Arrays, ArrayLists, and Collections

Rationale Suppose we have a program to compute the average of three numbers. We could write a simple little method to compute the average given the three numbers passed in.

/** * Suppose we want to compute the average of a set of numbers. We might be able to justify not * using arrays when we only need to compute the average for a couple of values, but when the * number of values we need to compute the average for gets larger the harder it is to compute * work with individual variables for each....hence the need for arrays. **/ public class RationaleForArrays { /** * Where we could do this for simple cases like computing the average for three numbers. **/ public double computeAverage(double n0, double n1, double n2) { return (n0 + n1 + n2)/3.0; } public static void main(String argv[]) { RationaleForArrays rationale = new RationaleForArrays(); /* * Although we could justify explicitly individual values when we are * dealing with a small number of variables... */ double number0 = 0.00 * 1.001; double number1 = 1.00 * 1.001; double number2 = 2.00 * 1.001; System.out.printf("Average for %3d numbers : %8.4f -- without array\n", 3, rationale.computeAverage(number0, number1, number2)); } OUTPUT: Average for 3 numbers : without array

What if it was more than 3 numbers … What if it was 100 numbers … or a 1,000 or 10,000 or even a million.. What if you did not know the number of numbers you need to compute the average of when you were writing your routine? We need the ability to get/set a list of these numbers … sounding like arrays

Java Arrays Arrays are a structured way to store multiple homogenous data in a single variable Index through the array starting with 0 to length-1 If you try to access outside the array’s valid range you get an ArrayIndexOutOfBoundsException Must be declared up front with some predetermined size and type Can be single or multidimensional –single dimensional are a series of values (think of a column of values) –multiple dimensional are implemented as an array of arrays (think of rows of columns) Can store primitives or object type data

Working with Arrays Declaring // single dimensional double score[];// same as double[] score; Declaring and Creating an Array doublescore[] = new double[100]; doublematrix[][] = new double[10][10]; Declaring and Initializing an Array double score[] = { 98.0, 87.5, }; double matrix[][] = { { 10.0, 20.0 }, { 30.0, 40.0 }, };

Some Examples of Arrays public void examplesOfArrays() { double score[] = { 98.0, 87.5, }; double matrix[][] = { { 10.0, 20.0 }, { 30.0, 40.0 }, }; System.out.println("\nSingle dimenstional array of scores:"); for(int i = 0; i < score.length; ++i) System.out.printf("Score %2d = %4.2f\n", i, score[i]); System.out.println("\n\nMatrix:"); for(int row = 0; row < matrix.length; ++row) { for(int col = 0; col < matrix[0].length; ++col) { System.out.printf(" %4.2f", matrix[row][col]); } System.out.println(); } OUTPUT: Single dimensional array of scores: Score 0 = Score 1 = Score 2 = Matrix:

So lets go back and look at that computing the average now with arrays …

/** * Suppose we want to compute the average of a set of numbers. We might be able to justify not * using arrays when we only need to compute the average for a couple of values, but when the * number of values we need to compute the average for gets larger the harder it is to compute * work with individual variables for each....hence the need for arrays. **/ public class RationaleForArrays { /** * As the number of values increase or non-deterministic at compile time. **/ public double computeAverage(double number[]) { double sum = 0.0; for(int i = 0; i < number.length; ++i) sum += number[i]; return sum/number.length; } public static void main(String argv[]) { RationaleForArrays rationale = new RationaleForArrays(); double number[] = null; number = new double[3]; for(int i = 0; i < 3; ++i) number[i] = i * 1.001; System.out.printf("Average for %3d numbers : %8.4f -- with array\n", number.length, rationale.computeAverage(number)); number = new double[100]; for(int i = 0; i < 100; ++i) number[i] = i * 1.001; System.out.printf("Average for %3d numbers : %8.4f -- with array\n", number.length, rationale.computeAverage(number)); /* * Did you notice that the code did not change when we changed * from 3 numbers to :-) */ } OUTPUT: Average for 3 numbers : with array Average for 100 numbers : with array

What else should we know about arrays? public void moreExamplesOfArrays() { String sa1[] = { "zero", "one", "two", "three", "four", "five" }; String sa2[] = { "0", "1", "2", "3", "4", "5" }; String sa3[] = { "d", "b", "f", "a", “c" }; /* indexing array elements */ System.out.printf("\n\nOriginal Strings\n"); for(int i = 0; i < sa1.length; ++i) System.out.printf("\t%s", sa1[i]); System.out.println(); /* copying array elements * System.arraycopy(src, srcPos, dest, destPos, length) */ System.arraycopy(sa2, 0, sa1, 0, 2); System.out.printf("After the copy \n"); for(int i = 0; i < sa1.length; ++i) System.out.printf("\t%s", sa1[i]); System.out.println(); /* sorting array elements */ System.out.printf("Before the sort \n"); for(int i = 0; i < sa3.length; ++i) System.out.printf("\t%s", sa3[i]); System.out.println(); Arrays.sort(sa3); System.out.printf("After the sort \n"); for(int i = 0; i < sa3.length; ++i) System.out.printf("\t%s", sa3[i]); System.out.println(); } OUTPUT: from arraycopy(…) Original Strings zero one two three four five After the copy 0 1 two three four five This is the same as setting the values individually in the code. Hence, we get no performance enhancement for doing this, just convince. OUTPUT: from Arrays.sort(…) Before the sort d b f a c After the sort a b c d f

Given what we know now … Can you think of how we could implement dynamic arrays?

Instead of us having to doing worrying about dynamically resizing, we can use an ArrayList

ArrayList Comparing ArrayList to Java arrays: –similar: a sequence of objects each object is index from 0 to size -1 –different: only can contain objects (no primitives) size() instead of length array[i] = value  array.set(i, value) can grow and shrink in size

Important ArrayList Methods Constructors –ArrayList(Collection) –ArrayList(initialSize) Methods –add(index, object)add an element to the list at a given location –add(object)add a given element to the list –addAll(Collection)add all the elements to the list –clear()remove all the elements –get(index)get the object at a specified index –remove(index)remove the element at a specified index –set(index, object)set a given object at a given index –size()number of elements in the list –toArray()convert to an array –trimToSize()trim to the current size

ArrayList Examples public void examplesOfArrayList() { ArrayList sal1 = new ArrayList (3); sal1.add("d"); sal1.add("b"); sal1.add("a"); /* indexing array elements */ System.out.printf("\n\nOriginal Strings\n"); for(int i = 0; i < sal1.size(); ++i) System.out.printf("\t%s", sal1.get(i)); System.out.println(); } OUTPUT: Original Strings d b a

ArrayList is a Collection As a result the Collections utility class can be used to help out. –binary search for an element –copy a list to another list –enumerate over a collection –fill a list with a given object –reverse –rotate –shuffle –sort –swap to elements in a list

Collections Examples public void examplesOfArrayList() { ArrayList sal1 = new ArrayList (3); sal1.add("d"); sal1.add("b"); sal1.add("a"); /* indexing array elements */ System.out.printf("\n\nOriginal Strings\n"); for(int i = 0; i < sal1.size(); ++i) System.out.printf("\t%s", sal1.get(i)); System.out.println(); Collections.sort(sal1); /* use of Collections utility */ System.out.printf("\n\nAfter the sort\n"); for(int i = 0; i < sal1.size(); ++i) System.out.printf("\t%s", sal1.get(i)); System.out.println(); Collections.reverse(sal1); /* use of Collections utility */ System.out.printf("\n\nAfter the reverse\n"); for(int i = 0; i < sal1.size(); ++i) System.out.printf("\t%s", sal1.get(i)); System.out.println(); } OUTPUT: Original Strings d b a After the sort a b d After the reverse d b a

public void examplesOfArrayList() { ArrayList sal1 = new ArrayList (3); sal1.add("d"); sal1.add("b"); sal1.add("a"); /* indexing array elements */ System.out.printf("\n\nOriginal Strings\n"); for(int i = 0; i < sal1.size(); ++i) System.out.printf("\t%s", sal1.get(i)); System.out.println(); Collections.sort(sal1); /* use of Collections utility */ System.out.printf("\n\nAfter the sort\n"); for(int i = 0; i < sal1.size(); ++i) System.out.printf("\t%s", sal1.get(i)); System.out.println(); Collections.reverse(sal1); /* use of Collections utility */ System.out.printf("\n\nAfter the reverse\n"); for(int i = 0; i < sal1.size(); ++i) System.out.printf("\t%s", sal1.get(i)); System.out.println(); Collections.rotate(sal1, 1); /* use of Collections utility */ System.out.printf("\n\nAfter the rotate\n"); for(int i = 0; i < sal1.size(); ++i) System.out.printf("\t%s", sal1.get(i)); System.out.println(); Collections.shuffle(sal1); /* use of Collections utility */ System.out.printf("\n\nAfter the shuffle\n"); for(int i = 0; i < sal1.size(); ++i) System.out.printf("\t%s", sal1.get(i)); System.out.println(); Collections.shuffle(sal1); /* use of Collections utility */ System.out.printf("\n\nAfter the shuffle\n"); for(int i = 0; i < sal1.size(); ++i) System.out.printf("\t%s", sal1.get(i)); System.out.println(); } OUTPUT: Original Strings d b a After the sort a b d After the reverse d b a After the rotate a d b After the shuffle b d a After the shuffle a d b

The Collections Framework Collection Interfaces –Set A collection of not duplicating elements Only contains methods exposed in Collection but adds uniqueness to the elements. Implementations: HashSet, TreeSet, LinkedHashSet –List A collection of elements (can contain duplicates) Additions to Collection: positional access, search, iteration, range view Implementations: Vector (retrofitted), ArrayList, LinkedList –Queue A collection of elements that follow a FIFO pattern Implementations: LinkedList, PriorityQueue –Map A collection of name-value pairs. Names can not be duplicated. Implementations: HashMap, TreeMap, LinkedHashMap Object Ordering –SortedSet – Iterator & toArray return the elements in “sorted” order –SortedMap - Iterator & toArray return keys, values, or entries in “sorted” order

The Collections Framework Hash Table Resizable Array Balanced Tree Linked List Hash Table + Linked List SetHashSetTreeSetLinkedHashSet ListArrayListLinkedList MapHashMapTreeMapLinkedHashMap