/**
 * A class implements the <code>Cloneable</code> interface to 
 * indicate to the {@link java.lang.Object#clone()} method that it 
 * is legal for that method to make a 
 * field-for-field copy of instances of that class. 
 * <p>
 * Invoking Object's clone method on an instance that does not implement the 
 * <code>Cloneable</code> interface results in the exception 
 * <code>CloneNotSupportedException</code> being thrown.
 * <p>
 * By convention, classes that implement this interface should override 
 * <tt>Object.clone</tt> (which is protected) with a public method.
 * See {@link java.lang.Object#clone()} for details on overriding this
 * method.
 * <p>
 * Note that this interface does <i>not</i> contain the <tt>clone</tt> method.
 * Therefore, it is not possible to clone an object merely by virtue of the
 * fact that it implements this interface.  Even if the clone method is invoked
 * reflectively, there is no guarantee that it will succeed.
 *
 * @author  unascribed
 * @version 1.17, 11/17/05
 * @see     java.lang.CloneNotSupportedException
 * @see     java.lang.Object#clone()
 * @since   JDK1.0
 */
public interface Cloneable { 
}
 

介面如上，但是裡面卻沒有任何方法。

在Object類裡，有clone方法：

    /**
     * Creates and returns a copy of this object.  The precise meaning 
     * of "copy" may depend on the class of the object. The general 
     * intent is that, for any object <tt>x</tt>, the expression:
     * <blockquote>
     * <pre>
     * x.clone() != x</pre></blockquote>
     * will be true, and that the expression:
     * <blockquote>
     * <pre>
     * x.clone().getClass() == x.getClass()</pre></blockquote>
     * will be <tt>true</tt>, but these are not absolute requirements. 
     * While it is typically the case that:
     * <blockquote>
     * <pre>
     * x.clone().equals(x)</pre></blockquote>
     * will be <tt>true</tt>, this is not an absolute requirement. 
     * <p>
     * By convention, the returned object should be obtained by calling
     * <tt>super.clone</tt>.  If a class and all of its superclasses (except
     * <tt>Object</tt>) obey this convention, it will be the case that
     * <tt>x.clone().getClass() == x.getClass()</tt>.
     * <p>
     * By convention, the object returned by this method should be independent
     * of this object (which is being cloned).  To achieve this independence,
     * it may be necessary to modify one or more fields of the object returned
     * by <tt>super.clone</tt> before returning it.  Typically, this means
     * copying any mutable objects that comprise the internal "deep structure"
     * of the object being cloned and replacing the references to these
     * objects with references to the copies.  If a class contains only
     * primitive fields or references to immutable objects, then it is usually
     * the case that no fields in the object returned by <tt>super.clone</tt>
     * need to be modified.
     * <p>
     * The method <tt>clone</tt> for class <tt>Object</tt> performs a 
     * specific cloning operation. First, if the class of this object does 
     * not implement the interface <tt>Cloneable</tt>, then a 
     * <tt>CloneNotSupportedException</tt> is thrown. Note that all arrays 
     * are considered to implement the interface <tt>Cloneable</tt>. 
     * Otherwise, this method creates a new instance of the class of this 
     * object and initializes all its fields with exactly the contents of 
     * the corresponding fields of this object, as if by assignment; the
     * contents of the fields are not themselves cloned. Thus, this method 
     * performs a "shallow copy" of this object, not a "deep copy" operation.
     * <p>
     * The class <tt>Object</tt> does not itself implement the interface 
     * <tt>Cloneable</tt>, so calling the <tt>clone</tt> method on an object 
     * whose class is <tt>Object</tt> will result in throwing an
     * exception at run time.
     *
     * @return     a clone of this instance.
     * @exception  CloneNotSupportedException  if the object's class does not
     *               support the <code>Cloneable</code> interface. Subclasses
     *               that override the <code>clone</code> method can also
     *               throw this exception to indicate that an instance cannot
     *               be cloned.
     * @see java.lang.Cloneable
     */
    protected native Object clone() throws CloneNotSupportedException;
 

當一個類實現Cloneable介面時，預設是不會重寫clone方法的，在eclipse新建類C：

public class C implements Cloneable {

}

所以，要主動去重寫clone方法，並且把clone方法設定為public，在方法內部，還要呼叫super.clone()

public class C implements Cloneable {

	@Override
	public Object clone() throws CloneNotSupportedException {
		return super.clone();
	}
	
}

這樣，就可以了。

clone方法是另一個構造方法，必須確保它不會傷害到原始物件，也就是說，要進行深clone。如果是淺clone，那麼clone出來的，只是原始物件的引用。

為了使得類是一個可克隆的，類裡的某些域，不能是final的，否則就無法賦值了。

介面有很多缺點，所以有些專家級的程式設計師，從來不去重寫clone方法。可以用一種拷貝構造方法來代替clone()：

public class C {
	private String name = null;
	
	public C(String name) {
		this.name = name;
	}
	
	public String getName() {
		return name;
	}

	public C(C c) {
		this.name = new String(c.getName());
	}
	
	public static void main(String[] args) {
		C c1 = new C("C");
		C c2 = new C(c1);
		System.out.println(c1 + ":" + c1.getName());
		System.out.println(c2 + ":" + c2.getName());
	}
}

上面的

	public C(C c) {
		this.name = new String(c.getName());
	}

就是拷貝構造方法，或者用靜態工廠方法public static C newInstance()來代替。