Java generics examples -- use generics in collection

Here are some most simple and common use of generics with collection.

Example 1:

List<String> names = new ArrayList<String>();
names.add("John");
System.out.printf("List<String> names: %s%n", names);

In the above example, names is a List of String. When retrieving elements from the List, the return value is of type String. So no need for casting, which is a big advantage over the old, non-parameterized collection.

Example 2:

Map<Integer, String> idToName = new HashMap<Integer, String>();
idToName.put(0, "John");
System.out.printf("Map<Integer, String> idToName: %s%n", idToName);

In the above example, idToName is a Map with a Integer key and String value. The output is:

Map<Integer, String> idToName: {0=John}

Example 3:

List<List<String>> listOfList = new ArrayList<List<String>>();
List<String> sublist1 = new ArrayList<String>();
sublist1.add("A String inside sublist1");
listOfList.add(sublist1);

List<String> sublist2 = new LinkedList<String>();
sublist2.add("A String inside sublist2");
listOfList.add(sublist2);
System.out.printf("List<List<String>> listOfList: %s%n", listOfList);

The above example shows a List whose elements are of type List, i.e., a List of List. The inner List declares that it can only hold String elements. The first inner list is an ArrayList of String, and the second is a LinkedList of String. Running this code snippet prints:

List<List<String>> listOfList: [[A String inside sublist1], [A String inside sublist2]]

Example 4:

private static <T> List<T> extractElements(List bag, Class<T> type) {
 List<T> result = new ArrayList<T>();
 for(Object e : bag) {
//if(e instanceof T)  can't use instanceof
if(type.isAssignableFrom(e.getClass())) {
    result.add((T) e);
}
 }
 return result;
}

This method takes a List of mixed elements and extracts those elements of the desired type. The following shows how to call this method:

List bag = new ArrayList();
bag.add(new Integer(0));
bag.add(new Integer(1));
bag.add(new Double(2008.5));
bag.add("a string");
List<Number> numbersInBag = extractElements(bag, Number.class);
System.out.printf("All elements in bag: %s%nNumber elements in bag: %s%n",
bag, numbersInBag);
List<Integer> integersInBag = extractElements(bag, Integer.class);
System.out.printf("All elements in bag: %s%nInteger elements in bag: %s%n",
bag, integersInBag);

-------- output -----------

All elements in bag: [0, 1, 2008.5, a string]
Number elements in bag: [0, 1, 2008.5]
All elements in bag: [0, 1, 2008.5, a string]
Integer elements in bag: [0, 1]