The classes Integer, Double, and Boolean wrap primitive values inside objects. These wrapper objects can be stored in ArrayLists.

The Double class is a typical number wrapper. There is a constructor that makes a Double object out of a double value:

Double r = new Double(8.2057);

Conversely, the doubleValue method retrieves the double value that is stored inside the Double object:

double d = r.doubleValue();

To add a primitive data type to a pre 1.5 ArrayList, you must first construct a wrapper object and then add the object. For example, the following code adds a floating-point number to an ArrayList:

ArrayList grades = new ArrayList();
double testScore = 93.45;
Double wrapper = new Double(testScore);
grades.add(wrapper);

Or the shorthand version:

grades.add(new Double(93.45));

To retrieve the number, you need to cast the return value of the get method to Double, and then call the doubleValue method:

wrapper = (Double)grades.get(0);
testScore = wrapper.doubleValue();

With Java 1.5, declare your ArrayList to only hold Doubles. With a new feature called auto-boxing in Java 1.5, when you define an ArrayList to contain a particular wrapper class, you can put the corresponding primitive value directly into the ArrayList without having to wrap it. You can also pull the primitive directly out.

ArrayList grades2 <Double> = new ArrayList <Double>();
grades2.add(93.45);
System.out.println("Value is " + grades2.get(0));