我目前正在学习Java中的不同DataStructures，其中一个是Array Bags和Linked Bags。我理解为什么数据结构如此重要以及泛型如何允许我们以统一的方式更容易地处理对象，无论它们的类型如何（如果我对使用数据结构的原因我的思考或理解是错误的，请告诉我）。
话虽如此，我还没有完全理解泛型和数据结构的一些方面。这个帖子没有特别提到那些，因为我将继续单独查看这些内容，但它可能会影响我为什么在正确实现允许我复制给定包的所有项目的方法时遇到问题
以下是我正在进行的实验室的代码。实验室要求我在此文件中完成某些方法，以便在编译和运行驱动程序时，将相应的答案返回到命令提示符。我相信当我运行驱动程序时，我到目前为止所做的是正确的。但是当我尝试完成duplicateAll()方法（下面文件中的倒数第二个方法）时，无论我如何尝试更改它，我都会继续遇到相同的错误。请记住，我的上一个方法removeDuplicates()并不完整，但我还没有参加实验室的那一部分。
以下是代码：
import java.util.Arrays;
import java.util.Random;
public final class ArrayBag<T> implements BagInterface<T> {
private final T[] bag;
private int numberOfEntries;
private static final int DEFAULT_CAPACITY = 25;
private boolean initialized = false;
private static final int MAX_CAPACITY = 10000;
/** Creates an empty bag whose initial capacity is 25. */
public ArrayBag() {
this(DEFAULT_CAPACITY);
} // end default constructor
/**
* Creates an empty bag having a given initial capacity.
*
* @param desiredCapacity The integer capacity desired.
*/
public ArrayBag(int desiredCapacity) {
if (desiredCapacity <= MAX_CAPACITY) {
// The cast is safe because the new array contains null entries.
@SuppressWarnings("unchecked")
T[] tempBag = (T[]) new Object[desiredCapacity]; // Unchecked cast
bag = tempBag;
numberOfEntries = 0;
initialized = true;
}
else
throw new IllegalStateException("Attempt to create a bag " +
"whose capacity exceeds " +
"allowed maximum.");
} // end constructor
/** Adds a new entry to this bag.
@param newEntry The object to be added as a new entry.
@return True if the addition is successful, or false if not. */
public boolean add(T newEntry) {
checkInitialization();
boolean result = true;
if (isArrayFull()) {
result = false;
} else { // Assertion: result is true here
bag[numberOfEntries] = newEntry;
numberOfEntries++;
} // end if
return result;
} // end add
/** Throws an exception if this object is not initialized.
*
*/
private void checkInitialization()
{
if (!initialized)
throw new SecurityException("ArrayBag object is not initialized " +
"properly.");
}
/** Retrieves all entries that are in this bag.
@return A newly allocated array of all the entries in the bag. */
public T[] toArray() {
// the cast is safe because the new array contains null entries
@SuppressWarnings("unchecked")
T[] result = (T[]) new Object[numberOfEntries]; // unchecked cast
for (int index = 0; index < numberOfEntries; index++) {
result[index] = bag[index];
} // end for
return result;
} // end toArray
/** Sees whether this bag is full.
@return True if the bag is full, or false if not. */
private boolean isArrayFull() {
return numberOfEntries >= bag.length;
} // end isArrayFull
/** Sees whether this bag is empty.
@return True if the bag is empty, or false if not. */
public boolean isEmpty() {
return numberOfEntries == 0;
} // end isEmpty
/** Gets the current number of entries in this bag.
@return The integer number of entries currently in the bag. */
public int getCurrentSize() {
return numberOfEntries;
} // end getCurrentSize
/** Counts the number of times a given entry appears in this bag.
@param anEntry The entry to be counted.
@return The number of times anEntry appears in the bag. */
public int getFrequencyOf(T anEntry) {
checkInitialization();
int counter = 0;
for (int index = 0; index < numberOfEntries; index++) {
if (anEntry.equals(bag[index])) {
counter++;
} // end if
} // end for
return counter;
} // end getFrequencyOf
/** Tests whether this bag contains a given entry.
@param anEntry The entry to locate.
@return True if the bag contains anEntry, or false if not. */
public boolean contains(T anEntry) {
checkInitialization();
return getIndexOf(anEntry) > -1;
} // end contains
/** Removes all entries from this bag. */
public void clear() {
while (!isEmpty()) {
remove();
}
} // end clear
/** Removes one unspecified entry from this bag, if possible.
@return Either the removed entry, if the removal was successful,
or null if otherwise. */
public T remove() {
checkInitialization();
// MODIFY THIS METHOD TO REMOVE A RANDOM ITEM FROM THE BAG
Random randomNum = new Random();
if(numberOfEntries > 0){
int randomKey = randomNum.nextInt(numberOfEntries);
T result = removeEntry(randomKey);
return result;
}else{
return null;
}
} // end remove
/** Removes one occurrence of a given entry from this bag.
@param anEntry The entry to be removed.
@return True if the removal was successful, or false if not. */
public boolean remove(T anEntry) {
checkInitialization();
int index = getIndexOf(anEntry);
T result = removeEntry(index);
return anEntry.equals(result);
} // end remove
// Removes and returns the entry at a given array index within the array bag.
// If no such entry exists, returns null.
// Preconditions: 0 <= givenIndex < numberOfEntries;
// checkInitialization has been called.
private T removeEntry(int givenIndex) {
T result = null;
if (!isEmpty() && (givenIndex >= 0)) {
result = bag[givenIndex]; // entry to remove
bag[givenIndex] = bag[numberOfEntries - 1]; // Replace entry with last entry
bag[numberOfEntries - 1] = null; // remove last entry
numberOfEntries--;
} // end if
return result;
} // end removeEntry
// Locates a given entry within the array bag.
// Returns the index of the entry, if located, or -1 otherwise.
// Precondition: checkInitialization has been called.
private int getIndexOf(T anEntry) {
int where = -1;
boolean stillLooking = true;
int index = 0;
while ( stillLooking && (index < numberOfEntries)) {
if (anEntry.equals(bag[index])) {
stillLooking = false;
where = index;
} // end if
index++;
} // end for
// Assertion: If where > -1, anEntry is in the array bag, and it
// equals bag[where]; otherwise, anEntry is not in the array
return where;
} // end getIndexOf
/** Override the equals method so that we can tell if two bags contain the same items
* the contents in the bag.
* @return a string representation of the contents of the bag */
public String toString() {
String result = "Bag{Size:" + numberOfEntries + " ";
for (int index = 0; index < numberOfEntries; index++) {
result += "[" + bag[index] + "] ";
} // end for
result += "}";
return result;
} // end toArray
/*********************************************************************
*
* METHODS TO BE COMPLETED
*
*
************************************************************************/
/** Check to see if two bags are equals.
* @param aBag Another object to check this bag against.
* @return True the two bags contain the same objects with the same frequencies.
*/
public boolean equals(ArrayBag<T> aBag) {
boolean result = false; // result of comparison of bags
boolean sameLength = false;
T[] thisBag = this.toArray();
T[] otherBag = aBag.toArray();
Arrays.sort(thisBag);
Arrays.sort(otherBag);
if (thisBag.length == otherBag.length){
sameLength = true;
}
if(sameLength){
for(int index = 0; index < otherBag.length; index++)
{
if(thisBag[index].equals(otherBag[index])){
result = true;
}
}
}
return result;
} // end equals
/** Duplicate all the items in a bag.
* @return True if the duplication is possible.
*/
public boolean duplicateAll() {
checkInitialization();
boolean success = false;
T[] thisBag = this.toArray();
if(!thisBag.isEmpty()){
int originalTotalEntries = numberOfEntries;
for(int index = 0; index < originalTotalEntries; index++){
success = thisBag.add(thisBag[index]);
numberOfEntries++;
}
}
return success;
} // end duplicateAll
/** Remove all duplicate items from a bag
*/
public void removeDuplicates() {
checkInitialization();
// COMPLETE THIS METHOD
return;
} // end removeDuplicates
} // end ArrayBag
当我尝试编译代码时，出现以下错误：
./ArrayBag.java:260: error: cannot find symbol
if(!thisBag.isEmpty()){
^
symbol: method isEmpty()
location: variable thisBag of type T[]
where T is a type-variable:
T extends Object declared in class ArrayBag
./ArrayBag.java:263: error: cannot find symbol
success = thisBag.add(thisBag[index]);
^
symbol: method add(T)
location: variable thisBag of type T[]
where T is a type-variable:
T extends Object declared in class ArrayBag
2 errors
我已尝试this.代替thisBag，如下所示（仅粘贴duplicateAll()方法：
public boolean duplicateAll() {
checkInitialization();
boolean success = false;
T[] thisBag = this.toArray(); //not needed anymore
if(!this.isEmpty()){
int originalTotalEntries = this.numberOfEntries;
for(int index = 0; index < originalTotalEntries; index++){
success = this.add(this[index]);
numberOfEntries++;
}
}
return success;
} // end duplicateAll
我在编译时遇到以下错误：
./ArrayBag.java:263: error: array required, but ArrayBag<T> found
success = this.add(this[index]);
^
where T is a type-variable:
T extends Object declared in class ArrayBag
1 error
所以我知道，由于我没有正确地投射this，我直接在上面得到错误。但是在最后一个上面发布的两个错误是我被挂断的地方。
我的问题是：
为什么我收到这些错误？我在上一节Java中收到了与cannot find symbol类似的错误，当我相信我没有正确实例化时？我认为我的方法是正确的，因为我投了T[] thisBag = this.toArray();，然后尝试使用thisBag和方法中代码中的条目。由于这是错误的，我不确定如何使用generics和ArrayBag / Bag DataStructures清除它们。
另外，我是否正确地接近通用包中的条目重复，或者是否有一种我不知道的更好的方式？我尝试在此文件中使用add()方法对我有利，但显然有困难。在线查看Java API的文档时，我发现可能有几条路由可以实现这种重复：
arrays.fill
arrays.setAll(T[] array, IntFunction<? extends T> generator)
arrays.clone
我认为我必须首先确保阵列中有足够的固定空间在内存中复制它。如果没有，我必须使用动态数组并将空间加倍并将内容复制两次到新数组中。这对我来说是新的，并没有亲自深入研究这种方法（只是学习了这个概念）。
与往常一样，我期待着听到你们所有人都会遇到类似的问题！并告诉我，我做错了什么！
ETA：添加提供的答案以及我的修复所遇到的另一个问题
在@gar建议如下之后，我实施了以下内容：
public boolean duplicateAll() {
checkInitialization();
boolean success = false;
T[] thisBag = this.toArray(); //not needed anymore
if(!this.isEmpty()){
int originalTotalEntries = this.numberOfEntries;
for(int index = 0; index < originalTotalEntries; index++){
success = this.add(thisBag[index]);
numberOfEntries++;
}
}
return success;
} // end duplicateAll
当我这样做时，我开始收到以下错误：
Exception in thread "main" java.lang.NullPointerException
at java.util.ComparableTimSort.countRunAndMakeAscending(ComparableTimSort.java:325)
at java.util.ComparableTimSort.sort(ComparableTimSort.java:188)
at java.util.Arrays.sort(Arrays.java:1246)
at ArrayBag.equals(ArrayBag.java:234)
at BagExtensionsTest.checkDuplicateAll(BagExtensionsTest.java:720)
at BagExtensionsTest.main(BagExtensionsTest.java:52)
我在这个网站上做了一些挖掘并读到这是由数组中的null个条目引起的（因此是java.lang.NullPointerException。添加了一些System.out.println行来查看我的大小变化复制它们时的数组并注意到第一个数组（包含一个条目）以三个条目结束。我从numberOfEntries++;方法的末尾删除了duplicateAll()，它纠正了我的问题。
只是想我会分享这些信息让其他人知道。
一如既往，很棒的信息！
答案 0 :(得分：2)
您对对象和数组的方法感到困惑。你提供的第二次尝试似乎非常接近。 isEmpty类位于ArrayBag类，而[...]访问器仅适用于数组。修改代码片段以获取要从数组而不是对象添加的条目，如下所示：
public boolean duplicateAll() {
checkInitialization();
boolean success = false;
T[] thisBag = this.toArray(); //not needed anymore
if(!this.isEmpty()){
int originalTotalEntries = this.numberOfEntries;
for(int index = 0; index < originalTotalEntries; index++){
success = this.add(thisBag[index]);
numberOfEntries++;
}
}
return success;
} // end duplicateAll
我还没有对此进行测试，因此可能存在其他错误，但希望它能让您走得更远。
答案 1 :(得分：1)
尝试tp替换它：
if(!thisBag.isEmpty()){
要：
if(thisBag.length>0){
并且：
success = this.add(this[index]);
要：
success = this.add(thisBag[index]);