Immutable types are types that when changed create a new version of the object in memory, rather than changing the existing object in memory. The simplest example of this is the built-in string
type.
Taking the following code, that appends " world" onto the word "Hello"
string myString = "hello";
myString += " world";
What is happening in memory in this case is that a new object is created when you append to the string
in the second line. If you do this as part of a large loop, there is the potential for this to cause performance issues in your application.
The mutable equivalent for a string
is a StringBuilder
Taking the following code
StringBuilder myStringBuilder = new StringBuilder("hello");
myStringBuilder.append(" world");
When you run this, you are modifying the StringBuilder
object itself in memory.