Does it make sense to copy an immutable type?

Question

Does it make sense to implement a copy method on an immutable type, returning a new instance? Or is it just the current instance?

I thought the type does not change, so why copy? Like no one is copying number 5, right?

Answer 1

There are certain cases where this makes sense. Java strings are a good example. When a string is created in Java, it refers to an array of character substitution (a char[]). He knows the offset in the char array and the length of the string. When you create a substring, it refers to the same base array. Now consider this code:

String x = buildVeryLongString();
String y = x.substring(0, 5);
// Use y a lot, but x is garbage collected

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String y = new String(x.substring(0, 5));

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StringBuilder builder = new StringBuilder(10000);
builder.Append("not a lot");
string x = builder.ToString();

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Answer 2

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Answer 3

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Answer 4

Java String :

String(String original)
      Initializes a newly created String object so that it represents the same 
      sequence of characters as the argument; in other words, the newly created 
      string is a copy of the argument string.

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Answer 5

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Answer 6

One advantage of immutable types is that they can be interned (like Java strings). Of course, you should not make extra copies if you can avoid this.