Asked  7 Months ago    Answers:  5   Viewed   32 times

I'm trying to get my head around mutable vs immutable objects. Using mutable objects gets a lot of bad press (e.g. returning an array of strings from a method) but I'm having trouble understanding what the negative impacts are of this. What are the best practices around using mutable objects? Should you avoid them whenever possible?



Well, there are a few aspects to this.

  1. Mutable objects without reference-identity can cause bugs at odd times. For example, consider a Person bean with a value-based equals method:

    Map<Person, String> map = ...
    Person p = new Person();
    map.put(p, "Hey, there!");
    map.get(p);       // => null

    The Person instance gets "lost" in the map when used as a key because its hashCode and equality were based upon mutable values. Those values changed outside the map and all of the hashing became obsolete. Theorists like to harp on this point, but in practice I haven't found it to be too much of an issue.

  2. Another aspect is the logical "reasonability" of your code. This is a hard term to define, encompassing everything from readability to flow. Generically, you should be able to look at a piece of code and easily understand what it does. But more important than that, you should be able to convince yourself that it does what it does correctly. When objects can change independently across different code "domains", it sometimes becomes difficult to keep track of what is where and why ("spooky action at a distance"). This is a more difficult concept to exemplify, but it's something that is often faced in larger, more complex architectures.

  3. Finally, mutable objects are killer in concurrent situations. Whenever you access a mutable object from separate threads, you have to deal with locking. This reduces throughput and makes your code dramatically more difficult to maintain. A sufficiently complicated system blows this problem so far out of proportion that it becomes nearly impossible to maintain (even for concurrency experts).

Immutable objects (and more particularly, immutable collections) avoid all of these problems. Once you get your mind around how they work, your code will develop into something which is easier to read, easier to maintain and less likely to fail in odd and unpredictable ways. Immutable objects are even easier to test, due not only to their easy mockability, but also the code patterns they tend to enforce. In short, they're good practice all around!

With that said, I'm hardly a zealot in this matter. Some problems just don't model nicely when everything is immutable. But I do think that you should try to push as much of your code in that direction as possible, assuming of course that you're using a language which makes this a tenable opinion (C/C++ makes this very difficult, as does Java). In short: the advantages depend somewhat on your problem, but I would tend to prefer immutability.

Tuesday, June 1, 2021
answered 7 Months ago

str is not an object, it's a reference to an object. "Hello" and "Help!" are two distinct String objects. Thus, str points to a string. You can change what it points to, but not that which it points at.

Take this code, for example:

String s1 = "Hello";
String s2 = s1;
// s1 and s2 now point at the same string - "Hello"

Now, there is nothing1 we could do to s1 that would affect the value of s2. They refer to the same object - the string "Hello" - but that object is immutable and thus cannot be altered.

If we do something like this:

s1 = "Help!";
System.out.println(s2); // still prints "Hello"

Here we see the difference between mutating an object, and changing a reference. s2 still points to the same object as we initially set s1 to point to. Setting s1 to "Help!" only changes the reference, while the String object it originally referred to remains unchanged.

If strings were mutable, we could do something like this:

String s1 = "Hello";
String s2 = s1;
s1.setCharAt(1, 'a'); // Fictional method that sets character at a given pos in string
System.out.println(s2); // Prints "Hallo"

Edit to respond to OP's edit:

If you look at the source code for String.replace(char,char) (also available in in your JDK installation directory -- a pro tip is to look there whenever you wonder how something really works) you can see that what it does is the following:

  • If there is one or more occurrences of oldChar in the current string, make a copy of the current string where all occurrences of oldChar are replaced with newChar.
  • If the oldChar is not present in the current string, return the current string.

So yes, "Mississippi".replace('i', '!') creates a new String object. Again, the following holds:

String s1 = "Mississippi";
String s2 = s1;
s1 = s1.replace('i', '!');
System.out.println(s1); // Prints "M!ss!ss!pp!"
System.out.println(s2); // Prints "Mississippi"
System.out.println(s1 == s2); // Prints "false" as s1 and s2 are two different objects

Your homework for now is to see what the above code does if you change s1 = s1.replace('i', '!'); to s1 = s1.replace('Q', '!'); :)

1 Actually, it is possible to mutate strings (and other immutable objects). It requires reflection and is very, very dangerous and should never ever be used unless you're actually interested in destroying the program.

Tuesday, June 1, 2021
answered 7 Months ago

Objects in hashsets should either be immutable, or you need to exercise discipline in not changing them after they've been used in a hashset (or hashmap).

In practice I've rarely found this to be a problem - I rarely find myself needing to use complex objects as keys or set elements, and when I do it's usually not a problem just not to mutate them. Of course if you've exposed the references to other code by this time, it can become harder.

Wednesday, June 23, 2021
answered 6 Months ago

Mutable means you can alter the collection in-place. So, if you have a collection c and you append an element with +=, then c has changed, and so has every other reference to that collection.

Immutable means that the collection object never changes; instead, you build new collection objects with operations such as + or ++, which return a new collection. This is useful in concurrent algorithms, since it requires no locking to add something to a collection. It may come at the cost of some overhead, but this property can be very useful. Scala's immutable collections are fully persistent data structures.

The difference is very similar to that between var and val, but mind you:

  1. You can modify a mutable collection bound to a val in-place, though you can't reassign the val
  2. you can't modify an immutable collection in-place, but if it's assigned to a var, you can reassign that var to a collection built from it by an operation such as +.

Not all collections necessarily exist in mutable and immutable variants; the last time I checked, only mutable priority queues were supported.

Wednesday, July 28, 2021
answered 5 Months ago


  • Use immutable types as much as possible.
  • Use thread-safe collections instead of explicit locks as much as possible.
  • Only use mutable types when you have no other reasonable choice.


  • Use thread pools as much as possible.
  • Use endless loops when thread pools aren't possible.
  • Manually start and stop threads as a last resort.

If you do have to use explicit locks, document them throughly. Especially when it comes to the order in which you lock objects. If you know that Foo objects are always locked before Bar objects and that Foo(key 100) is always locked before Foo(key = 200), you won't get deadlocks.

Thursday, September 23, 2021
answered 3 Months ago
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