String immutability in Java - Step-by-Step Execution

Concept Flow - String immutability

Create String s = "Hello"

↓

Try to change s by s.concat(" World")

↓

New String created with "Hello World"

↓

Original s still "Hello"

↓

Assign new String to s if desired

↓

s now points to new String or old String remains

When you try to change a String, Java creates a new String object instead of changing the original one.

Execution Sample

Java

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

This code shows that calling concat does not change the original String unless reassigned.

Execution Table

Step	Code executed	String s value	Action	Output
1	String s = "Hello";	"Hello"	Create s with "Hello"
2	s.concat(" World");	"Hello"	Create new String "Hello World" but do not assign
3	System.out.println(s);	"Hello"	Print s	Hello
4	s = s.concat(" World");	"Hello World"	Assign new String to s
5	System.out.println(s);	"Hello World"	Print s	Hello World

💡 Program ends after printing both values of s showing immutability effect

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
s	"Hello"	"Hello"	"Hello World"	"Hello World"

Key Moments - 2 Insights

Why does s.concat(" World") not change s immediately?

Because Strings are immutable in Java, concat creates a new String but does not change the original s unless you assign the result back to s (see step 2 and 4 in execution_table).

What happens if you print s right after s.concat(" World") without assignment?

The original s remains unchanged and prints "Hello" as shown in step 3 of execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of s after step 2?

A "Hello"

B "Hello World"

C null

D Compilation error

💡 Hint

Check the 'String s value' column at step 2 in execution_table

At which step does s actually change to "Hello World"?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Action' and 'String s value' columns in execution_table

If we remove the assignment in step 4, what will be printed at step 5?

A "Hello World"

B null

C "Hello"

D Runtime error

💡 Hint

Refer to step 3 and 5 outputs in execution_table when no assignment is done

Concept Snapshot

String immutability in Java means once a String is created, it cannot be changed.
Methods like concat() create new Strings instead of modifying the original.
To update a String variable, assign the new String back to it.
Example:
String s = "Hi";
s = s.concat(" there"); // s now "Hi there"

Full Transcript

This visual trace shows how Java Strings are immutable. When we create a String s with "Hello", it holds that value. Calling s.concat(" World") creates a new String "Hello World" but does not change s itself. Only when we assign the result back to s does s point to the new String. Printing s before assignment shows the original value. This teaches that String methods that seem to change the String actually return new Strings, preserving immutability.

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