Java Strings are Immutable - Here's What That Actually Means

Java String are Immutable - but what does that really mean?
We'll learn why Strings are immutable in Java, and what it means for your Java programs.
String immutability in Java means that String objects cannot be modified after they're created. Let's talk about why it matters that Strings are immutable, and why Java chose to make them immutable in this beginner video lesson tutorial.
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Hi, I'm John! I'm a Lead Java Software Engineer and I've been in the programming industry for more than a decade. I love sharing what I've learned over the years in a way that's understandable for all levels of Java learners.
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[@csgrinds]

Recommend if you're serious about mastering Java that you read e.g. Joshua Bloch rather than watching needlessly long youtube presentations. No criticism of this video intended.

[@MrYourSalvation]

Plus one interesting thing - due to String's immutability and presense of a string pool - the hashCode() method is calculated only the first time and saved as a value inside String object. Therefore when you call hashCode() method second time (e.g. on another variable with the same literal value) - it will just return stored value, no need to calculate hash code again.

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Really cool!

[@mohitsanghai5455]

Thanks a lot for the crisp and clear video, John. Just to summarize String Immutability Benefits - 1) Usage of String pool, saving tons of memory and hashCode calculation 2) safe for multithreading 3) Removes any security threat by not allowing hackers to change referenced string values to cause security issues in the application

[@bobfarker4001]

You basically summarized the summary.

[@Atarian6502]

Thanks for summary. Question regarding #1: Do I understand this correctly: the String pool saves the memory ONLY if you have two or more identical string variables (they point to the same piece of memory), but it wastes memory EVERY time you are changing the strings (it uses new piece of memory for the whole new string even if you change only one letter)?

[@swapode]

I think you're kinda missing the main reason why Strings are immutable: You can't really have sensible mutable Strings since you can't change their length without moving everything to newly allocated memory on the heap which is incredibly expensive. I absolutely see your points as benefits, in fact I think Rust has proven that the whole mutable by default approach is a mistake, but it might have been a good idea to go into the actual reason a bit.

[@XD-nj7bc]

I can think of multiple ways that you can change your String length during runtime without coping (at least most of the time). Lists, Allocating more than cou need and and only get new memory when you run out.

[@watertrainer3992]

@@XD-nj7bc allocating more than you need doesn't really solve the problem though, as you'd then just use more than you need, making it more expensive than immutable Strings

[@XD-nj7bc]

@@watertrainer3992 of course it is not cheaper than not mutable strings but you can then change the size without the copying he mentionend.

[@swapode]

@@XD-nj7bc I put the word "sensible" in there for a reason 🙂

[@garybuttherissilent5896]

@@XD-nj7bc yeah he put the word "sensible" in there for a reason bruh

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[@haoshecn]

Hi, John. Love your tutorials, but I think there is a little thing you didn’t explain in details here. When you new a string “John”, the object is indeed created in the heap, but “John” is still crated inside string pool. There is a reference inside the object pointing towards “John”. So, you actually created two objects when you use ‘new’ keyword to create a string.

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[@CodingWithJohn]

Those are good too (as they should be, coming directly from Oracle) and I've used them to do some research on various topics I've done videos for. But yeah, often it's easier to watch a video on it. Thanks!

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