Lesson 2

Generics, Lists, Maps

This lesson is describing creating and using classes which allows to pass a type as a parameter. In fact, maps are not a data structure, but for this lesson we will assume that they are.

I. Creating a class with a generic type


Task 0

Consider a class that contains String or Integer object. 


class StringWrapper {
  String value;
}
class IntegerWrapper {
  Integer value;
}

Those classes are very similar, so we would like to create something more universal by using a diamond: 

class Wrapper<T> {
  T value;
}
Wrapper<String> stringWrapper;
Wrapper<Integer> integerWrapper;

Create a class which contains a list of a specific type. Its size will be passed as the argument in a constructor. Don't use java's list, create your own.

II. Lists & maps


Array List Map
The fastest Slower (depends on implementation)
You have to provide a size Size is optional
Size is fixed Size is changing dynamic
Index of an object is an integer Index is of any type
No support for a diamond One parameter type required Two parameter types required (index, value)
List snippets 
// Creating a list of a specific type
List<Integer> list = new ArrayList<>();

// Adding an element to the list
list.add(5);

// Getting an element from the list (index as an argument)
list.get(0);

// Cheking if the list contains an element given as an argument
list.contains(5);

// Overriding the element with an index 0 with a new value 10
list.set(0, 10);

// Deleting an element with an index 0
list.remove(0);

// Deleting all elements from the list
list.clear();

Task 1

Create a program reading numbers via Scanner class from standard input. If the program receives 0, it will display:

  • smallest value
  • biggest value
  • average
  • median
Map sample code 
// Creating a mapping from String to String
Map<String, String> map = new HashMap<>();

// Putting a value “Warsaw” for a key “Poland”
map.put("Poland", "Warsaw");

// Getting a value for a key "Poland"
map.get("Poland");

// Checking if a key “Japan” exists
map.containsKey("Japan");

// Getting a set of all keys
map.keySet();

// Deleting a key (so its value)
map.remove("Poland");

// Deleting all keys (and values)
map.clear();
Task 2

There is a file with content:


Poland 2

Spain 3

Poland 1

Germany 10

Italy 123

China 40

China 78


Your task is to aggregate those informations. You need to display total value according to a country.

III. Nesting structures


Nesting structures Nesting structures
Nesting structures (color version) Nesting structures (color version)

Task 3

Write a program generating a structure like: university has many groups, there are many students in any group.

University, groups and students are classess with custom fields, which data should be randomized during constructing the object.

At the very end you should write whole content to .csv file.