Copy constructors
Copy constructors:
- 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()); } }
Example run of this class:
>java -cp . Galaxy
M101 mass: 15.0
M101Copy mass: 25.0
M101 mass: 15.0
M101CopyTwo mass: 35.0
See Also :
Would you use this technique?