Arrays in Java What, why and how Copyright 1998-2011 Curt Hill

What is an array? Multiple occurrences of values of one type One name many values Item is selected at run time by an integer index A true data type in Java, unlike C++ Always an object Always dynamic The mark of arrays is a pair of square brackets: [ ] Copyright 1998-2011 Curt Hill

Why do we need arrays? We often need to select a value by an index rather than just name If a picture has 240,000 pixels, do we really want 240,000 different variable names? Instead we find one name and use an integer subscript to choose which of the many we want Copyright 1998-2011 Curt Hill

Counting For Example static void process(Picture p){ // Enhance red Pixel [] pixels = p.getPixels(); for(int i=0;i<pixels.length;i++){ Pixel ap = pixels[i]; int r = 3 *ap.getRed()/2; ap.setRed(r); } Copyright 1998-2011 Curt Hill

Explanation A reference to pixels either refers to the whole object: Such as pixels.length which describes how many Otherwise a bracket selects which one: pixels[i].setRed(red); Set the ith pixel’s red value Copyright 1998-2011 Curt Hill

For All Example static void process(Picture p){ // Enhance red Pixel [] pixels = p.getPixels(); for(Pixel ap: pixels){ int r = 3 * ap.getRed()/2; ap.setRed(r); } Copyright 1998-2011 Curt Hill

Explanation This version of the for does two things for us It infers the length of the array It hides the subscripting of the pixel Copyright 1998-2011 Curt Hill

Arrays One name, many values Arrays elements demonstrate identical form and function The form is that each element has exactly the same type Polymorphism can come into play here, but not considered further until much later The function is that they are there for the same purpose Copyright 1998-2011 Curt Hill

What is needed? Declaration Allocation Access and reference Marked by the [] Allocation Determines the size Access and reference Indexing or subscripting Answers to these questions: What can we do to an array element? What can we do to the whole array? Copyright 1998-2011 Curt Hill

Array declaration Two similar declarations: int [ ] a, b, c[ ]; int d[]; Brackets may precede or follow the variable name No size is allowed in declaration Brackets are attached to variable name b is not an array, but a simple integer Even an array of primitives is an object Copyright 1998-2011 Curt Hill

Array Allocation Two step for objects or primitives Allocate the array handle This determines the size of the array Allocate (or initialize) the individual elements Primitives need to be initialized Objects are usualy initialized by the allocation In either case, usually done with a for Copyright 1998-2011 Curt Hill

Preface to Allocation At least for a time we will get an array from somewhere else A picture We will not use the new operator on it, nor will we initialize it by accessing individual elements We can create and allocate arrays with new The technique is seen in next slides Copyright 1998-2011 Curt Hill

Allocation examples Two statements One statement int x[]; x = new int [20]; One statement int a[] = new int[20]; The initialization of both of these still needs to be done Copyright 1998-2011 Curt Hill

Using whole arrays Suppose the following declaration int [] a = new int [5], b[]; Without subscript an array name is just an object handle b = a; Have both handles refer to same array a b Copyright 1998-2011 Curt Hill

Subscripting Using a subscript an element is obtained a[k] = a[j+2]-2; Anywhere an int can be used a[k] can be used First element has subscript zero Last element has subscript size minus one Copyright 1998-2011 Curt Hill

A Subscript The item in brackets chooses which element of array to be used The item may be a integral constant integral variable integral expression No doubles or float subscripts There is no element between [2] and [3] Must be in the correct range 0 to length of array-1 Copyright 1998-2011 Curt Hill

Subscript Errors Suppose: int [] a = new int [10]; . . . a[k] = 2; If k is less than zero or greater than 9 a subscript error occurs ArrayIndexOutOfBoundsException is thrown It is the programmers responsibility to guarantee that an index is in the correct bounds Copyright 1998-2011 Curt Hill

Subscript Traceback Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -2 at pd1.PictureDemo1.process(PictureDemo1.java:23) at pd1.PictureDemo1.main(PictureDemo1.java:16) ArrayIndexOut OfBounds is the error The -2 is the actual subscript Find the first (from the top) line that is in your code. In this case it is line 23 Copyright 1998-2011 Curt Hill

Discussion Since an array is one name with many values, an array element has a two piece name The array name and the subscript We may assign to this name or use in an expression Processing an array sequentially is most often done with a for statement Copyright 1998-2011 Curt Hill

Array extras Arrays can be parameters or results of methods Lowest subscript is always zero The number of elements of an array can be accessed by a property: a.length One larger than largest valid subscript Copyright 1998-2011 Curt Hill

The questions What can we do to an array element? Anything reasonable to do to a variable of the same type What can we do to the whole array? Pass as a parameter Return as a result Assign the handle Get the length Copyright 1998-2011 Curt Hill

Array Handles Like any object what you have is the handle This handle can be reused int [] ar = new int[20]; … ar = new int[40]; Thus the length of the array is dependent on the current version A different length depending on where it is checked Copyright 1998-2011 Curt Hill

Tracing Somewhat problematic Since one name has many values we have to be careful to keep track of them properly Moreover, we do not know how many elements there are until the allocation Copyright 1998-2011 Curt Hill

Discussion A primitive passed to a method just passes a copy The original cannot be changed An object just passes a handle The object may changed but not the handle Arrays may thus have their values changed Strings are the exception The string is immutable Copyright 1998-2011 Curt Hill

Conclusions Arrays are a valuable means to structure data They are an object without an object name They require a subscript (or index) to access individual elements Methods may change array parameters Multiple dimension arrays are also possible but will be discussed later Copyright 1998-2011 Curt Hill