Wrapper classes

Collections can only hold object references, not primitives
Each primitive type has a corresponding "wrapper class"
E.g. int has Integer as wrapper class
Collection can be of Wrapper class type: ArrayList<Integer>
To insert a primitive value into collection, first "wrap" it: Integer x = new Integer(17)
To access primtive value in a wrapper class object: x.intValue()
Prior to Java 5:

java.util.HashSet<Integer> numbers;
numbers = new java.util.HashSet<Integer>();
numbers.add(new Integer(3));
numbers.add(new Integer(5));
// a foreach loop to calculate the sum of all
// the items in the HashSet numbers:
int sum = 0;
for (Integer i: numbers) {
    sum = sum + i.intValue();
}
System.out.println("Sum is "+sum);

Starting with Java 5, language has a feature called "autoboxing" and "autounboxing": code doesn't need to explicitly wrap or unwrap primitives:

java.util.HashSet<Integer> numbers;
numbers = new java.util.HashSet<Integer>();
numbers.add(3);
numbers.add(5);
// a foreach loop to calculate the sum of all
// the items in the HashSet numbers:
int sum = 0;
for (Integer i: numbers) {
    sum = sum + i;
}
System.out.println("Sum is "+sum);

Lab 9 questions