Ways (Very Important)

1. Using String Literal (SCP)

String s1 = "Java";
String s2 = "Java";

Stored in String Constant Pool
Same object reused
Memory efficient

System.out.println(s1 == s2); // true

2. Using new Keyword (Heap)

String s3 = new String("Java");
String s4 = new String("Java");

Stored in Heap memory
Creates new object every time

System.out.println(s3 == s4); // false

String Immutability (Interview Favorite)

What Is Immutability?

Once a String object is created, it cannot be changed
Any modification creates a new object

String s = "Java";
s = s.concat(" World");

Result:

"Java" remains unchanged
New object "Java World" is created

Why Strings Are Immutable?

Security (classpath, passwords)
Thread safety
Performance optimization via SCP
Caching and hashcode stability

String Constant Pool (SCP)

Special memory area inside heap
Stores only unique string literals
Improves memory efficiency

String a = "Test";
String b = "Test"; // reused from SCP

== vs equals() (Very Important)

Using ==

Compares references (memory address)

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

Using equals()

Compares content

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

Commonly Used String Methods

Length

int len = s.length();

Concatenation

String s = "Java" + " Programming";
String s2 = s.concat(" Language");

Character Access

char ch = s.charAt(1);

Comparison

s.equals("Java");
s.equalsIgnoreCase("java");

Substring

String sub = s.substring(0, 4);

Contains / StartsWith / EndsWith

s.contains("av");
s.startsWith("Ja");
s.endsWith("va");

Replace

String r = s.replace("Java", "Core Java");

Trim

String t = " Java ".trim();

Case Conversion

s.toUpperCase();
s.toLowerCase();

String Concatenation Internals

String s = "Java" + "World";

Compiler optimizes literals
At runtime, uses StringBuilder for variables

String vs StringBuilder vs StringBuffer

Feature	String	StringBuilder	StringBuffer
Mutability	Immutable	Mutable	Mutable
Thread-Safe	Yes	No	Yes
Performance	Slow (modifications)	Fast	Slower than SB
Use Case	Read-only	Single-threaded	Multi-threaded

Common Beginner Mistakes

Using == instead of equals()
Excessive string concatenation in loops
Not understanding immutability
Using new String() unnecessarily
Ignoring performance impact

Interview-Ready Answers

Short Answer

String is an immutable class in Java used to represent a sequence of characters.

Detailed Answer

In Java, String is an immutable object stored in the String Constant Pool or heap. Any modification creates a new object. This design improves security, memory efficiency, and thread safety. Content comparison is done using equals() rather than ==.

Key Takeaway

The String class is simple on the surface but deep in behavior. Understanding immutability, SCP, memory behavior, and comparison methods is essential for writing efficient Java code and clearing interviews.

String Class Practice Examples

1. Creating a String Using Literal

String s = "Java";

Explanation

Stored in String Constant Pool (SCP).
Memory efficient.
Most commonly used approach.

2. Creating a String Using new Keyword

String s = new String("Java");

Explanation

Creates a new object in heap memory.
Not reused from SCP.
Rarely recommended unless explicitly needed.

3. Comparing Strings Using ==

String s1 = "Java";
String s2 = "Java";
System.out.println(s1 == s2);

Explanation

Compares references, not content.
Output: true (same SCP reference).

4. Comparing Strings Using .equals()

String s1 = new String("Java");
String s2 = new String("Java");
System.out.println(s1.equals(s2));

Explanation

Compares content.
Output: true.
Always use .equals() for string comparison.

5. Case-Insensitive Comparison

String s1 = "java";
String s2 = "JAVA";
System.out.println(s1.equalsIgnoreCase(s2));

Explanation

Ignores letter case.
Output: true.

6. Finding Length of String

String s = "Automation";
System.out.println(s.length());

Explanation

Returns number of characters.
Output: 10.

7. Access Character Using charAt()

String s = "Java";
System.out.println(s.charAt(1));

Explanation

Index starts from 0.
Output: a.

8. Loop Through String Characters

String s = "JAVA";
for (int i = 0; i < s.length(); i++) {
  System.out.println(s.charAt(i));
}

Explanation

Iterates character by character.
Common interview example.

9. Convert String to Uppercase

String s = "java";
System.out.println(s.toUpperCase());

Explanation

Creates a new String object.
Output: JAVA.

10. Convert String to Lowercase

String s = "JAVA";
System.out.println(s.toLowerCase());

Explanation

Original string remains unchanged.
Output: java.

11. Remove Leading and Trailing Spaces (trim())

String s = "  Java  ";
System.out.println(s.trim());

Explanation

Removes spaces from start and end only.
Output: Java.

12. Check if String Is Empty

String s = "";
System.out.println(s.isEmpty());

Explanation

Returns true if length is 0.

13. Check if String Is Blank (Java 11+)

String s = "   ";
System.out.println(s.isBlank());

Explanation

Returns true for whitespace-only strings.
Interview favorite.

14. Check if String Contains Substring

String s = "Selenium Java";
System.out.println(s.contains("Java"));

Explanation

Returns true if substring exists.

15. Replace Characters in String

String s = "Java";
System.out.println(s.replace('a', 'o'));

Explanation

Output: Jovo.
Original string remains unchanged.

16. Replace Substring

String s = "I love Java";
System.out.println(s.replace("Java", "Selenium"));

Explanation

Replaces entire substring.
Output: I love Selenium.

17. Split String into Array

String s = "Java Selenium TestNG";
String[] parts = s.split(" ");
for (String part : parts) {
  System.out.println(part);
}

Explanation

Splits based on delimiter.
Very common in automation.

18. Convert String to Char Array

String s = "Java";
char[] chars = s.toCharArray();
for (char c : chars) {
  System.out.println(c);
}

Explanation

Useful for character processing problems.

19. Check String Starts With / Ends With

String s = "automation@test.com";
System.out.println(s.startsWith("automation"));
System.out.println(s.endsWith(".com"));

Explanation

Used heavily in validations.

20. String Immutability Demonstration

String s = "Java";
s.concat(" Selenium");
System.out.println(s);

Explanation

Output: Java.
Strings are immutable.
concat() creates a new object.

21. Proper Way to Modify String

String s = "Java";
s = s.concat(" Selenium");
System.out.println(s);

Explanation

Reassignment required.
Output: Java Selenium.

22. Substring Extraction

String s = "Automation";
System.out.println(s.substring(0, 4));

Explanation

Extracts part of string.
Output: Auto.

23. Substring with Single Index

String s = "Automation";
System.out.println(s.substring(5));

Explanation

Starts from index to end.
Output: mation.

24. String Concatenation Using +

String s = "Java" + " " + "Selenium";
System.out.println(s);

Explanation

Compiler uses StringBuilder internally.
Simple and readable.

25. Interview Summary Example (Most Common)

String s1 = "Java";
String s2 = new String("Java");
System.out.println(s1 == s2);
System.out.println(s1.equals(s2));

Explanation

== → reference comparison → false.
.equals() → content comparison → true.
Very common interview question.