CS112-2012S-08 Arrays and Midterm Review 108-0: Arrays• ArrayLists are not part of Java proper• Library class• Created using lower-level Java construct: Array08-1: Arrays• Arrays are like a stripped-down ArrayList• Arrays are objects (like ArrayLists)• Access elements using [] notation (like python lists)• Can’t do anything fancy: no negative indices, no ranges• Need to declare the size of the array when it is created• Can’t change the size of an array once it is created• Get the length of the array using public length instance variable08-2: Arrays• Two ways to declare arrays:<typename>[] variableName;<typename> variableName[];• Examples:int A[]; // A is an array of integersint[] B; // B is an array if integersString C[]; // C is an array of strings08-3: Arrays: New• Like all other objects, Arrays are stored on the heap• int A[] just allocates space for a pointer• Need to call new to create the actual arraynew <type>[<size>]08-4: Arrays: New• Show contents of memory after each line:int A[];int B[];A = new int[10];B = new int[5];A[7] = 4;B[2] = 5;B[5] = 13; /// RUNTIME ERROR!CS112-2012S-08 Arrays and Midterm Review 208-5: Arraysvoid foo(){int i;int A[];A = new int[5]for (i = 0; i < 5; i++){A[i] = i;}}Trace through, show memory08-6: Arrays: Copyingint A[] = new int[SIZE];int B[] = new int[SIZE];// Code to store data in BA = B;• What do you think this code does?• What happens when we assign any object to another object?08-7: Arrays: Copyingint A[] = new int[SIZE];int B[] = new int[SIZE];// Code to store data in BA = B;• How could we copy the data from B into A• (A and B should point to different memory locations, have same values08-8: Arrays: Copyingint A[] = new int[SIZE];int B[] = new int[SIZE];// Code to store data in Bfor (int i = 0; i < B.length; i++){A[i] = B[i];}08-9: Array: CopyingCS112-2012S-08 Arrays and Midterm Review 3int A[] = new int[5];int B[] = new int[5];int C[];for (int i = 0; i < 5; i++)A[i] = i;for (int i = 0; i < 5; i++)B[i] = A[i];C = A;B[2] = 10;C[2] = 15;08-10: Arrays of Objects• We can have arrays of objects, as well as arrays of integers...Point pointArray[] = new Point[10];pointArray[3].setX(3.4);• What happens?• (refer to Java documentation for Point objects)08-11: Arrays of ObjectsPoint pointArray[] = new Point[10];for (int i = 0; i < 10; i++){pointArray[i] = new Point();}• Is this OK?08-12: Arrays of ObjectsPoint pointArray[] = new Point[10];for (int i = 0; i < 10; i++){pointArray[i] = new Point(i, i);}• Note that you can pass an integer to a parameter that expects a double (but not the other way around!)08-13: Arrays of ObjectsCS112-2012S-08 Arrays and Midterm Review 4Point pointArray[] = new Point[10];for (int i = 0; i < 10; i++){pointArray[i] = new Point(i, i);}• How would you calculate the average x value of all elements in the array?08-14: Arrays of Objects• How would you calculate the average x value of all elements in the array?Point pointArray[] = new Point[10];// Fill in pointArray//double sum = 0.0;for (int i = 0; i < pointArray.length; i++){sum = sum + pointArray[i].getX();}sum = sum / pointArray.length;08-15: Arrays of Objects• Arguments to Java program: What is this args variable?public static void main(String args[]){}• Array of strings of command line arguments• java MyProgram arg1 arg2• Using Run Dialog in Eclipse08-16: Arrays of Objects• Arguments to Java programpublic static void main(String args[]){for (int i = 0; i < args.length; i++){System.out.println(args[i]);}}08-17: 2D Arrays• We can create 2D arrays as well as 1D arrays• Like matricesCS112-2012S-08 Arrays and Midterm Review 5• 2D array is really just an array of arrays08-18: 2D Arraysint x[][]; // Declare a 2D arrayint[][] y; // Alternate way to declare 2D arrayx = new int[5][10]; // Create 50 spacesy = new int[4][4]; // create 16 spaces08-19: 2D Arraysint x[][]; // Declare a 2D arrayx = new int[5][5]; // Create 25 spacesx[2][3] = 11;x[3][3] = 2;x[4][5] = 7; // ERROR! Index out of bounds08-20: 2D Arrays• How would we create a 9x9 array, and set every value in it to be 3?08-21: 2D Arrays• How would we create a 9x9 array, and set every value in it to be 3?int board[][];board = new int[9][9];for (int i = 0; i < 9; i++)for int (j = 0; j < 9; j++)board[i][j] = 3;08-22: Using Arrays• Need to declare array size before using them• Don’t always know ahead of time how big our array needs to be• Allocate more space than we need at first• Maintain a second size variable, that has the number of elements in the array we actually care about• Classes that use arrays often will have an array instance variable, and a size instance variable (how much of thearray is used)08-23: Using Arrayspublic class StringArrayList{String data[];int listSize;opublic StringArrayList(){data = new String[10];listSize = 0;}/// other methods}CS112-2012S-08 Arrays and Midterm Review 608-24: Using Arrayspublic class StringArrayList{String data[];int listSize;int size(){// Fill me in!}// other methods}08-25: Using Arrayspublic class StringArrayList{String data[];int listSize;int size(){return listSize;}// other methods}08-26: Using Arrayspublic class StringArrayList{String data[];int listSize;void add(String newString){// Fill me in!}// other methods}08-27: Using Arrayspublic class StringArrayList{String data[];int listSize;void add(String newString){data[listSize] = newString;listSize++;}// other methods}08-28: Using Arrayspublic class StringArrayList{String data[];int listSize;void add(int index, String newString){// Fill me in!}// other methods}08-29: Using Arrayspublic class StringArrayList{String data[];int listSize;void add(int index, String newString){for (int i = listSize; i > index; i--){data[i] = data[i-1];}data[index] = newString;}// other methods}CS112-2012S-08 Arrays and Midterm Review 708-30: Midterm in 1 Week• Topics• Java Syntax• Using {}, knowing where ;’s go• Components of a class• data members (instance variables)• constructor(s)• methods08-31: Midterm in 1 Week• Topics• Methods• Return type• Parameter list• Method body• Calling methods• Pass by value (Objects can be tricky here!)08-32: Midterm in 1 Week• Topics• Variables• Variable Declaration, specifying type• Primative types versus reference types (objects)• NullPointerException08-33: Midterm in 1 Week• Topics• Object instatiation• When to use new• What happens when new is called08-34: Midterm in 1 Week•
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