Java Collections Generics

Java Collections and Generics java.util package By Waqas

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Java Collection Framework

Comparable Interface

Collection Framework Elements

Arrays Class

Collection Framework Interfaces

Comparator Interface

Collection Interface

Collections Class

Set Interface

Generics in Java

List Interface

Examples of Generic Errors

SortedSet Interface

Generic ArrayList

Map Interface

Generic TreeSet

SortedMap Interface

Generic TreeMap

Collection Framework Implementations

Generic Iterator

ArrayList Class LinkedList Class HastSet Class TreeSet Class HashMap Class TreeMap Class

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Java Collection Framework A Collection (sometimes called a Container) is simply an object that groups multiple objects into a single group.

The java collection framework standardizes the way in which groups of objects are handled by your programs by providing a unified architecture called “Collection Framework” By Waqas

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The Collection Framework is designed to meet several goals.

First, the framework provides high performance, fast and highly efficient way to work with groups of objects.

Second, the framework allows different types of collections to work in a similar manner with high By Waqas

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Collection Framework Elements The Collection Framework consists of elements: Interfaces, Implementation Algorithms.

three and

Interfaces Abstract data types to manipulate collections independent of implementation Collection, Set, List, Map etc. By Waqas

details.

e.g. 5

Implementations Concrete

classes

which

are

implementing

Collection interfaces, used as reusable data structures. e.g. HashMap, ArrayList, TreeSet, HashSet etc.

Algorithms Methods to perform operations on collections By Waqas such as sorting, searching, iterations.

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Collection Framework Interfaces

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Collection

Set

SortedSet

Map

List

SortedMap

Collection Framework

Interfaces By Waqas

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Collection Interface A Collection interface is the super interface in the collection interfaces hierarchy. A collection represents a group of objects, known as its elements. Java does not provide any direct implementation of this interface. It provides implementations of more specific subinterfaces like Set and List. By Waqas

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Set Interface A Set is a Collection that does not contain duplicate elements, so it’s a collection of unique elements. If you add duplicate element in Set then add( ) method simply ignore the element and return false. The elements in Set are not in order. The set interface does not define any additional methods, it inherits all the methods of Collection interface. By Waqas

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List Interface The List interface also extends Collection interface and declares the behavior of a collection that stores a sequence of objects in order. Elements can be inserted or retrieved by their position in the list, using a zero based index. List can accept duplicate values.

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SortedSet Interface The SortedSet interface extends Set and declares the behavior of a set in which objects are sorted in either their natural order or the order you specified in your custom object. SortedSet does not accept duplicate elements.

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Map Interface A Map is an object that maps keys to values. A map cannot contain duplicate keys but can contain duplicate values. Each key can map to at most one value. We can retrieve value by using its key. Elements are not orderedBy Waqas

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SortedMap Interface The SortedMap interface extends Map and declares the behavior of a map in which objects are sorted by their keys in their natural order or custom order.

SortedMap elements can not contain duplicate keys.

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Difference between Interfaces Interface

Duplicates

Order

Sorting

Set

Not Allowed

No Order

No Sorting

List

Allowed

Order by Index

No Sorting

Map

Not Allowed for Keys

No Order

No Sorting

Sorted Set

Not Allowed

By Natural Order

By Natural Order

Sorted Map

Not Allowed

By Natural Order

By Natural Order

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Collection Framework Implementations

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Set

SortedSet

HashSet

TreeSet

List

ArrayList

LinkedList Vector

Map

HashMap

SortedMap

Collection Framework Implementations

TreeMap By Waqas

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ArrayList Class ArrayList class implements the List interface. ArrayList is a dynamic array that can grow and shrink automatically as needed. Insertions and deletions are linear that’s why these operations are slow in ArrayList Searching is fast in ArrayList because Java performs search randomly. By Waqas

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LinkedList Class LinkedList class implements the List interface. Every element in LinkedList contains the data item and the pointer to the next node. Insertions and deletions are fast because they are linear in time. Searching is slow as java search LinkedList sequentially not randomly. By Waqas

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HashSet Class HastSet provides an implementation of Set

interface. Objects are not stored in order that’s why searching objects is very fast.

HashSet does not support duplicate elements. By Waqas

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TreeSet Class TreeSet provides an implementation of SortedSet interface. Objects are stored in sorted, ascending order. Access and retrieval times is not as fast as HashSet. TreeSet is an excellent choice when you want to store large amounts of sorted data items. By Waqas

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HashMap Class HashMap provides an implementation of Map interface. Objects are stored as key-value pairs. null objects are supported by the HashMap. Objects are not stored in order.

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TreeMap Class TreeMap class implements SortedMap interface. It provides an efficient means of storing key-value pairs in sorted order based on their keys.

As objects are sorted so random access or searching is little slower then HashMap. By Waqas

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Comparable Interface Many java collection framework classes such as TreeMap, TreeSet perform automatic sorting of objects when they added in the collection. For sorting objects they must be comparable. Java provides an interface to make two objects comparable. Custom classes should implement comparable interface to provide logic for class specific sorting By Waqas

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Comparable Interface class Student implements Comparable { int id; public int compareTo(Object obj) { Student s2 = (Student) obj; Integer st1 = new Integer(this.id); Integer st2 = new Integer(s2.id); return st1.compareTo(st2); By Waqas

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Arrays Class Arrays class contains manipulating arrays.

various

methods

for

The two most common methods are sorting and searching. To sort array pass the array in the sort method. This method sort all elements in their natural order. To search elements inside array use binarySearch 26 By Waqas method.

Comparator Interface Comparator interface is used to create objects which can be passed to Arrays.sort or Collection.sort methods or collections such as TreeMap and TreeSet. They are used to sort custom objects in collections. They are not needed for arrays and collections of primitive data types and for objects that have a natural sorting order such as String, Integer. By Waqas

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Comparator Interface class StudentComparator implements Comparator { public int compare(Object obj1, Object obj2) { Student s1 = (Student) obj1; Student s2 = (Student) obj2; Integer st1 = new Integer(s1.id); Integer st2 = new Integer(s2.id); By Waqas

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Comparator Interface Student students[] = new Student[5]; students[0] students[1] students[2] students[3] students[4]

= = = = =

new new new new new

Student(5, Student(2, Student(1, Student(4, Student(3,

By Waqasnew Arrays.sort(students,

"Simon"); "James"); "Peter"); "David"); "John");

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Collections Class Collections class contains various methods for manipulating collections.

sort( List ); binarySearch( List, Object ); min(List); max(List); replaceAll(List, Object, Object); reverse(List); shuffle(List); swap(List, int, int); By Waqas

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Java Generics

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Example of Generic Errors When we retrieve an element from a collection, we need to cast it to the right type otherwise compile time error occur. ArrayList list = new ArrayList(); String input = “London”; list.add(input); String output = list.get(0);

// Compiler Error

String output = (String) list.get(0); // Valid By Waqas

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Example of Generic Errors There is no way to ensure that we are casting to a correct type. Following code will compile but it will throw exception at runtime. ArrayList list = new ArrayList(); String input = “London”; list.add(input);

Integer output = list.get(0); // Runtime Error

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Java Generics Generics in Java allow us to create collections with a strong type. This means that if we are creating a collection to store Strings we will be forced to store and retrieve only Strings at compile time and runtime. Overall result of using generics in java collections is improved reliability, readability and performance. By Waqas

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Generic ArrayList ArrayList<E> list = new ArrayList<E>(); ArrayList<String> list = new ArrayList<String>(); list.add(“Simon”); list.add(“Peter”); list.add(new Integer(2));

// Compiler Error

String s1 = list.get(0); String s2 = list.get(1); Integer I = list.get(0);

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// Compiler Error

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Generic TreeSet TreeSet<E> set = new TreeSet<E>();

>();TreeSet set = new TreeSet
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Generic TreeMap TreeMap map = new TreeMap();

>();TreeMap map = new TreeMap
”); map.put(new Integer(2), “Simon map.put(new Integer(3), “Peter”); map.put(“1”, “Simon”); // Compiler Error map.put(“1”, new Integer(3)); // Compiler Error

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Generic Iterator Iterator<E> it = set.iterator(); >();TreeSet<String> set = new TreeSet<String set.add(“Simon”); set.add(“Peter”); Iterator<String> it = set.iterator(); String s; while(it.hasNext()) { s = it.next(); }

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