Attempt to create a caching system that can be integrated into rx

Cache.java

package com.scmspain.bluejobs.candidateagent;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import rx.Observable;

public abstract class Cache<I, T> {

  private Map<I, CachedObject<T>> cachedItems = new HashMap<>();
  private long expirationTime = 5 * 60 * 1000; // 5 minutes

  public Cache(long expirationTime) {
    this.expirationTime = expirationTime;
  }

  public Observable<T> retrieve(I index) {

    return Observable.from(cachedItems.values())
        .filter(cachedItem -> cachedItem.getIndex() == index)
        .filter(cachedItem -> cachedItem.getTimestamp() < System.currentTimeMillis() - expirationTime)
        .map(CachedObject::getItem);
  }

  public Observable.Transformer<T, T> save(I index, T item) {
    return input -> input.doOnNext(t -> cachedItems.put(index, new CachedObject<>(index, item, System.currentTimeMillis())));
  }

  private class CachedObject<T> {
    private final T item;
    private final I index;
    private final Long timestamp;

    public CachedObject(I index, T item, Long timestamp) {
      this.item = item;
      this.index = index;
      this.timestamp = timestamp;
    }

    public T getItem() {
      return item;
    }

    public I getIndex() {
      return index;
    }

    public Long getTimestamp() {
      return timestamp;
    }
  }
}