• provide an attractive alternative to the rather pathological clone method
• are easily implemented
• simply extract the argument's data, and forward to a regular constructor
• are unnecessary for immutable objects

public final class Galaxy {

  /**
  * Regular constructor.
  */
  public Galaxy(double aMass, String aName) {
    fMass = aMass;
    fName = aName;
  }

  /**
  * Copy constructor.
  */
  public Galaxy(Galaxy aGalaxy) {
    this(aGalaxy.getMass(), aGalaxy.getName());
    //no defensive copies are created here, since 
    //there are no mutable object fields (String is immutable)
  }

  /**
  * Alternative style for a copy constructor, using a static newInstance
  * method.
  */
  public static Galaxy newInstance(Galaxy aGalaxy) {
    return new Galaxy(aGalaxy.getMass(), aGalaxy.getName());
  }

  public double getMass() {
    return fMass;
  }

  /**
  * This is the only method which changes the state of a Galaxy
  * object. If this method were removed, then a copy constructor
  * would not be provided either, since immutable objects do not
  * need a copy constructor.
  */
  public void setMass(double aMass){
    fMass = aMass;
  }

  public String getName() {
    return fName;
  }

  // PRIVATE
  private double fMass;
  private final String fName;

  /** Test harness. */
  public static void main (String... aArguments){
    Galaxy m101 = new Galaxy(15.0, "M101");

    Galaxy m101CopyOne = new Galaxy(m101);
    m101CopyOne.setMass(25.0);
    System.out.println("M101 mass: " + m101.getMass());
    System.out.println("M101Copy mass: " + m101CopyOne.getMass());

    Galaxy m101CopyTwo = Galaxy.newInstance(m101);
    m101CopyTwo.setMass(35.0);
    System.out.println("M101 mass: " + m101.getMass());
    System.out.println("M101CopyTwo mass: " + m101CopyTwo.getMass());
  }
} 

>java -cp . Galaxy
M101 mass: 15.0
M101Copy mass: 25.0
M101 mass: 15.0
M101CopyTwo mass: 35.0