String immutability in Java - Step-by-Step Execution

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Concept Flow - String immutability

Create String s = "Hello"

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

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New String created with "Hello World"

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Original s still "Hello"

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Assign new String to s if desired

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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.

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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.

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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

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Variable Tracker

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

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Key Moments - 2 Insights

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

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

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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

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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"

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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.