string buffer

String does not allow appending. Each method you invoke on a String creates a new object and returns it. This is because String is immutable – it cannot change its internal state.

On the other hand StringBuilder is mutable. When you call append(..) it alters the internal char array, rather than creating a new string object.

Thus it is more efficient to have:

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 500; i ++) {

rather than str += i, which would create 500 new string objects.

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