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The {{Javadoc:SE|java/util|Map|get(java.lang.Object)|name=get}} method is used to access a key; for example, the value of the expression <code>phoneBook.get("Sally Smart")</code> is <code>"555-9999"</code>. This code uses a hash map to store the associative array, by calling the constructor of the {{Javadoc:SE|java/util|HashMap}} class. However, since the code only uses methods common to the interface {{Javadoc:SE|java/util|Map}}, a self-balancing binary tree could be used by calling the constructor of the {{Javadoc:SE|java/util|TreeMap}} class (which implements the subinterface {{Javadoc:SE|java/util|SortedMap}}), without changing the definition of the <code>phoneBook</code> variable, or the rest of the code, or using other underlying data structures that implement the <code>Map</code> interface.
The hash function in Java, used by HashMap and HashSet, is provided by the method {{Javadoc:SE|java/lang|Object|hashCode()}} method. Since every class in Java [[Inheritance (computer science)|inherits]] from {{Javadoc:SE|java/lang|Object}}, every object has a hash function. A class can [[Method overriding (programming)|override]] the default implementation of <code>hashCode()</code> to provide a custom hash function basedmore in accordance onwith the properties of the object.
The <code>Object</code> class also contains the method {{Javadoc:SE|name=equals(Object)|java/lang|Object|equals(java.lang.Object)}} thatmethod, which tests thean object for equality with another object. Hashed data structures in Java rely on objects maintaining the following contract between their <code>hashCode()</code> and <code>equals()</code> methods:
For two objects ''a'' and ''b'',
<syntaxhighlight lang=Java>
a.equals(b) == b.equals(a)
</syntaxhighlight>
In order to maintain this contract, a class that overrides <code>equals()</code> must also override <code>hashCode()</code>, and maybe vice versa, so that <code>hashCode()</code> is based on the same properties (or a subset of the properties) as <code>equals()</code>.
A further contract that a hashed data structuresstructure has with the object is that the results of the <code>hashCode()</code> and <code>equals()</code> methods will not change once the object has been inserted into the map. For this reason, it is generally a good practice to base the hash function on [[Immutable object|immutable]] properties of the object.
Analogously, TreeMap, and other sorted data structures, requiresrequire that an ordering be defined on the data type. Either the data type must already have defined its own ordering, by implementing the {{Javadoc:SE|java/lang|Comparable}} interface; or a custom {{Javadoc:SE|java/util|Comparator}} must be provided at the time the map is constructed. As with HashMap above, the relative ordering of keys in a TreeMap should not change once they have been inserted into the map.
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