Arrays

1. Arrays • array is a collection of elements • they have the same data type • indexed, i.e. accessible through an index • elements can be • objects of the same class or interface • values at a primitive type • index is an integer • numbering starts with 0 • the first element has index 0 • the last element has index N-1, where N is the size of the array • i.e. range of the indices is 0, 1, 2, ..., N-2, N-1 • the size of an array doesn't change

2. Array Syntax • there are two ways to declare it: • with [] before the name, e.g., String [] names; • with [] after the name, e.g., double scores []; • use index within [] to access an element • e.g., names [0] = "Jan"; • e.g., System.out.println ("Jan got " + scores [4]); • there are two ways to initialize an array: • by stating all of its elements within {}, e.g., • String [] names = {"Jan", "Bob", "Bob"}; • by declaring its size, e.g., • double [] scores = new double [10];

3. Arrays Initialization • initialization with new fills the array with the default values • e.g., new double [2] is the same as {0.0, 0.0}; • e.g., new String [3] is the same as {null, null, null}; • problem with array initialization • only in declaration you can use the {} form • e.g., this is illegal: • double grades [];grades = {3.33, 3.56, 1.29}; • but this is ok: • grades = new double [] {3.33, 3.56, 1.29};

4. Arrays (cont.) • an array is an object • e.g., String names [] = {"Jan", "Bob", "Bob"}; • then names references the object {"Jan", "Bob", "Bob"} • an entire array can be accessed, not only its elements • it can be assigned • e.g., if String names2 [] = names; • then names and names2 reference the same object • an array can be passed as a parameter to a method • e.g., void write (String names[]) {...} • every array has a length field • watch out, the element with index length doesn't exist! • the last index is length-1 • e.g., names[names.length-1] is the last element

5. Arrays in for-Loops • to go through an array: • for (int i = 0; i < names.length; i++) {System.out.println (names[i]);} • simpler way: "for-each" form: • for (String name: names) {System.out.println (name);} • this is equivalent • to go through an array backwards: • for (int i = names.length - 1; i >= 0; i--) {System.out.println (names[i]);}

6. Multidimensional Arrays • an array can hold other arrays • e.g. with • String sue [] = {"Sue", "Kim"}; String ev [] = {"Ev", "Man"}; • we can declare: • String persons[][] = {sue, ev}; • or simpler: • String persons[][] = {{"Sue", "Kim"}, {"Ev", "Man"}}; • think of rows and columns • "Sue", "Kim""Ev", "Man" • access as array • persons[1][0] // is Ev" • the length of subarrays can be different • e.g. triangular matrix • int matrix [][] = {{"1", "2", "3"}, {"4", "5"} {"6"}};

7. Arrays and Polymorphism • an array declared with a superclass type can hold objects of subclass type • class Square exends Shape {…}class Oval exends Shape {…}Shape shapes []{new Square(), new Oval()}; • then we can use it in a for-loop • for (Shape shape: shapes) {shape.draw();} • this will draw all the shapes • if subclass overridesdraw()thenitsdraw()will beused • similarly, an array declared with an interface type can hold objects of any class that implements it • class Square implementsColorable {…}class Oval implementsColorable {…}Colorable colorables []{new Square(), new Oval()};for (Colorable colorable: colorables) {colorable.setColor(Color.pink);}