因此，我在概念化以下问题的答案时遇到了很多困难。我不是在寻找答案，而是我可以采取的有用步骤，这将使我能够自己提出答案。注意：为了回答这些问题，我给出了引用的类和一个驱动程序。问题如下：
Q.1 为 ADT 包实现一个方法 replace，用给定的对象替换并返回当前在包中的任何对象。
Q.2. 编写一个 remove 方法来删除 ArrayBag 中元素的每个实例。
问题 3。至少给出两个适合使用固定包的情况的示例和两个适合使用可调整大小的包的情况的示例。
以下代码是我开始的，但不确定我的方向是否正确：
a.1 public T replace(T theItem) {
Random generator = new Random();
int randomPosition = generator.nextInt(numberOfEntries);
T result = null;
if (!isEmpty() && (randomPosition >= 0)) {
result = bag[randomPosition]; // Entry to remove
bag[randomPosition] = theItem; // Replace entry to remove with
// last entry
}
return result;
a.2 public void clear(T theItem) {
while (!this.isEmpty(ArrayBag))
this.remove();
a.3 not sure it should be related to coding examples or something else.
此外，ArrayBag 的类如下供参考：
import java.util.Arrays;
import java.util.Random;
public final class ArrayBag<T> implements BagInterface<T> {
private final T[] bag;
private int numberOfEntries;
private boolean initialized = false;
private static final int DEFAULT_CAPACITY = 25;
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 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
/**
* Retrieves all entries that are in this bag.
*
* @return A newly allocated array of all the entries in this bag.
*/
public T[] toArray() {
checkInitialization();
// 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;
// Note: The body of this method could consist of one return statement,
// if you call Arrays.copyOf
} // end toArray
/**
* Sees whether this bag is empty.
*
* @return True if this 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 this 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 this ba.
*/
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 this bag contains anEntry, or false otherwise.
*/
public boolean contains(T anEntry) {
checkInitialization();
return getIndexOf(anEntry) > -1; // or >= 0
} // end contains
/** Removes all entries from this bag. */
public void clear() {
while (!this.isEmpty())
this.remove();
} // end clear
/**
* Removes one unspecified entry from this bag, if possible.
*
* @return Either the removed entry, if the removal was successful, or null.
*/
public T remove() {
checkInitialization();
T result = removeEntry(numberOfEntries - 1);
return result;
} // 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
public boolean removeRandom() {
Random generator = new Random();
int randomPosition = generator.nextInt(numberOfEntries);
T result = removeEntry(randomPosition);
if (result == null) {
return false;
} else {
return true;
}
}
@Override
public boolean equals(Object obj) {
if (obj instanceof ArrayBag) {
ArrayBag<T> otherArrayBag = (ArrayBag<T>) obj;
if (numberOfEntries == otherArrayBag.numberOfEntries) {
// I create new arrays so that I can manipulate them
// and it will not alter this.bag or otherArrayBag.bag
T[] currentBagTempArray = toArray();
T[] otherBagTempArray = otherArrayBag.toArray();
Arrays.sort(currentBagTempArray);
Arrays.sort(otherBagTempArray);
for (int i = 0; i < numberOfEntries; i++) {
if (!currentBagTempArray[i].equals(otherBagTempArray[i])) {
return false;
}
}
return true;
} else {
return false;
}
} else {
return false;
}
}
public ResizableArrayBag<T> createResizableArray() {
T[] currentBagContents = toArray();
ResizableArrayBag<T> newBag = new ResizableArrayBag<T>(currentBagContents);
return newBag;
}
// Returns true if the array bag is full, or false if not.
private boolean isArrayFull() {
return numberOfEntries >= bag.length;
} // end isArrayFull
// 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 found = false;
int index = 0;
while (!found && (index < numberOfEntries)) {
if (anEntry.equals(bag[index])) {
found = true;
where = index;
} // end if
index++;
} // end while
// 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
// Removes and returns the entry at a given index within the array.
// If no such entry exists, returns null.
// Precondition: 0 <= givenIndex < numberOfEntries.
// Precondition: checkInitialization has been called.
private T removeEntry(int givenIndex) {
T result = null;
if (!isEmpty() && (givenIndex >= 0)) {
result = bag[givenIndex]; // Entry to remove
int lastIndex = numberOfEntries - 1;
bag[givenIndex] = bag[lastIndex]; // Replace entry to remove with
// last entry
bag[lastIndex] = null; // Remove reference to last entry
numberOfEntries--;
} // end if
return result;
} // end removeEntry
// Throws an exception if this object is not initialized.
private void checkInitialization() {
if (!initialized)
throw new SecurityException(
"ArrayBag object is not initialized properly.");
} // end checkInitialization
} // 结束 ArrayBag