Important library classes - Objects First with Java: A Practical Introduction Using BlueJ - page 536

K.2 The java.util package

The java.util package is a relatively incoherent collection of useful classes and interfaces.

package java.util-Summary of the most important classes and interfaces

This interface provides the core set of methods for most of the collection-based classes defined in the
java.util package, such as ArrayList , HashSet , and LinkedList . It defines signatures for
the add , clear , iterator , remove , and size methods.

interface Collection

interface Iterator

Iterator defines a simple and consistent interface for iterating over the contents of a collection. Its
three methods are hasNext , next , and remove .

interface List List is an extension of the Collection interface and provides a means to impose a sequence

on the selection. As such, many of its methods take an index parameter: for instance, add , get ,
remove , and set . Classes such as ArrayList and LinkedList implement List .

interface Map The Map interface offers an alternative to list-based collections by supporting the idea of associat-

ing each object in a collection with a key value. Objects are added and retrieved via its put and get
methods. Note that a Map does not return an Iterator , but its keySet method returns a Set of
the keys, and its values method returns a Collection of the objects in the map.

interface Set Set extends the Collection interface with the intention of mandating that a collection contain no

duplicate elements. It is worth pointing out that, because it is an interface, Set has no actual impli-
cation to enforce this restriction. This means that Set is actually provided as a marker interface to
enable collection implementers to indicate that their classes fulfill this particular restriction.

class ArrayList ArrayList is an implementation of the List interface that uses an array to provide efficient direct

access via integer indices to the objects it stores. If objects are added or removed from anywhere
other than the last position in the list, then following items have to be moved to make space or close
the gap. Key methods are add , get , iterator , remove , and size .

class Collections Collections contains many useful static methods for manipulating collections. Key methods are

binarySearch , fill , and sort .

class HashMap HashMap is an implementation of the Map interface. Key methods are get , put , remove , and

size . Iteration over a HashMap is usually a two-stage process: obtain the set of keys via its keySet
method, and then iterate over the keys.

class HashSet HashSet is a hash-based implementation of the Set interface. It is closer in usage to a Collection

than to a HashMap . Key methods are add , remove , and size .

class LinkedList LinkedList is an implementation of the List interface that uses an internal linked structure to store

objects. Direct access to the ends of the list is efficient, but access to individual objects via an index
is less efficient than with an ArrayList . On the other hand, adding objects or removing them from
within the list requires no shifting of existing objects. Key methods are add , getFirst , getLast ,
iterator , removeFirst , removeLast , and size .

class Random The Random class supports generation of pseudo-random values—typically, random numbers. The

sequence of numbers generated is determined by a seed value , which may be passed to a con-
structor or set via a call to setSeed . Two Random objects starting from the same seed will return
the same sequence of values to identical calls. Key methods are nextBoolean , nextDouble ,
nextInt , and setSeed .

class Scanner The Scanner class provides a way to read and parse input. It is often used to read input from the

keyboard. Key methods are next and hasNext .