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Utilities: Making a Collection or Map unmodifiable


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There are a number of other useful utilities in the Collections class:


Produces an old-style Enumeration for the argument.



Produces the maximum or minimum element in the argument using the natural comparison method of the objects in the Collection.

max(Collection, Comparator)

min(Collection, Comparator)

Produces the maximum or minimum element in the Collection using the Comparator.

nCopies(int n, Object o)

Returns an immutable List of size n whose handles all point to o.

subList(List, int min, int max)

Returns a new List backed by the specified argument List that is a window into that argument with indexes starting at min and stopping just before max.

Note that min( ) and max( ) work with Collection objects, not with Lists, so you don’t need to worry about whether the Collection should be sorted or not. (As mentioned earlier, you do need to sort( ) a List or an array before performing a binarySearch( ).)

Making a Collection or Map unmodifiable

Often it is convenient to create a read-only version of a Collection or Map. The Collections class allows you to do this by passing the original container into a method that hands back a read-only version. There are four variations on this method, one each for Collection (if you don’t want to treat a Collection as a more specific type), List, Set, and Map. This example shows the proper way to build read-only versions of each:


// Using the Collections.unmodifiable methods

package c08.newcollections;

import java.util.*;

public class ReadOnly

} ///:~

In each case, you must fill the container with meaningful data before you make it read-only. Once it is loaded, the best approach is to replace the existing handle with the handle that is produced by the “unmodifiable” call. That way, you don’t run the risk of accidentally changing the contents once you’ve made it unmodifiable. On the other hand, this tool also allows you to keep a modifiable container as private within a class and to return a read-only handle to that container from a method call. So you can change it from within the class but everyone else can only read it.

Calling the “unmodifiable” method for a particular type does not cause compile-time checking, but once the transformation has occurred, any calls to methods that modify the contents of a particular container will produce an UnsupportedOperationException.

Synchronizing a Collection or Map

The synchronized keyword is an important part of the subject of multithreading, a more complicated topic that will not be introduced until Chapter 14. Here, I shall note only that the Collections class contains a way to automatically synchronize an entire container. The syntax is similar to the “unmodifiable” methods:


// Using the Collections.synchronized methods

package c08.newcollections;

import java.util.*;

public class Synchronization

} ///:~

In this case, you immediately pass the new container through the appropriate “synchronized” method; that way there’s no chance of accidentally exposing the unsynchronized version.

The new collections also have a mechanism to prevent more than one process from modifying the contents of a container. The problem occurs if you’re iterating through a container and some other process steps in and inserts, removes, or changes an object in that container. Maybe you’ve already passed that object, maybe it’s ahead of you, maybe the size of the container shrinks after you call size( ) – there are many scenarios for disaster. The new collections library incorporates a fail fast mechanism that looks for any changes to the container other than the ones your process is personally responsible for. If it detects that someone else is modifying the container, it immediately produces a ConcurrentModificationException. This is the “fail-fast” aspect – it doesn’t try to detect a problem later on using a more complex algorithm.

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