Copy constructors

最新推荐文章于 2025-03-11 14:06:32 发布

最新推荐文章于 2025-03-11 14:06:32 发布 · 114 阅读

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本文介绍了一种使用深拷贝构造器实现对象复制的方法，并通过一个具体的 Galaxy 类实例展示了其用法。深拷贝构造器可以有效避免对象间的引用共享问题，确保每个对象拥有独立的数据副本。

Copy constructors :

[list]
[*] 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
[/list]

Example

public final class Galaxy {

  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());
  }
}