Lecture 22, CS 302-7 and 8, March 12

1. Reminders
1. Exam 1 – we’ll discuss Wednesday
2. Program 2 – March 16
2. Review
1. Recursion - optional
3. ArrayList motivation
1. Imagine you’re taking input from a user, but you don’t know how much they’ll give you
2. Could use an array, double size incrementally
3. Example – NamesArray.java
4. Note null – we’ll talk about that later.
4. ArrayList basics
1. An ArrayList is kind of like an array, with these differences:

i. Don’t need to worry about initial size!

ii. It can grow

1. consider example above

iii. It can shrink

1. Why is this important? Well, when you declare an array, it holds that space in memory, so you can’t use it for anything else. So, if it’s possible that it might hold 1,000,000 things, you need to declare it that size. ArrayLists can shrink as appropriate.

iv. Finally – it’s easier! Fewer things to worry about

1. aka ArrayIndexOutOfBounds exceptions

5. ArrayList declare/initialize
1. Think about an array list of strings. Steps to using:

i. Import ArrayList package

1. import java.util.ArrayList;

ii. Declare

1. ArrayList<String> words;

iii. Initialize

1. words =new ArrayList<String>();

iv. General form:

1. ArrayList<Type Parameter> name = new ArrayList<Type Parameter>();

v. What is the type parameter?

1. Tells the array list what kind of object to hold

vi. ***Note – you cannot pass primitives as Type Parameters

vii. If you want an arraylist of primitive types, instead pass wrapper classes

1. Integer

2. Double

3. Character

viii. Note - you can still fill them with normal ints etc in the above case: al.add(1)

ix. What do we call a class that can take a type parameter like this?

1. Generic class – because it’s general

6. ArrayLists – using
1. First, make sure you’ve initialized it before using!

i. words=new ArrayList<String>();

ii. Note the () – this calls the class ‘constructor’. We’ll explain this soon. Kind of like rand=new Random();

iii. If you don’t initialize first - error

2. When you initialize an ArrayList, it has size 0
3. How add elements?

i. words.add(“Rob”); words.add(“Bill”); words.add(“Mitch”);

ii. Generally, name.add(element);

iii. Note – this adds to the end of the array list

4. ArrayList size

i. words.size();

5. How get elements?

i. Say I wanted to get my name from the above ArrayList

1. words.get(1);

ii. Note – indexing starts at 0, just like arrays and string characters etc

iii. generally: name.get(index);

iv. Possible error – words.get(words.size())

6. Looping

i. Can use regular for loop or enhanced for loop

7. Example – Names.java
6. More things you can do with arraylists
1. Overwriting elements –

i. words.set(0, “Robert”);

ii. Generally:

1. name.set(index,new value);

iii. Note – this is just for overwriting existing values, not for adding new ones

2. Adding new elements in a given position:

i. words.add(1, “kevin”);

ii. generally-

1. name.add(index,new value);

iii. This inserts the new value at the given index, and ‘pushes everything back’ one spot.

iv. Thus, increases size of arraylist by 1

3. Removing elements:

i. words.remove(1);

ii. Decrease size by 1, move all elements down

iii. Generally –

1. name.remove(index);

8. HW – Work on P2, catch up on any reading you’ve missed