首先,下面是一个不正确地使用了默认clone方法的类的示例:
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package examples.cloning;
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/** An example class used to demonstrate what
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* happens when the default Object.clone method
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* is not overridden in a class with object
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* reference fields.
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*/
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public class IntArrayShared implements Cloneable {
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private int[] a;
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/** Class constructor method
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* @param size The maximum number of elements
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* @param initValue The initial value given
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*/
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public IntArrayShared( int size, int initValue ) {
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a = new int[size];
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for( int i=0; i < a.length; i++ ) {
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a[i] = initValue;
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}
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}
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/** Obtain the value of an array element
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* @param index the array index of interest
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* @return the value at the specified index
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*/
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public int elementAt( int index ) {
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return a[index];
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}
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/** Set the value of an array element
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* @param index the array index to be updated
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* @param newValue the new value given
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*/
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public void setValue( int index, int newValue ) {
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a[index] = newValue;
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}
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/** Converts the object into a String
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* @return The value of the array as a String
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*/
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public String toString() {
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StringBuffer sb = new StringBuffer( "[ " );
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for( int i=0; i < a.length; i++ ) {
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sb.append( a[i] + " " );
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}
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return sb.append( "]" ).toString();
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}
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/** The test method for the class
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* @param args Not used
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* @exception CloneNotSupportedException
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* The inherited clone method may
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* throw this exception
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*/
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public static void main( String[] args )
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throws CloneNotSupportedException {
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IntArrayShared x = new IntArrayShared( 5, 0 );
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IntArrayShared y = (IntArrayShared) x.clone();
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System.out.println( x );
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System.out.println( y );
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x.setValue( 2, 9999 );
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System.out.println( x );
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System.out.println( y );
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}
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}
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结果如下:
[ 0 0 0 0 0 ]
[ 0 0 0 0 0 ]
[ 0 0 9999 0 0 ]
[ 0 0 9999 0 0 ]
上面定义的引用域(是由整数构成的一个数组)不是不变的。因此,继承的Object.clone方法不能正确地克隆这个域。
下面是一个正确改写clone方法的示例:
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package examples.cloning;
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/** An example class used to demonstrate proper cloning
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* concepts
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*/
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public class IntArray implements Cloneable {
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private int[] a;
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/** Class constructor method
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* @param size The maximum number of elements
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* @param initValue The initial value given
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*/
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public IntArray( int size, int initValue ) {
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a = new int[ size ];
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for( int i=0; i<a.length; i++ ) {
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a[i] = initValue;
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}
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}
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/** Obtain the value of an array element
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* @param index the array index of interest
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* @return the value at the specified index
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*/
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public int elementAt( int index )
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{ return a[ index ]; }
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/** Set the value of an array element
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* @param index the array index to be updated
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* @param newValue the new value given
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*/
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public void setValue( int index, int newValue ) {
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a[ index ] = newValue;
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}
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/** Convert the object to a String
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* @return The value of the array as a String
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* object
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*/
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public String toString() {
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StringBuffer sb = new StringBuffer( "[ " );
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for( int i=0; i<a.length; i++ ) {
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sb.append( a[i] + " " );
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}
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return sb.append( "]" ).toString();
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}
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/** Provide a clone method specifically for the
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* IntArray class
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* @exception CloneNotSupportedException
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* Superclass may throw this
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* @return A clone of the object
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*/
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public Object clone()
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throws CloneNotSupportedException {
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IntArray newObject = (IntArray)super.clone();
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newObject.a = (int[])a.clone();
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return newObject;
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}
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/** The test method for the class
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* @param args Not used
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* @exception CloneNotSupportedException
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* The overiding clone method may
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* throw this
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*/
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public static void main( String[] args )
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throws CloneNotSupportedException {
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IntArray x = new IntArray( 5, 0 );
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IntArray y = (IntArray)x.clone();
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System.out.println( x );
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System.out.println( y );
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x.setValue( 2, 9999 );
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System.out.println( x );
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System.out.println( y );
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}
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}
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结果如下:
[ 0 0 0 0 0 ]
[ 0 0 0 0 0 ]
[ 0 0 9999 0 0 ]
[ 0 0 0 0 0 ]
希望我的这两个例子可以更好地帮助大家理解clone。