Java Reference
In-Depth Information
If your class is mutable, you should not be using objects of your class as keys in hash-based collections. If the
object has been used as a key changes after their use, you will not be able to locate the object in the collection because
locating an object in a hash based collection is based on its hash code. In such cases, you will have stranded objects in
the collection.
How should you implement a
hashCode()
method for a class? Here are some guidelines to write the logic for the
hashCode()
method for your class, which is reasonable for most of the purposes:
Start with a prime number, say 37.
•
int hash = 37;
•
Compute the hash code value for each instance variable of primitive data types separately
using the following logic. Note that you need to use only those instance variables in the hash
code computation, which are also part of the
equals()
method logic. Let's store the result of
this step in an
int
variable
code
. Let's assume that
value
is the name of the instance variable.
For
byte
,
short
,
int
, and
char
data types, use their integer value as
code = (int)value;
For
long
data type, use the
XOR
for two halves of 64-bit as
code = (int)(value ^ (value >>>32));
For
float
data type, convert its floating-point values to an equivalent integer value using
code = Float.floatToIntBits(value)
For
double
data type, convert its floating-point value to
long
using the
doubleToLongBits()
method of the
Double
class and then convert the long value to an
int
value using the
procedure as described above for the
long
data type.
long longBits = Double.doubleToLongBits(value);
code = (int)(longBits ^ (longBits >>>32));
For
boolean
data type, use
1
for
true
and
0
for
false
.
code = (value ? 1 : 0)
null
. Otherwise, call its
hashCode()
method to
get its hash code. Suppose
ref
is the name of the reference variable.
•
For a reference instance variable, use 0 if it is
code = (ref == null ? 0: ref.hashCode());
Compute the hash code using the following formula. Using 59 in the formula is an arbitrary
decision. Any other prime number, say 47, will work fine.
hash = hash * 59 + code;
hashCode()
•
Repeat the above three steps for all instance variables you want to include in your
computation.
hash
variable from your
hashCode()
method.
•
Finally, return the value contained in the
