CSE 116/504 – Intro. To Computer Science for Majors II Lecture 9: Arrays

Announcements Grades posted to UBLearns on regular basis May take few days, but allows you to track progress Lowest 3 homeworks dropped to calculate final grade Will also see lowest quiz grade excluded in calculations Homework for week due Mondays at 12:45PM Problems NOT optional If it takes longer than 15 minutes, get help ASAP

Inheritance Example public class SuperClass { public String getMyString() { return "SUPER"; } } public class SubClass extends SuperClass { public String getMyString() { return "sub " + super.getMyString(); } public String otherMethod() { return "other"; } public static void main(String[] args) { SubClass sub2, sub = new SubClass(); SuperClass super1 = new SuperClass(); System.out.println(sub.getMyString()); System.out.println(super1.getMyString()); System.out.println(sub.otherMethod()); super1 = sub; System.out.println(super1.getMyString()); SubClass sub2 = (SubClass)super1;

Declaring Array Variables Like all variables, must declare before use Type, brackets, & name declare array variable Still variables, so names follow usual rules Variable is array of the type, so use any legal type Each of the array's entries hold value of that type String[] bob; long[] johnSilver; VWCar[] errorProne;

Object[] parkingLot = new Car[17]; int[] bob; bob = new int[intVar]; Instantiating Arrays Cannot use until after instantiating array Type & length of array needed for instantiation Object[] parkingLot = new Car[17]; int[] bob; bob = new int[intVar]; Type is for elements & must be assignable to array’s type Size must be int since ½ a value hard to use

Object[] parkingLot = new Car[17]; int[] bob; bob = new int[intVar]; Instantiating Arrays Cannot use until after instantiating array Type & length of array needed for instantiation Object[] parkingLot = new Car[17]; int[] bob; bob = new int[intVar]; Type is for elements & must be assignable to array’s type Size must be int since ½ a value hard to use

Array Basics #1 Must specify size when allocated: Cannot grow array Cannot shrink array

Which Declaration is NOT Legal? int[] iBob = new int[23]; double[] dBob; Float fBob[17]; ArrayList[] alBob; int i=7; long[] lBob = new long[i]; 27

Which Declaration is NOT Legal? int[] iBob = new int[23]; double[] dBob; Float fBob[17]; ArrayList[] alBob; int i=7; long[] lBob = new long[i]; Convince your neighbor your answer is correct

Which Declaration is NOT Legal? int[] iBob = new int[23]; double[] dBob; Float fBob[17]; ArrayList[] alBob; int i=7; long[] lBob = new long[i]; 4

Which Declaration is NOT Legal? int[] iBob = new int[23]; double[] dBob; Float fBob[17]; ArrayList[] alBob; int i=7; long[] lBob = new long[i]; Need to use new to create array instances.

Which Declaration is NOT Legal? int[] iBob = new int[23]; double[] dBob; Float fBob[17]; Float[] fBob = new Float[17]; ArrayList[] alBob; int i=7; long[] lBob = new long[i]; Need to use new to create array instances.

Assigning Array Variables Can be set to null to mark there is no array Can be set to newly instantiated array Alias array by assigning to another array variable Does not copy array, but has both refer to same object Change to entry seen by both variables, since just aliases TV image is public domain: http://www.publicdomainpictures.net/pictures/70000/velka/tv-isolated-background-clipart.jpg Remote images by Dwight Burdette and used under Creative Commons 3.0 License. Original is at: https://commons.wikimedia.org/wiki/File:Philips_SRC206WM_Universal_Remote_Control.JPG

Array Key Concept #2 Array variable is reference variable target either array instance or null

Memory Trace Example int[] data = new int[3]; int[] al = data; for (int i = 0; i < data.length; i++) { data[i] = i * 2; } al[0] = 5; al = new int[1]; al[0] = 7;

Array Key Concept #3 Allocating array of object type DOES NOT allocate other objects for its entries

Entries Are Variables, Too! Within an array entries like individual variables int[] bob = new int[30]; Car[] parkingLot = new Car[300]; bob’s entries would be primitive variables parkingLot’s entries refer to instances Entries initialized like variable when array created bob’s entries would all be set to 0 null stored in all entries in parkingLot

Array entries act like fields Should initialize before using Array Key Concept #4 Array entries act like fields Should initialize before using

Entries Are Variables, Too! Object[] data = new Object[3]; Object[] al = data; for (int i = 0; i < data.length; i++) { data[i] = new Object(); } al[0] = new Object(); al = new Object[1]; al[0] = data[0]; data[0] = data[1];

Which is Correct Java Code? int[] a=new int(12); a[2] = 12; char[] b=new char[5]; b[] = “1234”; int[] c=new int[2]; c[2] = 12; int[] d=new int[20]; d(10) = 2.0; int s = 45; int[] d=new int[s]; d[s-4] = 32453; 26

Which is Correct Java Code? int[] a=new int(12); a[2] = 12; char[] b=new char[5]; b[] = “1234”; int[] c=new int[2]; c[2] = 12; int[] d=new int[20]; d(10) = 2.0; int s = 45; int[] d=new int[s]; d[s-4] = 32453; Convince your neighbor your answer is correct

Which is Correct Java Code? int[] a=new int(12); a[2] = 12; char[] b=new char[5]; b[] = “1234”; int[] c=new int[2]; c[2] = 12; int[] d=new int[20]; d(10) = 2.0; int s = 45; int[] d=new int[s]; d[s-4] = 32453;

Which is Correct Java Code? In new, size of array written in brackets int[] a=new int(12); a[2] = 12; char[] b=new char[5]; b[] = “1234”; int[] c=new int[2]; c[2] = 12; int[] d=new int[20]; d(10) = 2.0; int s = 45; int[] d=new int[s]; d[s-4] = 32453;

Which is Correct Java Code? int[] a=new int(12); a[2] = 12; char[] b=new char[5]; b[] = “1234”; int[] c=new int[2]; c[2] = 12; int[] d=new int[20]; d(10) = 2.0; int s = 45; int[] d=new int[s]; d[s-4] = 32453; Must assign array entries individually

Which is Correct Java Code? int[] a=new int(12); a[2] = 12; char[] b=new char[5]; b[] = “1234”; int[] c=new int[2]; c[2] = 12; int[] d=new int[20]; d(10) = 2.0; int s = 45; int[] d=new int[s]; d[s-4] = 32453; Entries at indices 0 - length - 1

Which is Correct Java Code? int[] a=new int(12); a[2] = 12; char[] b=new char[5]; b[] = “1234”; int[] c=new int[2]; c[2] = 12; int[] d=new int[20]; d(10) = 2.0; int s = 45; int[] d=new int[s]; d[s-4] = 32453; Specify entry by writing index in brackets

Which is Correct Java Code? int[] a=new int(12); a[2] = 12; char[] b=new char[5]; b[] = “1234”; int[] c=new int[2]; c[2] = 12; int[] d=new int[20]; d(10) = 2.0; int s = 45; int[] d=new int[s]; d[s-4] = 32453; Entry like variable of array's type

Which is Correct Java Code? int[] a=new int(12); a[2] = 12; char[] b=new char[5]; b[] = “1234”; int[] c=new int[2]; c[2] = 12; int[] d=new int[20]; d(10) = 2.0; int s = 45; int[] d=new int[s]; d[s-4] = 32453;

Array Basics #1 Must specify size when allocated: Cannot grow array Cannot shrink array

Array Key Concept #2 Array variable is reference variable target either array instance or null

Array Key Concept #3 Allocating array of object type DOES NOT allocate other objects for its entries

Array entries act like fields Should initialize before using Array Key Concept #4 Array entries act like fields Should initialize before using

For Next Lecture Discussing 1st (& most interesting) ADT Wednesday What is an ADT and why do ADTs drive the world? What is a MultiSet & why only in Google’s Java API? How does MultiSet differ from array or ArrayList? How to use MultiSet in our programs? There are week #4 homework problems on web Due by 12:45PM Monday for this week & all following