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//: concurrency/Pool.java

// Using a Semaphore inside a Pool, to restrict

// the number of tasks that can use a resource.

import java.util.concurrent.*;

import java.util.*;

public class Pool<T> {

private int size;

private List<T> items = new ArrayList<T>();

private volatile boolean[] checkedOut;

private Semaphore available;

public Pool(Class<T> classObject, int size) {

this.size = size;

checkedOut = new boolean[size];

available = new Semaphore(size, true);

// Load pool with objects that can be checked out:

for(int i = 0; i < size; ++i)

try {

// Assumes a default constructor:

items.add(classObject.newInstance());

} catch(Exception e) {

throw new RuntimeException(e);

}

}

public T checkOut() throws InterruptedException {

available.acquire();

return getItem();

}

public void checkIn(T x) {

if(releaseItem(x))

available.release();

}

private synchronized T getItem() {

for(int i = 0; i < size; ++i)

if(!checkedOut[i]) {

checkedOut[i] = true;

return items.get(i);

}

return null; // Semaphore prevents reaching here

}

private synchronized boolean releaseItem(T item) {

int index = items.indexOf(item);

if(index == -1) return false; // Not in the list

if(checkedOut[index]) {

checkedOut[index] = false;

return true;

}

return false; // Wasn't checked out

}

} ///:~