Java物件克隆

Java不提供克隆(複製)物件的自動機制。克隆物件意味著逐位複製物件的內容。要支援克隆操作，請在類中實現clone()方法。Object類中的clone()方法的宣告如下：

protected  Object clone()  throws   CloneNotSupportedException

clone()方法宣告為protected。 因此，不能從用戶端程式碼呼叫它。以下程式碼無效：

Object obj  = new Object();
Object clone = obj.clone(); // Error. Cannot  access protected clone() method

需要在類中宣告clone()方法為public來克隆類的物件。
它的返回型別是Object。 這意味著將需要使用clone()方法轉換返回值。
假設MyClass是可克隆的。 克隆程式碼將如下所示

MyClass mc  = new MyClass();
MyClass clone = (MyClass)mc.clone(); // Need to use  a  cast

Object類中的clone()方法會丟擲CloneNotSupportedException。要呼叫clone()方法，需要將呼叫放在try-catch塊中，或者重新丟擲異常。

範例

以下程式碼顯示了如何實現克隆方法。

class MyClass implements Cloneable {
  private double value;
  public MyClass(double value) {
    this.value = value;
  }

  public void setValue(double value) {
    this.value = value;
  }

  public double getValue() {
    return this.value;
  }

  public Object clone() {
    MyClass copy = null;
    try {
      copy = (MyClass) super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return copy;
  }
}

public class Main {
  public static void main(String[] args) {
    MyClass dh = new MyClass(100.00);

    MyClass dhClone = (MyClass) dh.clone();

    System.out.println("Original:" + dh.getValue());
    System.out.println("Clone :" + dhClone.getValue());

    dh.setValue(100.00);
    dhClone.setValue(200.00);

    System.out.println("Original:" + dh.getValue());
    System.out.println("Clone :" + dhClone.getValue());
  }
}

上面的程式碼生成以下結果。

Original:100.0
Clone :100.0
Original:100.0
Clone :200.0

範例-2

以下程式碼不從clone()方法返回物件型別，該方法僅在Java5或更高版本中編譯。

class MyClass  implements Cloneable  {
    public MyClass clone()  { 
       Object copy  = null;
       return  (MyClass)copy;
    }
}

下面的程式碼展示了如何做淺克隆。

class MyClass implements Cloneable {
  private double value;

  public MyClass(double value) {
    this.value = value;
  }

  public void setValue(double value) {
    this.value = value;
  }

  public double getValue() {
    return this.value;
  }

  public Object clone() {
    MyClass copy = null;
    try {
      copy = (MyClass) super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return copy;
  }
}

class ShallowClone implements Cloneable {
  private MyClass holder = new MyClass(0.0);

  public ShallowClone(double value) {
    this.holder.setValue(value);
  }

  public void setValue(double value) {
    this.holder.setValue(value);
  }

  public double getValue() {
    return this.holder.getValue();
  }

  public Object clone() {
    ShallowClone copy = null;
    try {
      copy = (ShallowClone) super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return copy;
  }
}

public class Main {
  public static void main(String[] args) {
    ShallowClone sc = new ShallowClone(100.00);
    ShallowClone scClone = (ShallowClone) sc.clone();

    System.out.println("Original:" + sc.getValue());
    System.out.println("Clone :" + scClone.getValue());

    sc.setValue(200.00);

    System.out.println("Original:" + sc.getValue());
    System.out.println("Clone :" + scClone.getValue());
  }
}

上面的程式碼生成以下結果。

Original:100.0
Clone :100.0
Original:200.0
Clone :200.0

範例-3

ShallowClone類的clone()方法中的程式碼與MyClass類的clone()方法相同。當ShallowClone類使用super.clone()呼叫Object類的clone()方法時，它會接收自身的淺拷貝。也就是說，它與其克隆共用其範例變數中使用的DoubleHolder物件。

在深克隆中，需要克隆物件的所有參照範例變數。

class MyClass implements Cloneable {
  private double value;

  public MyClass(double value) {
    this.value = value;
  }

  public void setValue(double value) {
    this.value = value;
  }

  public double getValue() {
    return this.value;
  }

  public Object clone() {
    MyClass copy = null;
    try {
      copy = (MyClass) super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return copy;
  }
}

class DeepClone implements Cloneable {
  private MyClass holder = new MyClass(0.0);

  public DeepClone(double value) {
    this.holder.setValue(value);
  }

  public void setValue(double value) {
    this.holder.setValue(value);
  }

  public double getValue() {
    return this.holder.getValue();
  }
  public Object clone() {
    DeepClone copy = null;
    try {
      copy = (DeepClone) super.clone();
      copy.holder = (MyClass) this.holder.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return copy;
  }
}

public class Main {
  public static void main(String[] args) {
    DeepClone sc = new DeepClone(100.00);
    DeepClone scClone = (DeepClone) sc.clone();

    System.out.println("Original:" + sc.getValue());
    System.out.println("Clone :" + scClone.getValue());

    sc.setValue(200.00);

    System.out.println("Original:" + sc.getValue());
    System.out.println("Clone :" + scClone.getValue());
  }
}

執行上面的程式碼，將生成以下結果 -

Original:100.0
Clone :100.0
Original:200.0
Clone :100.0