为什么 Java 不在 javadoc 中包含每个 function 的时间/空间复杂度？

Question

Hi I want to know what is the time complexity of the "replaceAll" function of the String class but I can't find any information on it.( http://docs.oracle.com/javase/6/docs/api/java/lang/String.html )

Wouldn't it be better for Java to include the complexities in the Javadoc? I believe it's a very important thing for someone to know.

Answer 1

Most functions have fairly straight forward time complexities. AFAIK, replaceAll is O(n)

IMHO. Nothing beats testing this yourself empirically eg with a profiler, because its highly likely that 99% of the methods you use have little to no impact on the performance of your application.

Answer 2

The complexity may be documented if guaranteed. For example, some of the collections classes document complexity guarantees. For example, fromHashMap :

This implementation provides constant-time performance for the basic operations (get and put)...

However, sometimes the complexity is:

• Not guaranteed, and free to change with modifications to the implementation.
• Obviously O(1).

Answer 3

The javadocs of the Java API specify a general contract of what must be done by each method, not how . Each implementor of the API (say, OpenJDK, Oracle's JDK, etc.) has a certain freedom on how to implement each contract, and that freedom may include making optimizations, even sacrifices in performance. So the javadocs in general don't specify details such as time/complexity of functions, unless it's absolutely necessary for a method to meet certain performance requirements.

Answer 4

If you're using the space / time complexity of basic operations to drive design decisions, then you're almost certainly doing it wrong.

First build a correct application, then profile it. Then optimize what the profiling process reveals as the bottlenecks.

Answer 5

The general answer is that the complexity typically depends on factors that are too difficult to analyse. This certainly applies for String.replaceAll , where the effective complexity depends critically on the regex string. (A poorly designed regex can make matching veerryyy slow.)