First Look at the Collections Framework: The ArrayList class

The ArrayList class is a generic class:
a list of type ArrayList<T> is a collection of objects of type T.

ArrayList<Circle> circles = new ArrayList<Circle>();
circles.add(new Circle(1)); // add circle with radius 1 to the list
circles.add(new Circle(2));

Note: There are brackets around the type Circle because this is a type parameter. You can replace "Circle" with any class, so we call ArrayList a generic class. We cannot replace "Circle" with a primitive type, so we cannot create an ArrayList of ints with ArrayList<int>.

ArrayList Methods

The size() method

When an ArrayList is first created, it has size 0. To get the size of an ArrayList, call the size() method:

int listSize = circles.size(); // listSize is 2

The get() method
To get an object at some specified position from an ArrayList, use the get() method. The argument for get() specifies the index of the element you want, where indexing into an ArrayList starts at 0:

Circle myFavoriteCircle = circles.get(1); // this gets the second circle in the list

Note: There is no element at index circles.size() - the following line will cause an out of bounds exception: myFavoriteCircle = circles.get(circles.size()); // exception thrown

The set() method

To assign or set a new value to an existing array list element, use the set() method.

Circle cir = new Circle(10); // create new Circle with radius 10
circles.set(1, cir); // Element at position 1 of circles array list is now the new circle

The add() method

We can use add() to add a new element to the end of an array list, or to add a new element at a specified position in the list. This method does not overwrite list elements - if we add an element at position i, the elements from position i on are moved down one position in the list.

circles.add(new Circle(4)); // add circle of radius 4 to end of array list (in position 2)
System.out.println("circle list size: " + circles.size()); // Output: 3
circles.add(1, new Circle(20)); // add new circle of radius 20 at position 1 in the list
System.out.println("circle list size: " + circles.size()); // Output: 4

The remove() method

To remove the array list element at a specified position, use the remove() method.

circles.remove(3); // removes the circle of radius 4 at position 3 (the end of the list)

Array Lists and Primitive Types

Array lists cannot contain elements of a primitive type. Fortunately, each primitive type has what is called a Wrapper Class.

A wrapper class instance contains a value of the corresponding primitive type. The primitive type value is passed to the constructor of the wrapper class:

Integer intWrap = new Integer(14);
             or, alternatively,
Integer intWrap = 14; // short-cut version of above - called "auto-boxing", only available with Java 5 and 6

It is also easy to convert the wrapper class object back to the primitive type:
int myInt = intWrap; // sets myInt to 14, called "auto-unboxing"... before Java 5, we used the intValue method:
int myInt = intWrap.intValue(); // needed before Java 5

Auto-boxing and auto-unboxing make it easy to perform operations on wrapper class objects:
intWrap = intWrap + 1; // intWrap now contains the integer 15

Array List Examples

Example: Write Java code that creates an array list of 10 strings. Then print the strings in the list.

Example: Create an array list of type Double. Add 3.14, 2.71, and 15 to it. Then add 0 at the beginning of the list.

The for each Loop

This new loop structure makes it easy to iterate through all elements in a list or array. It only allows us to traverse the entire array, starting from the first entry.

ArrayList<Square> sqrs = new ArrayList<Square>();
sqrs.add(new Square(1)); //add square with side length of 1
sqrs.add(new Square(2));
sqrs.add(new Square(3));

// loop through the list and print the area of each square
for(Square s: sqrs) // for each s in sqrs - s takes on each Square object in the list, one per iteration
    System.out.println("Area: " + s.area()); // print area of current square

int[] nums = {1, 2, 3, 4, 5, 6};
int sum = 0;

// compute sum of array elements using a for each loop
for(int i : nums)
    sum = sum + i;