Why does Java string have a copy constructor? [duplicate]

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What is the purpose of the expression “new St开发者_如何转开发ring(…)” in Java?

It's immutable, why would you need to invoke String.String(String str) ?

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From the API doc:

Unless an explicit copy of original is needed, use of this constructor is
unnecessary since Strings are immutable. 

The only reason I can think of is in the situation where:

1. You've created a very large String: A.
2. You've created a small substring: B based on A. If you look at the implementation of substring you'll see it references the same char[] array as the original string.
3. String A has gone out of scope and you wish to reduce memory consumption.
4. Creating an explicit copy of B will mean that the copy: B' no longer references the large char[]. Allowing B to go out of scope will allow the garbage collector to reclaim the large char[] that backs A and B, leaving only the small char[] backing B'.

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new String(s) can help garbase collection:

String huge = ...;
String small = s.substring(0,2); //huge.value char[] is not garbage collected here
String gcFriendly = new String(small); //now huge.value char[] can be garbage collected

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Just in case you need String that are not the same but equal.

Maybe for testing to make sure, people really do str.equals(str2) instead of (str == str2). But I never needed it.