IF value in cached THENreturn value from cache
ELSEcompute valuesave value in cachereturn value
END IF 

    //最容易产生的错误写法，先读取缓存，读不到就写缓存public Long getNumber(final long index) {if (cache.containsKey(index)) {return cache.get(index);}final long value = getNumber(index - 1) + getNumber(index - 2);cache.put(index, value);return value;}

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;public class NaiveCacheExample {private final Map<Long, Long> cache = new HashMap<>();private Object o=new Object();Lock lock =new ReentrantLock();public NaiveCacheExample() {cache.put(0L, 1L);cache.put(1L, 1L);}//最容易产生的错误写法，先读取缓存，读不到就写缓存public Long getNumber(final long index) {if (cache.containsKey(index)) {return cache.get(index);}final long value = getNumber(index - 1) + getNumber(index - 2);cache.put(index, value);return value;}//使用折返锁，使读写同步public Long getNumberByLock(final long index) {       long value =0;try {lock.lock();           if (cache.containsKey(index)) {return cache.get(index);}value = getNumberByLock(index - 1) + getNumberByLock(index - 2);cache.put(index, value);return value;}catch (Exception e){}finally{lock.unlock();}return 0l;}//使用同步，使读写同步public Long getNumberBySynchroniz(final long index) {synchronized (o){long value =0;try {if (cache.containsKey(index)) {return cache.get(index);}value = getNumberBySynchroniz(index - 1) + getNumberBySynchroniz(index - 2);cache.put(index, value);return value;}catch (Exception e){}finally{}}return 0l;}public static void main(final String[] args) {NaiveCacheExample naiveCacheExample =new NaiveCacheExample();Thread threadA =new Thread(new Runnable(){@Overridepublic void run() {System.out.println(naiveCacheExample.getNumberBySynchroniz(1000));                }},"Thread-A");threadA.start();final Thread threadB = new Thread(new Runnable() {public void run() {System.out.println(naiveCacheExample.getNumberBySynchroniz(1000));}}, "Thread-B");threadB.start();}
}

import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;public class GenericCacheExample<K, V> {private final ConcurrentMap<K, Future<V>> cache = new ConcurrentHashMap<>();private Future<V> createFutureIfAbsent(final K key, final Callable<V> callable) {Future<V> future = cache.get(key);if (future == null) {final FutureTask<V> futureTask = new FutureTask<V>(callable);future = cache.putIfAbsent(key, futureTask);if (future == null) {future = futureTask;futureTask.run();}}return future;}public V getValue(final K key, final Callable<V> callable) throws InterruptedException, ExecutionException {try {final Future<V> future = createFutureIfAbsent(key, callable);return future.get();} catch (final InterruptedException e) {cache.remove(key);throw e;} catch (final ExecutionException e) {cache.remove(key);throw e;} catch (final RuntimeException e) {cache.remove(key);throw e;}}public void setValueIfAbsent(final K key, final V value) {createFutureIfAbsent(key, new Callable<V>() {@Overridepublic V call() throws Exception {return value;}});}}