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<TITLE>Comparing Object Values Using Hash Codes (Java Developers Almanac Example)
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<h3>e57. Comparing Object Values Using Hash Codes</h3>
The hash code of an object is an integer value that's computed using
the value of the object. For example, for a <code>String</code> object, the
characters of the string are used to compute the hash code. For an
<code>Integer</code> object, the integer value is used to compute the hash code.
<P> Hash codes are typically used as an efficient way of comparing
the values of two objects. For example, if the hash code of the
string <code>"hello"</code> is 33, another <code>String</code> object with the same
contents would also a hash code of 33.
<P> If the hash codes of two object values are different, the
object values are guaranteed to be different. However, if the hash
codes of two object values are the same, the object values are
<font color="#0066ff"><i>not</i></font> guaranteed to be the same. An additional call to
<code>Object.equals()</code> must be made to confirm that the object values
are the same. A good hash code algorithm will minimize the chance of
two different values having the same hash code.
<P> The `<code>==</code>' operator is the most efficient way to determine if two
objects (rather than object values) are the same. However, in very
limited applications, it may be necessary to get the hash code of an
object (called the identity hash code) rather than of the object
value. For example, a hash table of objects requires the use of the
identity hash code.
<pre> File file1 = new File(<font color="#0066ff"><i>"a"</i></font>);
File file2 = new File(<font color="#0066ff"><i>"a"</i></font>);
File file3 = new File(<font color="#0066ff"><i>"b"</i></font>);
// Get the hash codes
int hc1 = file1.hashCode(); // 1234416
int hc2 = file2.hashCode(); // 1234416
int hc3 = file3.hashCode(); // 1234419
// Check if two object values are the same
if (hc1 == hc2 && file1.equals(file2)) {
// They are the same
}
// Get the identity hash codes
int ihc1 = System.identityHashCode(file1); // 1027049
int ihc2 = System.identityHashCode(file2); // 14642381
int ihc3 = System.identityHashCode(file3); // 6298545
</pre>
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e56. <a class="eglink" href="CloneObject.html?l=rel">
Cloning an Object
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e58. <a class="eglink" href="Wrap.html?l=rel">
Wrapping a Primitive Type in a Wrapper Object
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See also:
<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Arrays">
Arrays
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<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Assertions">
Assertions
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<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Classes">
Classes
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<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Commands">
Commands
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<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Numbers">
Numbers
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<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Strings">
Strings
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<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#System%20Properties">
System Properties
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<a class="eglink" href="/egs/java.lang/pkg.html?l=rel#Threads">
Threads
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