Coming up ArrayList ArrayList vs Array – Declaration – Insertion – Access – Removal Wrapper classes Iterator object

Lecture 8 über-Arrays OO

Coming up ArrayList ArrayList vs Array –D–Declaration –I–Insertion –A–Access –R–Removal Wrapper classes Iterator object

Problems with Arrays They don’t change size It’s a pain adding new elements if you don’t know how many are there already You have to use indexes ArrayIndexOutOfBoundsException OO

ArrayList to the rescue! We can fix (most of) those problems Sun have a library of helpful classes you can use for free They are downloaded with the JVM etc ArrayList is one of these library classes

So what is ArrayList An object Like an array OO

Coming up ArrayList ArrayList vs Array –D–Declaration –I–Insertion –A–Access –R–Removal Wrapper classes Iterator object

Array vs ArrayList ArrayArrayList They don’t change sizeChanges size as you add elements It’s a pain adding new elements if you don’t know how many are there already ArrayList has an add() method and takes care of its size itself You have to use indexes You can use indexes if you want, but you don’t have to ArrayIndexOutOfBoundsException Still thrown by ArrayList. Hey, it’s a fact of life, okay?

Cat[] catArray; catArray = new Cat[10]; catArray[0] = moggy1; catArray[1] = moggy2; callMethodOn(catArray[1]); catArray[0] = null; ArrayList catAList; catAList = new ArrayList(); catAList.add(moggy1); catAList.add(moggy2); callMethodOn(catAList.get(1)); catAList.remove(moggy1); InsertionAccessRemovalDeclaration ArrayList can use indexes too

So ArrayLists are quite similar... Why would we need them? ArrayList Advantages: – They grow and shrink when you add and remove things – arrays are fixed size – They have many useful methods...

many useful methods like....? Check out the api: – Application Programming Interface type ‘java api’ into google visit

Here’s a bit of the method summary for ArrayList...

Coming up ArrayList ArrayList vs Array –D–Declaration –I–Insertion –A–Access –R–Removal Wrapper classes Iterator object

ArrayLists and Primitives ArrayLists can’t store primitives So what if we want to store int s in an ArrayList? If ArrayList can only store objects, let’s make our primitives into objects..

Memory Wrappers Java has wrapper classes for each of the primitives myNumber int myNumber Integer

For every Primitive......there is a wrapper Watch out for the names, some of them are maybe a little different than you’d expect booleanBoolean byteByte charCharacter doubleDouble floatFloat intInteger longLong shortShort

How do I use one? int myInt; myInt = 5; Integer myInteger = new Integer(myInt); myArrayList.add(myInt);

What if I want the int back later? int getMyInt = myInteger.intValue(); Each Wrapper class has an intValue(), doubleValue(), longValue() etc method Do you mean to say that every time I want to put an int in an array, I have to wrap it when I put it in and then unwrap it when I get it out?!?

not quite So what happens if the following code is run? ArrayList arrl = new ArrayList(); arrl.add(1); arrl.add(2); for(int i=0; i<2; i++){ System.out.println(arrl.get(i)); }

It looks like it shouldn’t work...but it does Java recognises that storing ints and other primitives is useful So if you try and put a primitive in......it’ll box it up into an object for you

huh? In various (specified) situations, Java will autobox your primitives for you. Like putting ints into an ArrayList

A closer look at Collection Syntax Often you will see code written like: ArrayList This is: – the type of collection: ArrayList – the type of objects it will contain:

Why? This is a compiler thing It will check you code to make sure you don’t try to put an Integer into your ArrayList of Strings It’s called type checking, checking whether you are using the right data type.

BlueJ example BlueJ has a notebook example that uses an ArrayList

Object structures with collections Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

Adding a third note

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Removal may affect numbering

Coming up ArrayList ArrayList vs Array –D–Declaration –I–Insertion –A–Access –R–Removal Wrapper classes Iterator object

Collections design Collections are designed to store things So they are also designed to allow easy retrieval We can use loops to iterate though a collection But the java collections package comes with an iterator object My Collection My Collection My Collection Iterator My Collection Iterator

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Using an Iterator object Iterator it = myCollection.iterator(); while(it.hasNext()) { call it.next() to get the next object do something with that object } java.util.Iterator returns an Iterator object public void listNotes() { Iterator it = notes.iterator(); while(it.hasNext()) { System.out.println(it.next()); }

????! All collections have an iterator object that “comes with” them You call theCollectionImUsing.iterator(); to have this object returned to you. The iterator object has 3 methods:

These methods fit nicely in a while loop public void listNotes() { Iterator it=notes.iterator(); while(it.hasNext()) { System.out.println(it.next()); }

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling Index versus Iterator Ways to iterate over a collection: – for-each loop. Use if we want to process every element. – while loop. Use if we might want to stop part way through. Use for repetition that doesn't involve a collection. – Iterator object. Use if we might want to stop part way through. Often used with collections where indexed access is not very efficient, or impossible. Iteration is an important programming pattern.

Covered this lecture ArrayList ArrayList vs Array – Declaration – Insertion – Access – Removal Wrapper classes Iterator object