Count only currently active threads

I have a list of Thread and I want to count only currently active threads, but method isAlive() don’t work. How correctly is this or exist other method check for currently active threads.

public int activeSize() {
            int count = 0;
    
            for (Thread thread : list) {
                if (thread.isAlive()) {
                    count++;
                }
            }
    
            return count;
        }

Answer

tl;dr

Compare the state of the thread:

thread.getState().equals( Thread.State.RUNNABLE )

Here is an example making a stream from your list of Thread objects.

threads
.stream()
.filter( 
    thread -> thread.getState().equals( Thread.State.RUNNABLE ) 
)
.count()  // Or collect them: `.toList()`

Or count all active threads:

Thread.activeCount()

Details

I have no experience in this, but looking the Javadoc for Thread, I see getState. That method returns a Thread.State enum object. Documentation says:

A thread can be in one of the following states, as quoted from the doc:

• NEW
  A thread that has not yet started is in this state.
• RUNNABLE
  A thread executing in the Java virtual machine is in this state.
• BLOCKED
  A thread that is blocked waiting for a monitor lock is in this state.
• WAITING
  A thread that is waiting indefinitely for another thread to perform a particular action is in this state.
• TIMED_WAITING
  A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state.
• TERMINATED
  A thread that has exited is in this state.

So loop your Thread objects, fetch their State, and see if it is RUNNABLE.

for ( Thread thread : list) {
    if ( thread.getState().equals( Thread.State.RUNNABLE ) ) {
        …
    }
}

But if you just want a count of all active threads, call Thread.activeCount().