String in java

1. What is a String in Java?

A String in Java is a sequence of characters enclosed in double quotes.

String name = "Mapla";

👉 Internally, Java treats strings as objects, not primitive types.

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2.Why Strings are Important?

• Used for storing text data
• Helps in user input/output
• Used in file handling, web apps, APIs, etc.

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3.String is Immutable

In Java, Strings are immutable (cannot be changed).

String str = "Hello";
str.concat(" World");
System.out.println(str);

👉 Output: "Hello" (not changed
)

✔ Reason: A new object is created instead of modifying the original one.

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4.Common String Methods

• length()

Returns number of characters

String str = "Mapla";
System.out.println(str.length()); // 5

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• charAt()

Returns character at specific index

System.out.println(str.charAt(0)); // M

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• toUpperCase() & toLowerCase()

System.out.println(str.toUpperCase()); // MAPLA
System.out.println(str.toLowerCase()); // mapla

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• equals()

Compares two strings

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

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

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• contains()

Check substring exists or not

System.out.println(str.contains("pla")); // true

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• substring()

Extract part of string

System.out.println(str.substring(0, 3)); // Map

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• replace()

Replace characters

System.out.println(str.replace("Map", "Top")); // Topla

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1. String vs StringBuilder vs StringBuffer

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

👉 Use:

• String → normal usage
• StringBuilder → performance (single thread)
• StringBuffer → thread-safe apps

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1. Removing Duplicate Characters Example

String str = "aabbccdd";
String result = "";

for(int i = 0; i < str.length(); i++) {
    char c = str.charAt(i);
    if(result.indexOf(c) == -1) {
        result += c;
    }
}

System.out.println(result); // abcd

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1. Tips for Interviews

• String is immutable
• Stored in String Pool
• Use ".equals()" instead of "=="
• Prefer StringBuilder in loops