【发布时间】：2014-03-08 06:42:49
【问题描述】：
我无法理解课堂上的作业，描述如下：
创建三个名为 QueueExtendingVect 和 QueueContaingVect 的公共队列类。第一个应该使用 Vector 类通过扩展它来实现一个队列，并实现 Iterator 类型的迭代器；第二个应该使用 Vector 类通过包含 Vector 对象来实现队列，当然还要实现 Iterator 类型的迭代器。每个类都应该实现 iterator() 实例方法来创建一个 Iterator 实例。通过在调用时简单地抛出 java.lang.UnsupportedOperationException 在 Iterator 中实现 remove()。请注意，Iterator.next() 返回对存储的实际客户端对象的引用并推进迭代器。
通过扩展vector，我可以创建一个Iterator实例并使用它来遍历vector，并使用.next()和remove()方法。
这是我目前的代码...
import java.lang.reflect.InvocationTargetException;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Vector;
@SuppressWarnings("rawtypes")
public class QueueExtendingVector extends Vector implements Cloneable, Iterable{
/** no-args constructor*/
public QueueExtendingVector() {
}
public void remove(){
throw new UnsupportedOperationException();
}
/** adds element to end of Queue takes Object as arguement, calls addElement()*/
public void enqueue (Object c){
this.addElement(c);
}
/** dequeue() removes first element of the Queue
* @return */
public Object dequeue(){
if (this.size() !=0){
Object deleted = this.elementAt(0);
this.removeElementAt(0);
return deleted;
}
else{
return -1;
}
}
/** empty() returns true if Queue is empty*/
public boolean empty(){
return this.isEmpty();
}
/** makeNull sets all elements of Queue to null*/
public void makeNull(){
int s = this.size(); // number of elements not capacity
//System.out.println(s);
for ( int i = 0 ; i < s ; i++){
this.set(i, null);
}
}
/** equals() checks quality of two QEV object references, returns true if references point to same object and true if objects referred to are equal*/
public boolean equals(Object o ){
if( this == o ){ // if pointing at same object return true
return true;
}
if(!(o instanceof QueueExtendingVector)){ // if o not an instance of QEV return false
return false;
}
int mySize = this.size(); // size of this QEV
int compareToSize = ((QueueExtendingVector)o).size(); // size of object comparing to
if(mySize != compareToSize){ // if the sizes (number of elements) are not same returns false;
System.out.println("Sizes Unequal");
return false;
}
for ( int i = 0 ; i < mySize; i++ ) // loop through QEV
if( ((QueueExtendingVector)o).elementAt(i).equals(this.elementAt(i))){ // if equal (true) continue looping
}
else { // if not equal, break from loop return false
return false;
}
return true; // after loop finishes returns true
}
/** toString() method returns String*/
public String toString(){
String s = "";
for ( int i = 0 ; i < this.size(); i++){
s+= this.elementAt(i).toString() + " , ";
}
return s;
}
/** front() returns first element of Queue*/
public Object front(){
return this.firstElement();
}
public Object clone(){
Object o = null;
Object q = new QueueExtendingVector();
for(int i = 0 ; i < this.size(); i++){
try {
o = this.get(i).getClass().getMethod("clone", null).invoke(this.get(i), null);
((QueueExtendingVector)q).add(o);
}
// catch (Throwable e){
//}
catch (IllegalAccessException e) {
e.printStackTrace();
} catch (IllegalArgumentException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
System.out.println("element at index " + i + ": [" + this.get(i).getClass() + "]is not cloneable, shallow copy only preformed");
Object t = this.elementAt(i);
((QueueExtendingVector)q).add(t); // was add o
//throw new Error(this.elementAt(i).getClass() + " must implement Cloneable");
//e.printStackTrace();
} catch (NoSuchMethodException e) {
//e.printStackTrace();
} catch (SecurityException e) {
//e.printStackTrace();
}
}
return q;
}
in a class called A1, which we were given, and to use exactly as was such.
import java.util.Date;
import java.util.Iterator;
import java.util.Vector;
public class A1 {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
QueueExtendingVector qev1 = new QueueExtendingVector();
QueueExtendingVector qev2 = new QueueExtendingVector();
QueueExtendingVector qev3;
QueueContainingVector qcv1 = new QueueContainingVector();
QueueContainingVector qcv2 = new QueueContainingVector();
QueueContainingVector qcv3;
CInteger ci = new CInteger(88);
qev1.enqueue(ci);
qev2.enqueue(ci);
qcv1.enqueue(ci);
qcv2.enqueue(ci);
for ( int i = 0 ; i < 5; i++)
if ( i%2 == 0){
qev1.enqueue(new CInteger(i+1));
qev2.enqueue(new CInteger(i+1));
qcv1.enqueue(new CInteger(i+1));
qcv2.enqueue(new CInteger(i+1));
}
else{
qev1.enqueue(new Date(i*i));
qev2.enqueue(new Date(i*i));
qcv1.enqueue(new Date(i*i));
qcv2.enqueue(new Date(i*i));
}
// QueueExtendingVector ******************************************************************************************************
if (qev1.equals(qev2)){
System.out.println("qev1.equals(qev2) is true");
}
else{
System.out.println("qev1.equals(qev2) is false");
}
System.out.println("but........");
if (qev1 == qev2){
System.out.println("qev1 == qev2");
}
else{
System.out.println("qev != qev2");
}
if(qev1.empty()){
System.out.println("qev1 empty");
}
else{
System.out.println("qev1 not empty");
}
qev3 = (QueueExtendingVector)qev1.clone();
ci.seti(44);
System.out.println("first element of qev1 is " + qev1.front());
System.out.println("qev3 stringified(hopefully didn't change) = " + qev3);
System.out.println("First elt of qev1 is: " + qev1.front());
System.out.println(qev1.size());
// ********** that first Iteratorstuff goes here!
try{
qev1.iterator().remove();
}
catch (UnsupportedOperationException e){
System.out.println("Calling Iterator.remove() and throwing Exception");
}
Iterator i = qev1.iterator();
for ( int j = 0 ; i.hasNext(); j ++){
System.out.println("qev1[" + j + "] = " + i.next());
}
}
}
}
我不明白的是台词：
try{
qev1.iterator().remove();
}
catch (UnsupportedOperationException e){
System.out.println("Calling Iterator.remove() and throwing Exception");
}
i get the illegal state exception which is thrown when , .next() is not called before .remove() is called
我不明白 qev1.iterator().remove() 是如何工作的？ 我需要重写迭代器方法吗？
oracle 文档指出： IllegalStateException - 如果下一个方法还没有被调用，或者在最后一次调用下一个方法之后已经调用了 remove 方法
【问题讨论】：