Understanding String and String Methods in Java

Introduction

In Java, String is one of the most commonly used classes. It represents a sequence of characters and is widely used for handling text such as names, messages, and data processing.

Strings in Java are immutable, which means once a String object is created, it cannot be changed.

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What is a String in Java?

A String is an object that represents a sequence of characters.

Example:

String name = "Ponvel";

Here, "Ponvel" is stored as a String object.

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Important Concept: Immutability

Strings are immutable in Java.

Example:

String str = "Hello";
str.concat(" World");

System.out.println(str);

Output:

Hello

Even though we tried to add " World", the original string is not changed.

Correct Way:

str = str.concat(" World");

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String Creation Methods

1. Using Literal

String s1 = "Java";

2. Using new Keyword

String s2 = new String("Java");

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Difference: Literal vs New Keyword

Literal	new Keyword
Stored in String pool	Stored in heap
Memory efficient	More memory used
Faster	Slightly slower

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Common String Methods (VERY IMPORTANT)

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1. length()

Returns the length of the string.

String str = "Java";
System.out.println(str.length());

Output: 4

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2. toUpperCase() & toLowerCase()

String str = "java";

System.out.println(str.toUpperCase()); // JAVA
System.out.println(str.toLowerCase()); // java

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3. charAt()

Returns character at specific index.

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

Output: a

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4. indexOf()

Finds position of a character.

String str = "Java";
System.out.println(str.indexOf('a'));

Output: 1

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5. substring()

Extracts part of string.

String str = "Java Programming";

System.out.println(str.substring(5));
System.out.println(str.substring(0, 4));

Output:

Programming
Java

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🔹 6. equals() vs == (VERY IMPORTANT )

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

System.out.println(a == b);       // true
System.out.println(a == c);       // false
System.out.println(a.equals(c));  // true

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Difference: equals() vs ==

equals()	==
Compares values	Compares memory
Recommended	Not safe

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7. contains()

Checks if string contains value.

String str = "Java Programming";
System.out.println(str.contains("Java"));

Output: true

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8. replace()

Replaces characters.

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

Output: Jovo

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9. trim()

Removes spaces.

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

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10. split()

Splits string into array.

String str = "Java,Python,C++";

String[] arr = str.split(",");

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

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StringBuilder vs StringBuffer vs String

Difference Table:

Feature	String	StringBuilder	StringBuffer
Mutable	No	Yes	Yes
Thread Safe	No	No	Yes
Performance	Slow	Fast	Medium

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When to Use What?

• Use String → When data is constant
• Use StringBuilder → Fast operations (single thread)
• Use StringBuffer → Multi-threaded applications

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Interview Important Questions

Why String is Immutable?

• Security
• Performance (String pool)
• Thread safety

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What is String Pool?

• Special memory area to store unique string literals

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What is difference between StringBuilder and StringBuffer?

Answer:

• Both are mutable
• StringBuffer is thread-safe
• StringBuilder is faster

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Conclusion

Strings are a fundamental part of Java programming. Understanding how they work, especially concepts like immutability, string pool, and string methods, is essential for both coding and interviews.

Mastering String methods will help you solve many real-world problems such as:

• Data processing
• Validation
• Parsing

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