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HASH TABLES
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[CHAP. 8
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The first call to println() invokes the HashMap.toString() method, printing the entire Map object. The second call to println() invokes the HashMap.size() method, showing that the Map object has six key/value elements. The next call to println() invokes the HashMap.keySet() method, which returns a Set object containing all the keys (the six German words). The next call to println() invokes the HashMap.values() method, which returns a Collection object containing all the values (the six English words). The next call to println() invokes the HashMap.get() method, which returns the value for a given key. This call returns the value object "clock" for the key object "Uhr". The next call to println() invokes the HashMap.remove() method, which deletes the ("Rad","wheel") pair, which is confirmed by the next call: map.get("Rad") returns null, indicating that there is no key/value pair in map whose key is "Rad". The last two lines prints the entire map again and its size, revealing that the ("Rad","wheel") pair has indeed been deleted.
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The order in which the key/value pairs were stored in the HashMap object in Example 8.1 seems to be random and unrelated to the order in which the pairs were inserted. The next example verifies this. EXAMPLE 8.2 Java HashMap Objects Are Hash Tables
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This program creates two independent HashMap objects and loads them with the same key/value pairs but in different orders:
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public class TestHashTable { public static void main(String[] args) { Map map1 = new HashMap(); map1.put("Tor","gate"); map1.put("Rad","wheel"); map1.put("Tag","day"); map1.put("Uhr","clock"); map1.put("Hut","hat"); map1.put("Ohr","ear"); System.out.println("map1=" + map1); Map map2 = new HashMap(); map2.put("Rad","wheel"); map2.put("Uhr","clock"); map2.put("Ohr","ear"); map2.put("Tag","day"); map2.put("Tor","gate"); map2.put("Hut","hat"); System.out.println("map2=" + map2); }
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The output is:
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map1={Rad=wheel, Uhr=clock, Ohr=ear, Tor=gate, Hut=hat, Tag=day} map2={Rad=wheel, Uhr=clock, Ohr=ear, Tor=gate, Hut=hat, Tag=day} The order in which the key/value pairs are stored in the HashMap table is reflected by the output from the toString() method. That stored order is same in both tables, independent of the order in which they were inserted. Note that it is also the same stored order in the HashMap table in Example 8.1.
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JAVA HASH CODES The order in which the key/value pairs are stored in a HashMap table depends only upon the capacity of the table and the values of objects the hash codes. Recall ( 4) that every object in Java is given an intrinsic hash code, which is computed from the actual hard data stored in the object. The Object.hashCode() method returns that code for each object.
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HASH TABLES
EXAMPLE 8.3 Hash Codes of Some String Objects
This program prints the intrinsic hash codes of the String objects stored in the previous programs:
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public class TestHashCodes { public static void main(String[] args) { printHashCode("Rad"); printHashCode("Uhr"); printHashCode("Ohr"); printHashCode("Tor"); printHashCode("Hut"); printHashCode("Tag"); } private static void printHashCode(String word) { System.out.printf("%s: %s%n", word, word.hashCode()); } }
The output is:
Rad: Uhr: Ohr: Tor: Hut: Tag: 81909 85023 79257 84279 72935 83834
The fact that all six codes are relatively close 5-digits integers reflects the fact that these String objects all have length 3.
HASH TABLES A hash table is a table that uses a special function to compute the location of data values from their key values instead of storing the keys in the table. The special function is called the hash function for the table. Since the lookup time is independent of the size of the table, hash tables have very fast access time. Java defined a Hashtable class in its java.util package. But it has essentially been upgraded to the HashMap class. That is, a HashMap table can do everything that a Hashtable object can do. Moreover, the HashMap class is more consistent with rest of the Java Collections Framework. A general hash table looks like the one pictured in Figure 8.2: an array of Objects indexed by their hash values. This requires that the range of the hash function match the range of index values in the array. This is Figure 8.2 Hash table almost always managed by simply using the remainder operator modulo the size of the array: EXAMPLE 8.4 Mapping Keys into a Hash Table of Size 11
This program prints hash code values for String objects to be stored in a hash table of size 11:
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public class TestHashing { private static final int MASK = 0x7FFFFFFF; // private static final int CAPACITY = 11;
= 2^32-1
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