Java Reference
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System.out.println(map.get(p1)); // Output: first
System.out.println(map.get(new Point(10, 20))); //
Output: null
Listing5-20 ' s hashCode() methodisalittleverboseinthatitassignseachof x and
y to local variable hc , rather than directly using these fields in the hash code calcula-
tion. However, I decided to follow this approach to more closely mirror the hash code
When you run this application, its last two lines of output are of the most interest.
Insteadofpresenting first point followedby null ontwoseparatelines,theap-
plicationnowcorrectlypresents first point followedby first point onthese
Note LinkedHashMap isasubclassof HashMap thatusesalinkedlisttostoreits
entries. As a result, LinkedHashMap 's iterator returns entries in the order in which
they were inserted. For example, if Listing 5-19 had specified Map<String, In-
teger> argMap = new LinkedHashMap<>(); ,theapplication's outputfor
java HashMapDemo how much wood could a woodchuck chuck if
a woodchuck could chuck wood would have been {how=1, much=1,
wood=2, could=2, a=2, woodchuck=2, chuck=2, if=1} followedby
Number of distinct arguments = 8 .
The IdentityHashMap class provides a Map implementation that uses reference
equality( == )insteadofobjectequality( equals() )whencomparingkeysandvalues.
This is an intentional violation of Map 's general contract, which mandates the use of
equals() when comparing elements.
IdentityHashMap obtains hash codes via System 's static int iden-
tityHashCode(Object x) methodinsteadofviaeachkey's hashCode() meth-
od. identityHashCode() returns the same hash code for x as returned by Ob-
ject 's hashCode() method,whetherornot x 'sclassoverrides hashCode() .The
hash code for the null reference is zero.
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