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e21cbe82a9
Where possible update Java documentation links to Java 11. Additionally update some other links to use HTTPS.
57 lines
2.1 KiB
XML
57 lines
2.1 KiB
XML
<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p>Classes that define an <code>equals</code> method but no <code>hashCode</code> method
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can lead to unexpected results if instances of those classes are stored in a hashing data structure.
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Hashing data structures expect that hash codes fulfill the contract that two objects that
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<code>equals</code> considers equal should have the same hash code. This contract is likely to be
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violated by such classes.</p>
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</overview>
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<recommendation>
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<p>Every class that implements a custom <code>equals</code> method should also provide
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an implementation of <code>hashCode</code>.</p>
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</recommendation>
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<example>
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<p>In the following example, class <code>Point</code> has no implementation of <code>hashCode</code>.
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Calling <code>hashCode</code> on two distinct <code>Point</code> objects
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with the same coordinates would probably result in different hash codes.
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This would violate the contract of the <code>hashCode</code> method, in which case
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objects of type <code>Point</code> should not be stored in hashing data structures.</p>
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<sample src="HashedButNoHashBad.java" />
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<p>In the modification of the above example, the implementation of <code>hashCode</code> for class
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<code>Point</code> is suitable because the hash code is computed from exactly the same fields that are considered
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in the <code>equals</code> method. Therefore, the contract of the <code>hashCode</code> method is fulfilled.
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</p>
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<sample src="HashedButNoHash.java" />
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</example>
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<references>
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<li>
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J. Bloch, <em>Effective Java (second edition)</em>, Item 9. Addison-Wesley, 2008.
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</li>
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<li>
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Java API Specification:
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<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#equals(java.lang.Object)">Object.equals</a>,
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<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#hashCode()">Object.hashCode</a>.
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</li>
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<li>
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IBM developerWorks: <a href="https://www.ibm.com/developerworks/java/library/j-jtp05273/index.html">Java theory and practice: Hashing it out</a>.
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</li>
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</references>
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</qhelp>
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