Possible Duplicate: is python slower than java/C#? Ignoring all the characteristics of each languages and focusing SOLELY on speed, which language is better performance-wise? You'd think this ... More on stackoverflow.com

All things being equal, yes Java is usually faster than Python. That said, there are some pretty important qualifiers to that statement: Much of the "heavy lifting" done by Python is actually implemented in super-optimised C and C++ code which is (again, all other things being equal) faster than Java code, e.g. the Numpy library is a good example of this. Many programmes - e.g. most web apps - are I/O Bound (meaning that their performance is limited by the speed of input/output operations such as sending messages over the network or retrieving information from a database), meaning that, in practice, the speed difference between Java and Python is irrelevant, as the programme spends 99% of its execution time on I/O. Speed of implementation is often more important than speed of language. An inefficient Java implementation will often be orders of magnitude slower than an efficient Python on. Developer time is often relevant. Python advocates often state (rightly or wrongly) that as Python is more concise and "simpler" than Java, meaning that it allows Python developers to be more productive than developers using other languages, which is relevant if CPU cycles are cheaper than engineer wages. More on reddit.com

Both Python and Java compile the source files to bytecode. The difference is in how they to run this bytecode. In both languages, the bytecode is basically a binary representation of the textual source code, not an assembly program that can run on a CPU. You have a different program accepts the bytecode and runs it. How does it run it? Python has an interpreter, i.e a program that keeps a "world model" of a Python program (which modules are imported, which variables exist, which objects exist...), and runs the program by loading bytecodes one by one and executing each one separately. This means that a statement such as y = x + 1 is executed as a sequence of operations like "load constant 1", "load x" "add the two values" "store the result in y". Each of these operations is implemented by a function call that does something in C and often reads and updates dictionary structures. This is slow, and it's slower the smaller the operations are. That's why numerical code in Python is slow - numerical operations in Python convert single instructions into multiple function calls, so in this type of code Python can be even 100x slower than other languages. Java compiles the bytecode to machine code. You don't see it because it happens at runtime (referred to as JIT), but it does happen. Since Java also knows that x in y = x + 1 is an integer, it can execute the line using a single CPU instruction. There's actually an implementation of Python that also does JIT compilation. It's called PyPy and it's five times faster than CPython on average, depending what exactly you do with it. It will run all pure Python code, I think, but it still has problems with some libraries. More on reddit.com

The question is meaningless outside of context. What data is it? What format is it in? What processing? It is common to call into libraries written in compiled languages and heavily optimised to do the heavy lifting, e.g numpy calling into linear algebra libraries that are optimised for the CPU you are using. There is not going to be an objective answer X is faster than Y because there will always be a but. In practice both python and java either native or hooking together libraries are popular solutions for a lot of pipelines. More on reddit.com