java - 如何修复 java 哈希表断言错误？

import java.math.BigInteger; import java.util.ArrayList; import java.util.Collection; public class Hashtable<V> { private Object[] arr; //an array of Pair objects, where each pair contains the key and value stored in the hashtable private int max; //the size of arr. This should be a prime number private int itemCount; //the number of items stored in arr private final double maxLoad = 0.6; //the maximum load factor public static enum PROBE_TYPE { LINEAR_PROBE, QUADRATIC_PROBE, DOUBLE_HASH; } PROBE_TYPE probeType; //the type of probe to use when dealing with collisions private final BigInteger DBL_HASH_K = BigInteger.valueOf(8); /** * Create a new Hashtable with a given initial capacity and using a given probe type * @param initialCapacity * @param pt */ public Hashtable(int initialCapacity, PROBE_TYPE pt) { max = nextPrime(initialCapacity); probeType = pt ; arr = new Object[max]; } /** * Create a new Hashtable with a given initial capacity and using the default probe type * @param initialCapacity */ public Hashtable(int initialCapacity) { max = nextPrime(initialCapacity); probeType = PROBE_TYPE.LINEAR_PROBE; arr = new Object[max]; } /** * Store the value against the given key. If the loadFactor exceeds maxLoad, call the resize * method to resize the array. the If key already exists then its value should be overwritten. * Create a new Pair item containing the key and value, then use the findEmpty method to find an unoccupied * position in the array to store the pair. Call findEmmpty with the hashed value of the key as the starting * position for the search, stepNum of zero and the original key. * containing * @param key * @param value */ public void put(String key, V value) { int i = hash(key); int finalPos = findEmpty(i,1, key); if (this.getLoadFactor() > this.maxLoad) { this.resize(); } if (!hasKey(key)) { itemCount++; } arr[finalPos] = new Pair(key, value); } /** * Get the value associated with key, or return null if key does not exists. Use the find method to search the * array, starting at the hashed value of the key, stepNum of zero and the original key. * @param key * @return */ public V get(String key) { int i = hash(key); if( arr[i] == null) { return null; } else {return find(hash(key),key,0);} } /** * Return true if the Hashtable contains this key, false otherwise * @param key * @return */ public boolean hasKey(String key) { if (find(hash(key),key,0) == null) { return false; }else { return true; } } /** * Return all the keys in this Hashtable as a collection * @return */ public Collection<String> getKeys() { ArrayList<String> collection = new ArrayList<String>(); for(int i =0;i<arr.length;i++) { if(arr[i] != null) { Pair p = (Pair) arr[i]; collection.add(p.key); } } return collection; } /** * Return the load factor, which is the ratio of itemCount to max * @return */ public double getLoadFactor() { return itemCount / (double) max; } /** * return the maximum capacity of the Hashtable * @return */ public int getCapacity() { return this.max; } /** * Find the value stored for this key, starting the search at position startPos in the array. If * the item at position startPos is null, the Hashtable does not contain the value, so return null. * If the key stored in the pair at position startPos matches the key we're looking for, return the associated * value. If the key stored in the pair at position startPos does not match the key we're looking for, this * is a hash collision so use the getNextLocation method with an incremented value of stepNum to find * the next location to search (the way that this is calculated will differ depending on the probe type * being used). Then use the value of the next location in a recursive call to find. * @param startPos * @param key * @param stepNum * @return */ private V find(int startPos, String key, int stepNum) { Pair p = (Pair) arr[startPos]; if(p == null) { return null; }else if (p.key.equals(key)) { return p.value; }else { stepNum++; int nextLocation = getNextLocation(startPos, stepNum, key); return find(nextLocation, key , stepNum); } } /** * Find the first unoccupied location where a value associated with key can be stored, starting the * search at position startPos. If startPos is unoccupied, return startPos. Otherwise use the getNextLocation * method with an incremented value of stepNum to find the appropriate next position to check * (which will differ depending on the probe type being used) and use this in a recursive call to findEmpty. * @param startPos * @param stepNum * @param key * @return */ private int findEmpty(int startPos, int stepNum, String key) { if (arr[startPos] == null || ((Pair) arr[startPos]).key.equals(key)) { return startPos; }else { stepNum++; int nextPos = getNextLocation(startPos,stepNum,key); return findEmpty(nextPos,stepNum,key); } } /** * Finds the next position in the Hashtable array starting at position startPos. If the linear * probe is being used, we just increment startPos. If the double hash probe type is being used, * add the double hashed value of the key to startPos. If the quadratic probe is being used, add * the square of the step number to startPos. * @param i * @param stepNum * @param key * @return */ private int getNextLocation(int startPos, int stepNum, String key) { int step = startPos; switch (probeType) { case LINEAR_PROBE: step++; break; case DOUBLE_HASH: step += doubleHash(key); break; case QUADRATIC_PROBE: step += stepNum * stepNum; break; default: break; } return step % max; } /** * A secondary hash function which returns a small value (less than or equal to DBL_HASH_K) * to probe the next location if the double hash probe type is being used * @param key * @return */ private int doubleHash(String key) { BigInteger hashVal = BigInteger.valueOf(key.charAt(0) - 96); for (int i = 0; i < key.length(); i++) { BigInteger c = BigInteger.valueOf(key.charAt(i) - 96); hashVal = hashVal.multiply(BigInteger.valueOf(27)).add(c); } return DBL_HASH_K.subtract(hashVal.mod(DBL_HASH_K)).intValue(); } /** * Return an int value calculated by hashing the key. See the lecture slides for information * on creating hash functions. The return value should be less than max, the maximum capacity * of the array * @param key * @return */ private int hash(String key) { int hashVal = key.charAt(0); for(int i=0; i<key.length();i++) { int c = key.charAt(i); hashVal =(hashVal * 27 +c) % max; } return hashVal; } /** * Return true if n is prime * @param n * @return */ private boolean isPrime(int n) { if(n<=2) return true; if(n % 2 == 0) return false; for(int i=3; i*i<n; i+=2) { if(n % i == 0) return false; } return true; } /** * Get the smallest prime number which is larger than n * @param n * @return */ private int nextPrime(int n) { if(isPrime(n)) { return n; }else { n++; return nextPrime(n); } } /** * Resize the hashtable, to be used when the load factor exceeds maxLoad. The new size of * the underlying array should be the smallest prime number which is at least twice the size * of the old array. */ private void resize() { int tableSize = nextPrime(max*2); Object[] oldArr = arr; arr = new Object[tableSize]; itemCount = 0; max = tableSize; for (int i=0;i<oldArr.length;i++) { if(oldArr[i] != null) { Pair p = (Pair) oldArr[i]; put(p.key, p.value); } } } /** * Instances of Pair are stored in the underlying array. We can't just store * the value because we need to check the original key in the case of collisions. * @author jb259 * */ private class Pair { private String key; private V value; public Pair(String key, V value) { this.key = key; this.value = value; } }

@Test public void testInsert() { Hashtable<Boolean> h = new Hashtable<>(1000, PROBE_TYPE.DOUBLE_HASH); for(int i=0;i<2000;i++) { for(int j=2000;j>0;j--) { h.put(i+":"+j, true); } } for(int i=0;i<2000;i++) { for(int j=2000;j>0;j--) { assertTrue(h.hasKey(i+":"+j)); } } } @Test public void testGet() { Hashtable<String> h = new Hashtable<>(9); int c = 0; for(int i=0;i<10;i++) { for(int j=10;j>0;j--) { h.put(i+":"+j, j+":"+i); c++; } } for(int i=0;i<10;i++) { for(int j=10;j>0;j--) { assertEquals(h.get(i+":"+j), j+":"+i); } } }