guava/src/com/google/common/base/Stopwatch.java
@CanIgnoreReturnValue
  public Stopwatch start() {
    checkState(!isRunning, "This stopwatch is already running.");
    isRunning = true;
    startTick = ticker.read();
    return this;
  }
@CanIgnoreReturnValue
  public Stopwatch start() {
    checkState(!isRunning, "This stopwatch is already running.");
    isRunning = true;
    startTick = ticker.read();
    return this;
  }
Starts the stopwatch.

@return this {@code Stopwatch} instance
@throws IllegalStateException if the stopwatch is already running.