Introductory Android Development

Transcription

1 Introductory Android Development Quick Links & Text References Overview Pages Declaring ArrayList Pages Fill with Test Data Pages Add Elements Pages Number of Elements Pages Retrieve Element Pages Remove Element Pages Change Element Value Pages Loop Through Elements Pages Sort ArrayList Pages Search for Element Pages Overview ArrayList is a class build in to Android Java It is a subclass of Collections (similar to.net Collections) Basically a dynamically sized array Declaring an ArrayList ArrayList<type> thelist = new ArrayList<type>(); type must be class name (Integer, Double, String, etc.) Instantiation is optional here, but must be done before trying to add items to the ArrayList Filling and ArrayList with (Test) Values Collections.addAll(theList, value1, value2, etc); Filling an ArrayList with literals probably wouldn t be done in real applications, but it s handy for testing/demonstration value1, value2, etc. must be the same type as the ArrayList definition If the ArrayList is a collection of class objects, use new objname(), new objname() Page 1 of 5

2 Add Elements to the ArrayList thelist.add(item); item must be the same type as the ArrayList Determining the Number Elements in the ArrayList thelist.size() Retrieving an ArrayList Element thelist.get(index) ArrayList elements cannot be retrieved by simply using an index: thelist[i] Removing an ArrayList Element thelist.remove(text); thelist.remove(index); If the ArrayList contains strings, you can remove an element using the first example, using the string value. Otherwise, remove the element by designating its index. Changing / Setting the Value of an ArrayList Element thelist.set(index, newvalue); Again, you can t simply access the element using square brackets: thelist[2] = "new value"; newvalue type must match the ArrayList type Looping Through ArrayList Elements for(int i=0; i<thelist.size(); i++) //don't forget to use get or set for(type item : thelist) type must match the element type Page 2 of 5

3 Sorting an Array List Native type sort Collections.sort(theList); Only works for Integer, Double and String String case-insensitive sort Collections.sort(theList, String.CASE_INSENSITIVE_ORDER); Class element sort Collections.sort(theList, new Comparator<className>() { public int compare(classname value1, classname value2){ //Code that returns -1 if value1 < value2 0 if value1 = value2 1 if value1 > value2 //Actually -1 and 1 can be any negative or positive //value //end compare //end Comparator Needed to sort class elements stored in an ArrayList Page 3 of 5

4 Alternatively, you can define the Comparator in the object class definition itself public class MyClass { String firstname; //Sample text field int birthyear; //Sample numeric field //Normal stuff: more members, constructors, getters, settersz public static Comparator<ListItem> birthyearcomparator = new Comparator<ListItem>() { public int compare(listitem item1, ListItem item2){ return item1.birthyear - item2.birthyear; ; public static Comparator<ListItem> namecomparator = new Comparator<ListItem>() { public int compare(listitem item1, ListItem item2){ return item1.firstname.compareto(item2.firstname); ; You can add as many Comparators as you want. Notice these two have different names. Just like the Comparator in the previous section, each Comparator must include a compare function the returns a negative value, zero value or positive value. These examples demonstrate how to accomplish this for numbers and Strings. To sort using the class Comparators: Collections.sort(theList, MyClass.nameComparator); or Collections.sort(theList, MyClass.birthYearComparator); Searching for an Element If the ArrayList contains numbers or text, you can use the indexof method index = thelist.indexof("barney"); //String ArrayList index = thelist.indexof(100); //numeric ArrayList Like most indexof methods, this method returns -1 if the item is not found in the list. If the list is sorted, you can use Collections.binarySearch(theList, "findme") Returns a negative number (not necessarily -1) if not found Page 4 of 5

5 Searching an array of class objects I haven t found an easy to search an ArrayList of objects, but the following method that implements a simple Linear Search will work. private int indexoflistitem(string findme) { int foundat = -1; for (int i=0; i<thelist.size(); i++) if(thelist.get(i).getfirstname().equals(findme)) { foundat=i; break; //end for return foundat; Sorted is NOT required If you just want to know if the item is there or not (Contains equivalent) private boolean duplicateitem(string newitem) { boolean isdup = false; for(listitem item : listitems) { if(item.getitemtext().touppercase().equals(newitem.touppercase())) isdup = true; return isdup; Binary search will work if ArrayList is sorted Comparator is defined You re searching for the same type of object as is in the list (not just text or number) Page 5 of 5