String immutability in Java

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.

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.

Examples

This 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);

Assigning 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"

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);
    }
}

Important Notes

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

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

Summary

Strings cannot be changed after creation.

Changing a string creates a new string object.

Immutability helps keep programs safe and predictable.

Practice

1. What does it mean that a String in Java is immutable?

easy

A. Once created, the string cannot be changed.

B. The string can be changed anytime.

C. The string changes automatically when printed.

D. The string can only be changed inside loops.

Solution

1. Step 1: Understand the meaning of immutability

   Immutability means the object cannot be changed after creation.

2. Step 2: Apply this to Java Strings

   Java strings cannot be modified once created; any change creates a new string.

3. Final Answer:

   Once created, the string cannot be changed. -> Option A

4. Quick Check:

   String immutability = cannot change after creation [OK]

Hint: Immutable means no change allowed after creation [OK]

Common Mistakes:

• Thinking strings can be changed directly
• Confusing immutability with constant variables
• Assuming strings change when printed

2. Which of the following is the correct way to create a new string by changing a character in an existing string str?

easy

A. str.charAt(0) = 'A';

B. str[0] = 'A';

C. str = "A" + str.substring(1);

D. str.replaceChar(0, 'A');

Solution

1. Step 1: Check syntax for changing characters in strings

   Strings are immutable, so direct assignment like str.charAt(0) = 'A' is invalid.

2. Step 2: Use substring to create a new string

   Concatenate "A" with the substring from index 1 to form a new string.

3. Final Answer:

   str = "A" + str.substring(1); -> Option C

4. Quick Check:

   Use substring + concat to change string [OK]

Hint: Use substring and concat to create new string [OK]

Common Mistakes:

• Trying to assign directly to string characters
• Using array-like indexing on strings
• Using non-existent methods like replaceChar

3. What is the output of the following code?

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

medium

A. HELLO

B. hello

C. null

D. HELLOhello

Solution

1. Step 1: Understand that strings are immutable

   The method toUpperCase() returns a new string but does not change the original string s.

2. Step 2: Check what is printed

   Since s was not reassigned, printing s outputs the original string "hello".

3. Final Answer:

   hello -> Option B

4. Quick Check:

   Original string unchanged = hello [OK]

Hint: toUpperCase returns new string; original stays same [OK]

Common Mistakes:

• Assuming toUpperCase changes original string
• Expecting uppercase output without reassignment
• Confusing method effect with print output

4. Find the error in this code snippet:

String s = "test";
s.charAt(0) = 'T';
System.out.println(s);

medium

A. Cannot assign to charAt() result; strings are immutable.

B. Missing semicolon after declaration.

C. charAt() method does not exist for String.

D. System.out.println() cannot print strings.

Solution

1. Step 1: Check usage of charAt()

   The method charAt() returns a character but cannot be assigned a new value.

2. Step 2: Understand string immutability

   Strings cannot be changed by assigning to characters; this causes a compile error.

3. Final Answer:

   Cannot assign to charAt() result; strings are immutable. -> Option A

4. Quick Check:

   charAt() result is read-only [OK]

Hint: charAt() returns char, cannot assign to it [OK]

Common Mistakes:

• Trying to assign to charAt()
• Thinking strings are mutable
• Ignoring compile errors on assignment

5. You want to replace all occurrences of the letter 'a' with 'b' in a string input. Which code correctly does this considering string immutability?

hard

A. input.replace('a', 'b'); System.out.println(input);

B. input.charAt(0) = 'b'; System.out.println(input);

C. input.toCharArray()[0] = 'b'; System.out.println(input);

D. input = input.replace('a', 'b'); System.out.println(input);

Solution

1. Step 1: Understand replace() returns new string

   The replace() method returns a new string with replacements but does not change the original.

2. Step 2: Assign the new string back to input

   To update input, assign the result of replace() back to it before printing.

3. Final Answer:

   input = input.replace('a', 'b'); System.out.println(input); -> Option D

4. Quick Check:

   Assign replace result to variable to update string [OK]

Hint: Assign replace() result to variable to update string [OK]

Common Mistakes:

• Calling replace() without assignment
• Trying to assign to charAt() or array elements
• Expecting original string to change automatically