Why is System.out.println so slow?

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println is not slow, it’s the underlying PrintStream that is connected with the console, provided by the hosting operating system.

You can check it yourself: compare dumping a large text file to the console with piping the same textfile into another file:

cat largeTextFile.txt
cat largeTextFile.txt > temp.txt

Reading and writing are similiar and proportional to the size of the file (O(n)), the only difference is, that the destination is different (console compared to file). And that’s basically the same with System.out.

The underlying OS operation (displaying chars on a console window) is slow because

  1. The bytes have to be sent to the console application (should be quite fast)
  2. Each char has to be rendered using (usually) a true type font (that’s pretty slow, switching off anti aliasing could improve performance, btw)
  3. The displayed area may have to be scrolled in order to append a new line to the visible area (best case: bit block transfer operation, worst case: re-rendering of the complete text area)

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