ListHelpers.java

package ch21concurrent.examples;

import java.util.*;

import ch21concurrent.annotations.*;

/**
 * ListHelder
 * <p/>
 * Examples of thread-safe and non-thread-safe implementations of
 * put-if-absent helper methods for List
 *
 * @author Brian Goetz and Tim Peierls
 */

@NotThreadSafe
class BadListHelper<E> {
    public List<E> list = Collections.synchronizedList(new ArrayList<E>());

    public synchronized boolean putIfAbsent(E x) {
        boolean absent = !list.contains(x);
        if (absent)
            list.add(x);
        return absent;
    }
}

@ThreadSafe
class GoodListHelper<E> {
    public List<E> list = Collections.synchronizedList(new ArrayList<E>());

    public boolean putIfAbsent(E x) {
        synchronized (list) {
            boolean absent = !list.contains(x);
            if (absent)
                list.add(x);
            return absent;
        }
    }
}