String immutability in Java

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Introduction

Strings in Java cannot be changed once created. This helps keep data safe and programs reliable.

When you want to store text that should not change, like a username or ID.

When sharing text data between different parts of a program without risk of accidental changes.

When using strings as keys in collections like HashMap to avoid unexpected behavior.

When you want to improve performance by reusing string objects safely.

When working with multithreaded programs to prevent data conflicts.

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Syntax

Java

String text = "Hello";
// text cannot be changed after this

Strings are objects but behave like fixed text.

Any change creates a new String object instead of modifying the original.

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Examples

line_end_arrow_notchThis looks like changing the string, but actually a new string is created and greeting points to it.

Java

String greeting = "Hi";
greeting = greeting + " there!";
System.out.println(greeting);

line_end_arrow_notchAssigning one string to another copies the reference, not the string itself.

Java

String name = "Alice";
String copy = name;
// Both name and copy point to the same string object "Alice"

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Sample Program

This program shows that the original string stays the same after trying to replace a character. A new string is created instead.

Java

public class Main {
    public static void main(String[] args) {
        String original = "Cat";
        String changed = original.replace('C', 'B');
        System.out.println("Original: " + original);
        System.out.println("Changed: " + changed);
    }
}

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Important Notes

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Because strings are immutable, methods like replace() return new strings.

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Use StringBuilder if you need to change text many times for better performance.

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Summary

Strings cannot be changed after creation.

Changing a string creates a new string object.

Immutability helps keep programs safe and predictable.