我目前正在学习 Java 中的不同 DataStructures，一种是 Array Bags 和 Linked Bags。我理解数据结构如此重要的原因，以及泛型如何让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（如果我对该陈述的想法或理解由于使用数据结构背后的原因是错误的，请告诉我）。
话虽如此，我还没有完全理解泛型和数据结构的某些方面。这个线程没有特别提到那些，因为我将继续单独查找这些东西，但它可能会影响我为什么在正确实施允许我复制给定包的所有项目的方法时遇到问题。
下面是我正在研究的实验室的代码。实验室要求我完成此文件中的某些方法，以便在编译和运行驱动程序时，将适当的答案返回到命令提示符。当我运行驱动程序时，我相信我到目前为止所做的事情是正确的。但是当我尝试完成该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，我的最后一个方法removeDuplicates()不完整，但我还没有进入实验室的那个部分。
这是代码：
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
我试过this.代替thisBag以下方法（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
我在编译时收到以下错误：
./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this正确投射，所以我直接在上面得到了错误。但是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：
为什么我会收到这些错误？我cannot find symbol在上一堂 Java 课程中收到了类似的错误，那是我认为我没有正确实例化的时候？我认为我的方法是正确的，因为我进行了转换T[] thisBag = this.toArray();，然后尝试在thisBag方法中使用和代码中的条目。由于这是错误的，我不确定如何使用genericsArrayBag/Bag DataStructures 清除这些。
另外，我是在适当地处理通用包中的条目重复，还是有更好的方法我不知道？我正在尝试使用add()此文件中的方法来发挥自己的优势，但显然遇到了困难。在在线查看 Java API 的文档时，我发现我可以采取一些方法来实现这种复制：
arrays.fill
arrays.setAll(T[] array, IntFunction<? extends T> generator)
arrays.clone
我在想我必须首先确保数组中有足够的固定空间来在内存中复制它。如果没有，我将不得不使用动态数组并将空间加倍并将内容复制两次到一个新数组中。这对我来说是新的，并且没有亲自深入研究过这种方法（刚刚学习了这个概念）。
与往常一样，我期待听到你们如何解决类似的问题！并告诉我我做错了什么！
ETA：添加提供的答案和我的修复遇到的另一个问题
在@gar 提出以下建议后，我实现了以下内容：
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null数组中的条目引起的（因此java.lang.NullPointerException。添加了一些System.out.println行以查看复制数组时数组大小的变化，并注意到第一个数组（包含一个条目）以三个条目。我numberOfEntries++;从方法的末尾删除了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！