keys vs. values

[Kevin Knowles]

In HashMap and other classes and in general, what is the difference between keys and values?

[Michael Morris]

A key is something like a postal address and a value like a package that is located there. Using an array you could think of an array index as being a key and whatever is stored at that index as a value. For example:

int[] array = {1, 2, 3, 4};
int i = array[1];

In the code above, using the array analogy, the key is 1 (the subscript) and the value is 2, so the variable i is set to 2. Here is the way you could do the same thing with a Map:

Map map = new HashMap();
for (int j = 0; j < 4; j++) {
    map.put(new Integer(j), new Integer(j + 1));
}
int i = ((Integer) map.get(new Integer(1)).intValue();

That code is a lot messier than the first, but it shows the relationship between keys and values. The real power of a Map though is the ability to associate any object key to any object value. For example, say we have a group of students with average grades and we want to associate the student's name with the grade:

String[] names = {"John Smith", "Mary Brown", "Jack Jones"};
Float[] grades = {new Float(77.3F), new Float(95.6F), new Float(82.8F)};
Map map = new HashMap();
for (int j = 0; j < names.length; j++) {
    map.put(names[j], grades[j]);
}
//Print Mary's name and grade:
System.out.println(names[1] + " " + map.get(names[1]);

[Stan James]

Smalltalk used more the more intuitive name Dictionary instead of the implementation-revealing Hashmap. Given a word you can look up the definition. Given a key you can look up the value.
Definition = Dictionary.lookup( Word )
Value = Hashmap.get( Key )
myCapitolCity = myHashmap.get( "Virginia" )

[Maulin Vasavada]

A key is something like a postal address and a value like a package that is located there.
thanks michael. i tried to answer this question first but i couldn't come with this example
i like it.
regards
maulin