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In Java, any variable that refers to an Object is a reference variable. The variable stores the memory address of the actual Object.

String x = new String("Chuck"); String y = x; x and y store the same memory address.

"Chuck"

x y 0x2B3 0x2B3 0x2B3

String x = "Chuck"; String y = "Chuck"; x and y store the same memory address.

"Chuck"

x y 0x2B7 0x2B7 0x2B7

"Chuck" "Chuck"

String x = new String("Chuck"); String y = new String("Chuck"); x and y store different memory addresses.

x y 0x2B7

String x = "Chuck"; String y = "Chuck";

"Chuck"

x y 0x2B7 0x2B7 0x2B7

x = null;

Collection, List, and Set all have methods that return iterators. Iterators allow you to go from item to item through a collection. Map does not have an iterator, but it does have a keySet() method that returns a Set of all keys. You can get an iterator from the Set.

An Iterator provides a standard way to access all of the references stored in a collection. For some Collections, TreeMap and HashSet for instance, the underlying data structures are not sequentially organized like an array. For example, a tree has nodes all over the place.

By using the Iterator, the references from a Collection can be accessed in a more standard sequential-like manner without having to manipulate the underlying Collection data structure.

1 2 3 4

Iterator frequently used methods

Name Use

next() returns a reference to the next item

remove() removes the last ref returned by next

hasNext() checks to see there are more items

import java.util.Iterator;

ArrayList<String> words; words = new ArrayList<String>(); words.add("at"); words.add("is"); words.add("of"); words.add("us"); Iterator<String> it = words.iterator(); System.out.println(it.next());

OUTPUTat

at is of us

Iterator it = list.iterator();

method next() { oldRef = currRef currRef = next ref in the collection return oldRef }

it it.next();

next moves the iterator up one spot and returns a reference to the 1st item.

at is of us

ArrayList<String> words; words = new ArrayList<String>(); words.add("at"); words.add("is"); words.add("of"); words.add("us"); Iterator<String> it = words.iterator(); System.out.println(it.next()); System.out.println(it.next()); System.out.println(it.next()); System.out.println(it.next());

OUTPUT

at is of us

ArrayList<String> words; words = new ArrayList<String>(); words.add("at"); words.add("is"); words.add("of"); words.add("us"); Iterator<String> it = words.iterator(); while(it.hasNext()) { System.out.println(it.next()); }

OUTPUTat is of us

ArrayList<String> words; words = new ArrayList<String>(); words.add("at"); words.add("is"); words.add("of"); Iterator<String> it = words.iterator(); System.out.println(it.next()); it.remove(); System.out.println(it.next()); System.out.println(words);

OUTPUT

at is [is, of]

at is of us

Iterator it = list.iterator();

it it.next();

at is of us

it it.remove();

remove always modifies the last reference returned by next.

at is of us

ArrayList<String> words; words = new ArrayList<String>(); words.add("at"); words.add("is"); words.add("of"); Iterator<String> it = words.iterator(); System.out.println(it.next()); it.remove(); it.remove();

OUTPUT

at error

at is of us

it.next();

at is of us

it.remove();

remove always modifies the last reference returned by next.

is of us at

it.remove();

remove call blows up because there was no call to next; thus, there was no reference to modify.

is of us

ListIterator frequently used methods

Name Use

next() returns a reference to the next item

remove() removes the last ref returned by next or previous

hasNext() checks to see there are more items

add() adds in a new item

set() sets the last ref returned by next or previous

previous() goes back and returns a ref to prev item

import java.util.ListIterator;

ArrayList<String> words; words = new ArrayList<String>(); words.add("at"); words.add("is"); words.add("of"); words.add("us"); ListIterator<String> it = words.listIterator(); System.out.println(it.next()); System.out.println(it.next());

OUTPUTat is

ArrayList<String> words; words = new ArrayList<String>(); words.add("at"); words.add("is"); words.add("of"); words.add("us"); ListIterator<String> it = words.listIterator(); System.out.println(it.next()); System.out.println(it.next()); System.out.println(it.previous());

OUTPUTat is is

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ArrayList<String> words; words = new ArrayList<String>(); words.add("at"); words.add("up"); words.add("or"); ListIterator<String> it = words.listIterator(); it.next(); it.next(); it.next(); System.out.println(it.previous()); System.out.println(it.previous()); it.set("33"); System.out.println(words);

OUTPUT or up [at, 33, or]

ArrayList<String> words; words = new ArrayList<String>(); words.add("at"); words.add("is"); words.add("us"); ListIterator<String> it = words.listIterator(); System.out.println(it.next()); it.set("###"); System.out.println(it.next()); System.out.println(words);

OUTPUT at is [###, is, us]

it.next();

it.set("###");

set always modifies the last reference returned by next or previous.

it.next();

next moves the iterator up one spot and returns a reference to the 2nd item.

ArrayList<String> words; words = new ArrayList<String>(); words.add("is"); words.add("us"); ListIterator<String> it = words.listIterator(); it.add("##"); System.out.println(it.next()); System.out.println(it.next()); System.out.println(it.previous()); it.set("##"); System.out.println(words);

OUTPUT is us us [##, is, ##]

it.add("##");

add always adds the new item in front of the current spot.

it.next();

next moves the iterator up one spot and returns a reference to the 2nd item.

it.next();

next moves the iterator up one spot and returns a reference to the 3rd item.

it.previous();

previous backs the iterator up one spot and returns a reference to the 3rd item.

it.set("##");

set always modifies the last reference returned by next or previous.

Modifications through an Iterator or ListIterator are always applied to the reference returned by the last next or previous call. Pay attention to the direction you are going. Iterator only goes one direction. ListIterator can go either direction.

int[] array = {4,5,6,7}; int sum = 0; for(int i=0; i<array.length; i++) { sum += array[i]; }

int array[] = {4,9,6,2,3}; int sum = 0; for (int num : array) sum = sum + num; System.out.println(sum);

ArrayList<Integer> list; list = new ArrayList<Integer>(); list.add(3); list.add(9); for (Integer num : list) System.out.print(num + " ");

ArrayList<Integer> list; list = new ArrayList<Integer>(); list.add(3); list.add(9); for (int num : list) System.out.print(num + " ");

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