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Given the scenario: This class is intended to allow users to write a series of messages, so that each message is identified with a timestamp and the name of the thread that wrote the message:

public class Logger {
  private StringBuilder contents = new StringBuilder();
  public void log(String message) {
   contents.append(System.currentTimeMillis());
   contents.append(": ");
   contents.append(Thread.currentThread().getName());
   contents.append(message);
   contents.append("\n");
  }
  public String getContents() { return contents.toString(); }
}

How can we ensure that instances of this class can be safely used by multiple threads?

This class is already thread-safe.
Replacing StringBuilder with StringBuffer will make this class thread-safe.
Synchronize the log() method only.
Synchronize the getContents() method only.
Synchronize both log() and getContents().
This class cannot be made thread-safe.

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