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

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.