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HashDemo.java
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HashDemo.java
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class Entry {
private String key;
private String value;
private int hash;
private Entry next;
Entry getNext() {
return next;
}
void setNext(Entry e) {
next = e;
}
String getKey() {
return key;
}
String getValue() {
return value;
}
void setValue(String v) {
value = v;
}
public String toString() {
return String.format("entry: hash %d key %s, value %s\n", hash, key, value);
}
Entry(String k, String v, int h, Entry n) {
key = k;
value = v;
hash = h;
next = n;
}
}
class HashTable {
private int capacity;
private Entry table[];
private static int hash(String str) {
return str.hashCode();
}
private void trace(String format, Object... args) {
System.out.printf(format, args);
}
private void trace(String str) {
System.out.print(str);
}
private Entry getEntry(String key) {
int hash = hash(key);
int index = hash & (capacity - 1);
Entry e = table[index];
while (e != null) {
if (e.getKey().equals(key) == true)
return e;
e = e.getNext();
}
return null;
}
void put(String key, String value) {
Entry e = getEntry(key);
if (e != null) {
e.setValue(value);
return;
}
else {
int hash = hash(key);
int index = hash & (capacity - 1);
Entry current = table[index];
table[index] = new Entry(key, value, hash, current);
}
}
String get(String key) {
Entry e = getEntry(key);
if (e != null)
return e.getValue();
else
return null;
}
void remove(String key) {
int hash = hash(key);
int index = hash & (capacity - 1);
Entry e = table[index];
Entry previous = null;
while (e != null) {
if (e.getKey().equals(key) == true) {
if (previous == null)
table[index] = e.getNext();
else
previous.setNext(e.getNext());
return;
}
previous = e;
e = e.getNext();
}
}
void show() {
for (int i = 0; i < capacity; i++) {
trace("%4d : ", i);
if (table[i] == null)
trace("null\n");
else {
Entry e = table[i];
trace(e.toString());
while ((e = e.getNext()) != null)
trace(" " + e.toString());
}
}
trace("\n");
}
HashTable(int capacity) {
this.capacity = capacity;
table = new Entry[capacity];
java.util.Arrays.fill(table, null);
}
}
class HashDemo {
public static void main(String args[]) {
HashTable ht = new HashTable(8); /* choose 4, 8, 16, 32 etc...*/
System.out.println("Empty hash table:");
ht.show();
System.out.println("Fill with key-value pairs:");
ht.put("alpha", "aaa");
ht.put("beta", "bbb");
ht.put("gamma", "ggg");
ht.put("delta", "ddd");
ht.put("epsilon", "eee");
ht.show();
System.out.println("Overwrite epsilon value:");
ht.put("epsilon", "new_eee");
ht.show();
System.out.println("Get existing values:");
System.out.println(ht.get("alpha"));
System.out.println(ht.get("gamma"));
System.out.println("Get nonexistent value:");
System.out.println(ht.get("omega"));
System.out.println("Remove gamma:");
ht.remove("gamma");
ht.show();
System.out.println("More complicated test. Add new entries:");
for(char i = 'a'; i <= 'z'; i++) {
String str = new String();
str += i;
ht.put(str, str);
}
ht.show();
System.out.println("Delete this entries:");
for(char i = 'a'; i <= 'z'; i++) {
String str = new String();
str += i;
ht.remove(str);
}
ht.show();
System.out.println("Delete other entries:");
ht.remove("alpha");
ht.remove("beta");
ht.remove("gamma");
ht.remove("delta");
ht.remove("epsilon");
ht.show();
}
}