Chapter 5 Arrays

Arrays Arrays Chapter 5 Outline  Declaring and using array  Array of objects  Sorting and seaching elements in an array  Two-dimensional array  The ArrayList class Array • An array is a data structure that stores a collection of values of the same type • Arrays are accessed with [ ]  Array indices are zero-based  Example: scores[5] returns the 5 th value in the array 0 1 2 3 4 5 6 7 8 9 79 87 94 82 67 98 87 81 74 91 scores The entire array has a single name Each value has an index Declaring an array • Syntax: • Example:  int[] scores = new int[100]; • an array that can hold 100 integers  String[] names; names = new names[20]; • an array that can hold 20 String objects • You can create an array object and supply initial values at the same time  int[] smallPrimes = { 2, 3, 5, 7, 11, 13 };  char[] letterGrades = {'A', 'B', 'C', 'D', ’F'}; DataType[] arrayName = new DataType[size]; DataType[] arrayName; // only declares the variable arrayName = new DataType[size]; See ebook page 328 for other syntax Using the array • If a is an array, to process all elements in a, using: • Example:  for (int i=0; i < scores.length; i++) System.out.println (scores[i]);  for (int score : scores) System.out.println (score);  for (String n : names) System.out.println (n); • See BasicArray.java, page 322. Modify the program to input values for (int i=0; i < a.length; i++) // process a[i] for (Type x : a) // process x length field returns the number of elements Bounds checking • Once an array is created, it has a fixed size • The index value must be in range 0 to N-1, so you cannot access indexes outside this limit  The Java interpreter throws an exception named ArrayIndexOutOfBoundsException if an array index is out of bounds • This is called automatic bounds checking • It’s common error when using arrays: for (int i=0; i <= codes.length; i++) codes[i] = i*50 + epsilon; problem Array as Parameters • An entire array can be passed as a parameter to a method • Because an array is an object, when an entire array is passed as a parameter, a copy of the reference to the original array is passed (they become aliases of each other)  Therefore, changing an array element within the method changes the original • Example: write inputArray and ouputArray methods, see Mang1Chieu.java Outline  Declaring and using array  Array of objects  Sorting and seaching elements in an array  Two-dimensional array  The ArrayList class Array of objects • The elements of an array can be object references  String[] words = new String[10]; • words object stores 10 references to String objects  Rectangle[] rects = new Rectangle[5]; • rects object stores 5 references to Rectangle objects • Initially an array of objects holds null references • Each object stored in an array must be instantiated separately Array of String • The words array when initially declared: words - - - - - • At this point, the following reference would throw a NullPointerException: System.out.println (words[0]); String[] words = new String[10]; [...]... rectangle Rectangle[] rects = new Rectangle [5] ; rects[0] = new HinhChuNhat (5, 4); rects[1] = new HinhChuNhat(7,3); rects[2] = new HinhChuNhat(100 ,50 ); (5, 4) rects (7,3) (100 ,50 ) - Example: Array of objects • Now let's look at an example that manages a collection of CD objects  See CD.java (page 340)  See CDCollection.java (page 337)  See Tunes.java (page 3 35) • Modify main method to add a CD object... (page 341)  insertion sort (page 3 45) • To sort an array of numbers, you can use one of the sort methods in the Arrays class (using a tuned QuickSort algorithm)  Arrays. sort(type[] a): a is an array of type int, long, short, char, byte, float or double  Example: • int[] a = new int[10000]; … Arrays. sort(a); • How to sort an array of objects by using sort method in the Arrays class? Sorting an array of... other.getArea()) return -1; else if (getArea() > other.getArea()) return 1; return 1; } } Searching value in the array • To search a value in a sorted array, you can use binarySearch method in the Arrays class  int Arrays. binarySearch( type[] a, type v ) • a: a sorted array of type int, long, short, char, byte, float or double • v: a value of the same type as the elements of a • If it is found, its index... using array  Array of objects  Sorting and seaching elements in an array  Two-dimensional array  The ArrayList class Two-Dimensional Arrays • A two-dimensional array can be thought of as a table of elements, with rows and columns two int[][] scores = new int[12] [50 ]; dimensions • A two-dimensional array is declared by specifying the size of each dimension separately: • A array element is referenced... a = new int[10000]; … Arrays. sort(a); • How to sort an array of objects by using sort method in the Arrays class? Sorting an array of objects • To sort an array of objects by using sort method in the Arrays class, the objects must belong to classes that implement the Comparable interface  Example: you want to sort an array of Rectangle order by area ascending public class Rectangle implements Comparable... dimension separately: • A array element is referenced using two index values: value = scores[3][6]; Example public class TwoDArray { public static void main (String[] args) { int[][] table = new int [5] [10]; for (int row=0; row < table.length; row++) for (int col=0; col < table[row].length; col++) table[row][col] = row * 10 + col; for (int row=0; row < table.length; row++) { for (int col=0; col < table[row].length;... array  Array of objects  Sorting and seaching elements in an array  Two-dimensional array  The ArrayList class Why do we use ArrayList class? • Problem:  In C/C++, you have to fix the sizes of all arrays at compile time • forces users into uncomfortable  In Java, the situation is much better You can set the size of an array at run time int size= ; Rectangle[] rects = new Rectangle[size]; • once... array, but it automatically adjusts its capacity as you add and remove elements, without your needing to write any code Declaring an ArrayList object • Syntax: ArrayList name = new ArrayList(); • As of JDK 5. 0, ArrayList is a generic class with a type parameter, so you must to specify the type of the element objects that the array list holds ArrayList name = new ArrayList();  ArrayList . Arrays Arrays Chapter 5 Outline  Declaring and using array  Array of objects  Sorting and seaching elements in an array  Two-dimensional array  The ArrayList class Array • An array. returned Outline  Declaring and using array  Array of objects  Sorting and seaching elements in an array  Two-dimensional array  The ArrayList class Two-Dimensional Arrays • A two-dimensional array can be thought. int[10000]; … Arrays.sort(a); • How to sort an array of objects by using sort method in the Arrays class? Sorting an array of objects • To sort an array of objects by using sort method in the Arrays