Java threads are not actually executed in parallel?

Question

Until now I was under the impression that 2 threads that start in the same time are also executed in parallel (both running their piece of codes in the same time), but I read some documentation recently and I understood that they actually take turns on the execution of their code, so there is no piece of code for first thread executed in the same time as a piece of code from the second thread. Is my understanding correct?

If yes, then how multi-threading is faster then one thread execution? I'm asking this because the only difference is that a single thread executes the code sequential, while multithreading can take turns on the execution, but still should take the same amount of time since it's nothing done in parallel

Answer 1

a) on multi-processor machines, threads can actually run in parallel (one per CPU)

b) If your thread calls Thread.sleep() while waiting for IO etc., it makes resources available to other threads. So multi-threaded applications are actually faster than single-threaded ones when dealing with external resources

Answer 2

Java threads are executed in parallel if there are enough CPUs available for a JVM. You can't run 2 computations on a machine with a single computing element at the same time, so this computing element is used either by first, or by second computation at any given time. Probably what you've read concerned this circumstances.

Answer 3

No, Java threads are executed in parallel (unlike some other platforms like CPython). However, whether that gives performance improvements depends on the code you execute.

If you test with easily parallelizable & CPU intensive tasks like calculating PI with a parallelizable algorithm or resizing lots of images etc., you can easily demonstrate that performance can be increased basically linearly (if you have 2 CPUs = x2, 4 CPUs = x4 etc.)

EDIT:
When you only have one CPU, multi-threading is still beneficial. For example, you can have one thread reading images from the disk while the other thread resizes the images. This will also improve the performance because you can utilize the CPU without waste.

EDIT2: When you read and resize images (note the plural) in a single thread, then you will see that CPU usage won't be 100% at all times. This is because while the thread is reading from file, it can't perform the resizing. If you had more than one thread, by the time a resize has finished another file would have been ready in-memory. If you are dealing with big images, it's relatively easy to peg the CPU at 100% with this design.