Java - Most efficient way to convert a TreeSet into a String[]?

Question

I was doing this:

(String[]) myTreeSet.toArray();

but that gives me a ClassCastException at runtime.

The only thing I can think of doing is just making an array first and then iterating through each element in myTreeSet and adding it to the array. It seems like there must be a better way than this. Is there or should I just do this?

Thanks.

http://forums.devshed.com/java-help-9/treeset-to-an-array-401225.html — ta.speot.is, Feb 12 '12 at 06:13

score 21 · Accepted Answer · answered Feb 12 '12 at 06:19

21

String[] result = myTreeSet.toArray(new String[myTreeSet.size()]);

answered Feb 12 '12 at 06:19

Louis Wasserman

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Just out of curiosity, could someone explain why my way threw the exception, and why this way doesn't? – Tim Feb 12 '12 at 06:30
2

http://stackoverflow.com/questions/5690351/java-stringlist-toarray-gives-classcastexception – Peter Svensson Feb 12 '12 at 06:56
how about reverse – Kalpesh Soni Oct 04 '18 at 15:55

score 3 · Answer 2 · answered Feb 12 '12 at 10:59

When using toArray() the following happens: a new Object array is being allocated by using new Object[size] and each element from the array will be a reference to one of your strings element. Even if actually points to strings, it's an array with the type Object.

When using toArray(T[] a) the following happens: a new T arrays is being allocated by using

java.lang.reflect.Array
              .newInstance(a.getClass().getComponentType(), size)

and each from the array will be a reference to one of your strings because a.getClass().getComponentType will return String.class in your case. This time, it's an array with the type String.

Java - Most efficient way to convert a TreeSet into a String[]?

2 Answers2