我想初始化一个项目数组，但无法弄明白。
这是我的代码。
public class HashTable<Item> {
private int m; // hash table size
private Item[] T; // hash table
HashTable(int M)
{
m = M;
T = new Item[M];
for(int i=0;i<M;i++){
Item T[i] = null;
}
}
...
...
的解 的
T = (Item[])new Object[M];
答案 0 :(得分：4)
我认为你需要的是：
for(int i=0;i<M;i++){
T[i] = new Item(); // call some constructor here
}
你有
Item T[i] = ...
，而它应该只是
T[i] = ...
请尝试这些提示。
也这样做：
T = (Item[])new Object[M];
事情是，Item在这里并不是真正的类型。你需要
阅读如何将泛型实际编译成字节码，
你会看到引擎盖下会发生什么。
答案 1 :(得分：1)
您正在尝试创建Generic类型的数组，请查看此帖子。
答案 2 :(得分：0)
假设你创建了类项目，你可以像这样调用它：
public class HashTable
{
private int tableSize; // hash table size
private Item[] table; // hash table
public static void main(String[] args)
{
// where 10 is the number of nodes
Item[] myTable = createHashTable(10);
}
private static Item[] createHashTable(int size)
{
Item[] table = new Item[size];
for(int i = 0; i < table.length; i++)
{
table[i] = new Item(i);
}
}
}
但是，如果您想查看完整HashTable实现的示例：
/*
* HashTable.java
*
*
*/
/**
* A class that implements a hash table that employs open addressing
* using either linear probing, quadratic probing, or double hashing.
*/
public class HashTable {
/* Private inner class for an entry in the hash table */
private class Entry {
private String key;
private LLList valueList; // all of the values with this key
private boolean hasBeenRemoved; // has this entry been removed?
private Entry(String key, int value) {
this.key = key;
valueList = new LLList();
valueList.addItem(value, 0);
hasBeenRemoved = false;
}
}
// parameters for the second hash function -- see h2() below
private static final int H2_MIN = 5;
private static final int H2_DIVISOR = 11;
// possible types of probing
public static final int LINEAR = 0;
public static final int QUADRATIC = 1;
public static final int DOUBLE_HASHING = 2;
public static final int NUM_PROBE_TYPES = 3;
private Entry[] table; // the hash table itself
private int probeType = LINEAR; // the type of probing
// keeps track of how many times we perform a probe of a given length
private int[] probeLengthCount;
public HashTable(int size, int probeType) {
if (probeType >= 0 && probeType < NUM_PROBE_TYPES)
this.probeType = probeType;
else
throw new IllegalArgumentException("invalid probeType: " +
probeType);
table = new Entry[size];
probeLengthCount = new int[size + 1];
for (int i = 0; i <= size; i++)
probeLengthCount[i] = 0;
}
public HashTable(int size) {
// Call the other constructor to do the work.
this(size, LINEAR);
}
/* first hash function */
private int h1(String key) {
int h1 = key.hashCode() % table.length;
if (h1 < 0)
h1 += table.length;
return h1;
}
/* second hash function */
private int h2(String key) {
int h2 = key.hashCode() % H2_DIVISOR;
if (h2 < 0)
h2 += H2_DIVISOR;
h2 += H2_MIN;
return h2;
}
/*
* probeIncrement - returns the amount by which the current index
* should be incremented to obtain the nth element in the probe
* sequence
*/
private int probeIncrement(int n, int h2) {
if (n <= 0)
return 0;
switch (probeType) {
case LINEAR:
return 1;
case QUADRATIC:
return (2*n - 1);
case DOUBLE_HASHING:
default:
return h2;
}
}
/*
* probe - attempt to find a slot in the hash table for the specified key.
*
* If key is currently in the table, it returns the index of the entry.
* If key isn't in the table, it returns the index of the first empty cell
* in the table.
* If overflow occurs, it returns -1.
*/
private int probe(String key) {
int i = h1(key); // first hash function
int h2 = h2(key); // second hash function
int positionsChecked = 1;
// keep probing until we get an empty position or a match
while (table[i] != null && !key.equals(table[i].key)) {
if (positionsChecked == table.length) {
probeLengthCount[positionsChecked]++;
return -1;
}
i = (i + probeIncrement(positionsChecked, h2)) % table.length;
positionsChecked++;
}
probeLengthCount[positionsChecked]++;
return i;
}
/**
* insert - insert the specified (key, value) pair in the hash table
*/
public void insert(String key, int value) {
if (key == null)
throw new IllegalArgumentException("key must be non-null");
int i = h1(key);
int h2 = h2(key);
int positionsChecked = 1;
int firstRemoved = -1;
while (table[i] != null && !key.equals(table[i].key)) {
if (table[i].hasBeenRemoved && firstRemoved == -1)
firstRemoved = i;
if (positionsChecked == table.length)
break;
i = (i + probeIncrement(positionsChecked, h2)) % table.length;
positionsChecked++;
}
probeLengthCount[positionsChecked]++;
if (table[i] != null && key.equals(table[i].key))
table[i].valueList.addItem(value, 0);
else if (firstRemoved != -1)
table[firstRemoved] = new Entry(key, value);
else if (table[i] == null)
table[i] = new Entry(key, value);
else
throw new RuntimeException("overflow occurred");
}
/**
* search - search for the specified key, and return the
* associated list of values, or null if the key is not in the
* table
*/
public LLList search(String key) {
if (key == null)
throw new IllegalArgumentException("key must be non-null");
int i = probe(key);
if (i == -1 || table[i] == null)
return null;
else
return table[i].valueList;
}
/**
* remove - remove from the table the entry for the specified key
*/
public void remove(String key) {
if (key == null)
throw new IllegalArgumentException("key must be non-null");
int i = probe(key);
if (i == -1 || table[i] == null)
return;
table[i].key = null;
table[i].valueList = null;
table[i].hasBeenRemoved = true;
}
/**
* printStats - print the statistics for the table -- i.e., the
* number of keys and items, and stats for the number of times
* that probes of different lengths were performed
*/
public void printStats() {
int numProbes = 0;
int probeLengthSum = 0;
int numKeys = 0;
for (int i = 0; i < table.length; i++) {
if (table[i] != null && !table[i].hasBeenRemoved)
numKeys++;
}
System.out.println("\n" + numKeys + " keys");
System.out.println("probe-length stats:");
System.out.println("length\tcount");
for (int i = 1; i <= table.length; i++) {
if (probeLengthCount[i] != 0)
System.out.println(i + "\t" + probeLengthCount[i]);
numProbes += probeLengthCount[i];
probeLengthSum += (probeLengthCount[i] * i);
}
System.out.println("average probe length = " +
(double)probeLengthSum / numProbes);
}
}
这是Linked-Linked-List
的第二个文件 /*
* LLList.java
*
*
*/
import java.util.*;
/**
* A class that implements our simple List interface using a linked list.
* The linked list includes a dummy head node that allows us to avoid
* special cases for insertion and deletion at the front of the list.
*/
public class LLList implements List {
// Inner class for a node. We use an inner class so that the LLList
// methods can access the instance variables of the nodes.
private class Node {
private Object item;
private Node next;
private Node(Object i, Node n) {
item = i;
next = n;
}
}
private Node head; // dummy head node
private int length; // # of items in the list
/**
* Constructs a LLList object for a list that is initially empty.
*/
public LLList() {
head = new Node(null, null);
length = 0;
}
/*
* getNode - private helper method that returns a reference to the
* ith node in the linked list. It assumes that the value of the
* parameter is valid.
*
* If i == -1, it returns a reference to the dummy head node.
*/
private Node getNode(int i) {
Node trav = head;
int travIndex = -1;
while (travIndex < i) {
travIndex++;
trav = trav.next;
}
return trav;
}
/** getItem - returns the item at position i in the list */
public Object getItem(int i) {
if (i < 0 || i >= length)
throw new IndexOutOfBoundsException();
Node n = getNode(i);
return n.item;
}
/**
* addItem - adds the specified item at position i in the list,
* shifting the items that are currently in positions i, i+1, i+2,
* etc. to the right by one. Always returns true, because the list
* is never full.
*
* We don't need a special case for insertion at the front of the
* list (i == 0), because getNode(0 - 1) will return the dummy
* head node, and the rest of insertion can proceed as usual.
*/
public boolean addItem(Object item, int i) {
if (i < 0 || i > length)
throw new IndexOutOfBoundsException();
Node newNode = new Node(item, null);
Node prevNode = getNode(i - 1);
newNode.next = prevNode.next;
prevNode.next = newNode;
length++;
return true;
}
/**
* removeItem - removes the item at position i in the list,
* shifting the items that are currently in positions i+1, i+2,
* etc. to the left by one. Returns a reference to the removed
* object.
*
* Here again, we don't need a special case for i == 0 (see the
* note accompanying addItem above).
*/
public Object removeItem(int i) {
if (i < 0 || i >= length)
throw new IndexOutOfBoundsException();
Node prevNode = getNode(i - 1);
Object removed = prevNode.next.item;
prevNode.next = prevNode.next.next;
length--;
return removed;
}
/** length - returns the number of items in the list */
public int length() {
return length;
}
/**
* isFull - always returns false, because the linked list can
* grow indefinitely and thus the list is never full.
*/
public boolean isFull() {
return false;
}
/**
* toString - converts the list into a String of the form
* [ item0 item1 ... ]
*/
public String toString() {
String str = "[ ";
Node trav = head.next; // skip over the dummy head node
while (trav != null) {
str += (trav.item + " ");
trav = trav.next;
}
str += "]";
return str;
}
/**
* iterator - returns an iterator for this list
*/
public ListIterator iterator() {
return new LLListIterator();
}
/*
*** private inner class for an iterator over an LLList ***
*/
private class LLListIterator implements ListIterator {
private Node nextNode; // the next node to visit
private Node lastVisitedNode; // the most recently visited node
public LLListIterator() {
nextNode = head.next;
lastVisitedNode = null;
}
/**
* hasNext - does the iterator have additional items to visit?
*/
public boolean hasNext() {
return (nextNode != null);
}
/**
* next - returns a reference to the next Object in the iteration
*/
public Object next() {
if (nextNode == null)
throw new NoSuchElementException();
Object item = nextNode.item;
lastVisitedNode = nextNode;
nextNode = nextNode.next;
return item;
}
}
}