android/guava/src/com/google/common/cache/CacheBuilder.java
public CacheBuilder<K, V> ticker(Ticker ticker) {
    checkState(this.ticker == null);
    this.ticker = checkNotNull(ticker);
    return this;
  }
public CacheBuilder<K, V> ticker(Ticker ticker) {
    checkState(this.ticker == null);
    this.ticker = checkNotNull(ticker);
    return this;
  }
Specifies a nanosecond-precision time source for this cache. By default, {@link
System#nanoTime} is used.

<p>The primary intent of this method is to facilitate testing of caches with a fake or mock
time source.

@return this {@code CacheBuilder} instance (for chaining)
@throws IllegalStateException if a ticker was already set