Sams Teach Yourself Java in 24 Hours, 7th Edition (2014)

To add an object, call the array list’s add(Object) method with that object as the argument. Here are statements that add five strings:

servants.add("Bates");
servants.add("Anna");
servants.add("Thomas");
servants.add("Mrs. O'Brien");
servants.add("Daisy");

Each element is added to the end of the list, so the first string in servants is “Bates” and the last is “Daisy”.

There’s a corresponding remove(Object) method that takes the object out of the list:

servants.remove("Mrs. O'Brien");

The size of an array list is the number of elements it currently holds. Retrieve this information by calling the list’s size() method, which returns an integer:

int servantCount = servants.size();

When you have used generics to specify the class the list contains, it’s simple to use a for loop to iterate through each element of the list:

for (String servant : servants) {
System.out.println(servant);
}

The first argument to for is a variable where an element should be stored. The second is the array list. Other data structures can employ the same loop.

The add(Object) method stores the object at the end of the list. Objects also can be added to a list by specifying the position within the list where the object should be stored. This requires the add(int, Object) method, which takes the position as the first argument:

ArrayList<String> aristocrats = new ArrayList<String>();
aristocrats.add(0, "Lord Robert");
aristocrats.add(1, "Lady Mary");
aristocrats.add(2, "Lady Edith");
aristocrats.add(3, "Lady Sybil");
aristocrats.add(0, "Lady Grantham");

The last statement in the preceding example adds “Lady Grantham” at the top of the list instead of the bottom, putting her above “Lord Robert” and the others.

The position specified as the first argument must be no greater than the size() of the list. If “Lord Robert” had been added with 1 as the position instead of 0, the program would fail with an IndexOutOfBoundsException.

An element can be removed from a list by specifying its position as the argument to remove(int):

aristocrats.remove(4);

The element at a specified position in a list can be retrieved by calling get(int) with that position. Here’s a for loop that pulls each string out of a list and displays it:

for (int i = 0; i < aristocrats.size(); i++) {
String aristocrat = aristocrats.get(i);
System.out.println(aristocrat);
}

Often it is necessary to find out whether an array list contains a specific object. This can be determined by calling the list’s indexOf(Object) method with that object as the argument. The method returns the position of the object or –1 if it cannot be found in the list:

int hasCarson = servants.indexOf("Carson");

The hour’s first project employs these techniques on a simple game in which shots are fired at (x,y) points on a 10-by-10 grid. Some points contain a target and others do not.

The targets are represented by the Point class in the java.awt package. A point is created by calling the Point(int, int) constructor with the x and y coordinates as the two arguments.

This statement creates a point at (5,9):

Point p1 = new Point(5,9);

Here’s a 10-by-10 grid with that point marked by an X and empty spaces marked by a “.” period character:

Output

1 2 3 4 5 6 7 8 9
1 . . . . . . . . .
2 . . . . . . . . X
3 . . . . . . . . .
4 . . . . . . . . .
5 . . . . . . . . .
6 . . . . . . . . .
7 . . . . . . . . .
8 . . . . . . . . .
9 . . . . . . . . .

Columns go from left to right and represent the x coordinate. Rows extend from top to bottom and mark the y coordinate.

Before this project, you saw how array lists could hold strings. They can hold Point or any other class of objects. This statement creates a list of points:

ArrayList<Point> targets = new ArrayList<Point>();

The Java compiler won’t allow any class other than Point or its subclasses to be added to the array list.

In NetBeans or another programming tool, create a Java file named Battlepoint and designate com.java24hours as its package. Enter the text of Listing 17.1 into the file.

LISTING 17.1 The Full Text of Battlepoint.java