Generics and The ArrayList Class

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• Generics and The ArrayList Class
• Lecture 13

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Part 1

• The ArrayList Class

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The ArrayList Class

• ArrayList is a class in the standard Java
  libraries.
• Unlike arrays, which have a fixed length once
  they have been created, an ArrayList is an object
  that can grow and shrink while your program is
  running.
• In general, an ArrayList serves the same purpose
  as an array, except that an ArrayList can change
  length while the program is running.

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ArrayLists Disadvantages

• An ArrayList is less efficient than an array.
• It does not have the convenient square bracket
  notation.
• The base type of an ArrayList must be a class
  type (or other reference type) it cannot be a
  primitive type (int, double, or char).

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Using the ArrayList Class

• In order to make use of the ArrayList class, it
  must first be imported from the package
  java.util.
• An ArrayList is created and named in the same
  way as object of any class, except that you
  specify the base type as follows
• ArrayListltBaseTypegt aList new
  ArrayListltBaseTypegt()
• An initial capacity can be specified when
  creating an ArrayList as well.
• e.g. The following code creates an ArrayList
  that stores objects of the base type String with
  an initial capacity of 20 items.
• ArrayListltStringgt list new ArrayListltStringgt(20)
  

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The ArrayList Methods

• The tools for manipulating arrays consist only
  of the square brackets and the instance variable
  length.
• ArrayLists, however, come with a selection of
  powerful methods that can do many of the things
  for which code would have to be written in order
  to do them using arrays.

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The ArrayList Methods
Example ArrayListltDoublegt list1 new
ArrayListltDoublegt(30) ArrayListltStringgt list2
new ArrayListltStringgt()
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The ArrayList Methods
Example ArrayListltStringgt list new
ArrayListltStringgt() list.add(Simab) list.add(
Shahid) list.add(0,Semester 081)
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The ArrayList Methods
Example ArrayListltStringgt list new
ArrayListltStringgt() int index 1
list.add(Hi) list.add(There) string S1
list.set(index, Hello) List.add(S1) String S2
list.get(index)
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Example 01

• ArrayListltintegergt list new ArrayListltintegergt()
  
• int index 2
• list.add(1) // will add at index 0
• list.add(2) // will add at index 1
• index 1
• integer i list.set(index, 7)
• System.out.println(i)
• Output
• 2
• The set() function of ArrayList class set the
  index with the given value and returned the value
  previously stored at that index

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Example 01

• ArrayListltintegergt list new ArrayListltintegergt()
  
• int index 2
• list.add(1) // will add at index 0
• list.add(2) // will add at index 1
• index 1
• integer i list.get(index)
• System.out.println(i)
• Output
• 2
• The get() function of ArrayList class returned
  the returned the value stored at that index

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The ArrayList Methods
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The ArrayList Methods
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Example

• ArrayListltIntegergt list new ArrayListltIntegergt
  (20)
• list.add(1)
• list.add(2)
• list.add(3)
• int index 1
• list.remove(index)
•   for(Integer i list)
• System.out.println(i " ")
• Output
• 1
• 3
• In this code the remove function has removed the
  value stored at index 1.

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Example

• ArrayListltIntegergt list new ArrayListltIntegergt
  (20)
• list.add(1)
• list.add(2)
• list.add(3)
• int index 1
• Integer i list.remove(index)
•  
• System.out.println(i " ")
• Output
• 2

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Example

• ArrayListltIntegergt list new ArrayListltIntegergt
  (20)
• list.add(1)
• list.add(2)
• list.add(3)
• list.clear()
• for(Integer i list)
• System.out.println(i " ")
• Output
• It will not print any thing because the clear()
  will delete all the elements of the list.

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The ArrayList Methods
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Example

• ArrayListltIntegergt list new ArrayListltIntegergt
  (20)
• list.add(1)
• list.add(2)
• list.add(3)
• int index
• index list.indexOf(2) System.out.println(
  Index of 2 is index)
• Output
• Index of 2 is 1

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Example

• ArrayListltIntegergt list new ArrayListltIntegergt
  (20)
• list.add(4)
• list.add(2)
• list.add(3)
• list.add(4)
• int index
• index list.lastIndexOf(4)
  System.out.println(Last Index of 4 is
  index)
• Output
• Index of 4 is 3

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The ArrayList Methods
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The ArrayList Methods
22
The ArrayList Methods
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Why are Some Parameters of Type Base_Type and
Others of type Object

• When looking at the methods available in the
  ArrayList class, there appears to be some
  inconsistency.
• In some cases, when a parameter is naturally an
  object of the base type, the parameter type is
  the base type.
• However, in other cases, it is the type Object.
• This is because the ArrayList class implements a
  number of interfaces, and inherits methods from
  various classes.
• These interfaces and classes specify that certain
  parameters have type Object.

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For-each Loop for ArrayList Objects

• The ArrayList class is an example of a collection
  class.
• Starting with version 5.0, Java has added a new
  kind of for loop called a for-each or enhanced
  for loop.
• This kind of loop has been designed to cycle
  through all the elements in a collection (like an
  ArrayList).
• Syntax
• for (Array_Base_Type Variable
  ArrayList_Object)
• Statement
• Example
• ArrayListltIntegergt list new
  ArrayListltIntegergt(10)
• for (Integer element list)
• element 20

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Use trimToSize to Save Memory

• An ArrayList automatically increases its capacity
  when needed.
• However, the capacity may increase beyond what a
  program requires.
• In addition, although an ArrayList grows
  automatically when needed, it does not shrink
  automatically.
• If an ArrayList has a large amount of excess
  capacity, an invocation of the method trimToSize
  will shrink the capacity of the ArrayList down to
  the size needed.

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Example (ArrayList class)

• import java.util.
• class ArrayListDemo
• public static void main(String args)
• ArrayListltStringgt list new ArrayListltStringgt
  (20)
• list.add("AB") list.add("CD")
  list.add("GH")
• list.add("IJ") list.add("KL")
  list.add("MN")
• list.add("QR") list.add("ST")
  list.add("WX")
• list.add("YZ")
• // Print the elements of the ArrayList
• System.out.println("The following is the
  initial ArrayList")
• for(String s list)
• System.out.print(s " ")
• // Add elements to the ArrayList
• list.add(2 , "ef") list.add(7 , "op")
  list.add(10 , "uv")
• // Print the new ArrayList
• System.out.println(" ")
• System.out.println("The following is the new
  ArrayList")

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Example (ArrayList class)

• // Print the element at position index1 in the
  ArrayList
• System.out.println(" ")
• int index111
• String element list.get(index1)
• System.out.println("The element at position
  " " " index1 " " "is" " " element)
• // Print the index of an element of the
  ArrayList
• System.out.println(" ")
• int index list.indexOf("CD")
• System.out.println("The element CD is at
  position " " index)

Output The following is the initial
ArrayList AB CD GH IJ KL MN QR ST WX YZ The
following is the new ArrayList AB CD ef GH IJ KL
MN op QR ST uv WX YZ The element at position
11 is WX The element CD is at position 1
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Parameterized Classes and Generics

• The class ArrayList is a parameterized class.
• It has a parameter, denoted by Base_Type, that
  can be replaced by any reference type to obtain a
  class for ArrayLists with the specified base
  type.
• Starting with version 5.0, Java allows class
  definitions with parameters for types.
• These classes that have type parameters are
  called parameterized class or generic
  definitions, or, simply, generics.