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This compliant solution uses a private object lock to synchronize the method bodies of the increment() and getCount methods, to ensure atomicity. For more information on private object locks, see CON04-J. Use the private lock object idiom instead of the Class object's intrinsic locking mechanismlock.
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public class KeyedCounter {
private final Map<String,Integer> map = new HashMap<String,Integer>();
private final Object lock = new Object();
public void increment(String key) {
synchronized (lock) {
Integer old = map.get(key);
int value = (old == null) ? 1 : old.intValue() + 1;
map.put(key, value);
}
}
public Integer getCount(String key) {
synchronized (lock) {
return map.get(key);
}
}
}
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