CodeGym/Java Course/Java Syntax/What are generics?

What are generics?

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"Another cool topic."

"The surprises just keep coming! Is it my birthday?"

"Today, I'll tell you about generics. Generics are types that have a parameter. In Java, container classes let you indicate the type of their inner objects."

"When we declare a generic variable, we indicate two types instead of one: the variable type and the type of the data it stores."

"ArrayList is a good example. When we create a new ArrayList object, it's convenient to indicate the type of the values that will be stored inside this list."

Code	Explanation
`ArrayList<String> list = new ArrayList<String>();`	Create an ArrayList variable called `list`. Assign an ArrayList object to it. This list can only store String objects.
`ArrayList list = new ArrayList();`	Create an ArrayList variable called `list`. Assign an ArrayList object to it. This list can store any values.
`ArrayList<Integer> list = new ArrayList<Integer>();`	Create an ArrayList variable called `list`. Assign an ArrayList object to it. This list can only store `Integer` and `int` values.

"Sounds super interesting. Especially the part about storing values of any type."

"It only seems like that's a good thing. In reality, if we put strings into an ArrayList in one method and then expect it to contain numbers in another method, the program will crash (terminate with an error)."

"I see."

"For now, we won't create our own classes with type parameters. We'll just use the existing ones."

"Can any class be a type parameter, even one that I write?"

"Yes. Any type except for primitive types. All type parameters must inherit from the Object class."

"You mean I can't write ArrayList&ltint>?"

"Indeed, you can't. But Java developers have written wrapper classes for each of the primitive types. These classes inherit Object. This is how it looks:"

Primitive type	Class	List
int	Integer	ArrayList<Integer>
double	Double	ArrayList<Double>
boolean	Boolean	ArrayList<Boolean>
char	Character	ArrayList<Character>
byte	Byte	ArrayList<Byte>

"You can easily assign primitive classes and their analogs (wrapper classes) to each other:"

Examples
`int a = 5; Integer b = a; int c = b;`
`Character c = 'c'; //the literal c is a char char d = c;`
`Byte b = (byte) 77; // The literal 77 is an int`
`Boolean isOk = true; // the literal true is a boolean`
`Double d = 1.0d; // The literal 1.0d is a double`

"Great. I think I'll try to use ArrayList more often."

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Fadhil Radhian Software Engineer at Accenture

25 March 2023, 02:42

Generic also present in Typescript

Anonymous #10782038 Java Developer at Microsoft

18 March, 18:58

How is it at accenture? I got an offer from them but I have heard bad things.

Stephen Haokip

Level 8 , imphal, India

18 June 2020, 09:54

i progressing thru the lesson is because i have some java knowledge before. this app gives u work n then teaches you. which makes it beginners unfriendly.

Mihai Bone

Level 8 , Bucharest, Romania

20 October 2020, 22:43

They do it on purpose to make you research on google for solutions, is a feature not a bug :D

+17

Clint

Level 22 , cincinnati, united states

18 November 2020, 20:28

no other book or course does it this backwards

Hoist

Level 32 , San Diego, United States

20 February, 02:35

many paying $ for receiving the Pain of researching unfamiliar topics ;

Gellert Varga

Level 23 , Szekesfehervar, Hungary

26 April 2020, 18:51

"if we put strings into an ArrayList in one method and then expect it to contain numbers in another method, the program will crash." Notice: 1.) I don't got a program-crash (with JDK8) if i wrote this:

ArrayList mixedList = new ArrayList();
	mixedList.add(cat1);
	mixedList.add("Blah-blah");
	mixedList.add(1);

The program worked flawlessly. However, the output added a comment: "Notice: .... unsafe operation." 2.) With JDK8, when i did the declaration of the mixed list this way:

ArrayList<Object> mixedList = new ArrayList<>();
	mixedList.add(cat1);
	mixedList.add("Blah-blah");
	mixedList.add(1);

	for (Object obj : mixedList)
	    System.out.println(obj);

The program worked flawlessly. And the output didn't add any comment! 3.) With JDK-11, when i wrote this without any diamond operator:

ArrayList mixedList = new ArrayList();

the program worked well even then, and the output didn't add any comment.

AfEi

Level 19

12 May 2022, 17:00

crash:

public static void main(String[] args) {
        ArrayList list = new ArrayList();
        list.add(123);
        list.add("abc");
        System.out.println(totals(list));
    }
    public static int totals(ArrayList list) {
        int sum = 0;
        for (int i = 0; i < list.size(); i++) {
            sum += (int)list.get(i);
        }
        return sum;
    }

Gellert Varga

Level 23 , Szekesfehervar, Hungary

18 May 2022, 18:13

Yes, I probably didn't interpret that sentence well. (Unfortunately, I don't have a high level of English.) Your example code clearly shows in which case the program will crash. I would modify the quoted sentence in the lesson in this way, in this form I would have understood it better already at first reading: "if we put strings into an ArrayList in one method and then another method want to work with this list as if it contains numbers, the program will crash."

Krzysztof Kosała

Level 19 , Wejherowo, Poland

19 November 2023, 14:57

It's not crushed because cat1, "Blah-blah" and 1 - everything of this extends Object class. And object class has default toString() method. So if you only want to display it, it's ok but in another more complicated operation will throw exception.

hidden #10426208

Level 11

2 January 2020, 13:42

hidden #10362262

Level 9

31 July 2019, 04:43

A good additional reading on ArrayLists and Wrapper Classes is at Javanotes 8.1 (textbook online).

+17

Joe M

Level 47 , Owings Mills, United States

20 November 2020, 01:29

That is a good book!

Finn

Level 11

20 May 2019, 09:33

Hey, what would happen if i do this?

ArrayList<String> myList = new ArrayList<>();

Would myList reference a ArrayList that can just store Strings or any Value?

Berkson

Level 17 , Fortaleza, Brazil

22 May 2019, 23:29

Only store Strings because you declared the diamond operator <String> . If you declare :

ArrayList list = new ArrayList();

You can store any value. Ex:

list.add(23);
list.add("teste");

Will work without a problem. You can iterate over the list with a Iterator without declare the diamond operator too, but if you use, for example, the first value on a method that uses a string as argument you'll must do a downcast to string, cause when you add something to a list of this type, without diamond operator specifying the type "<String>", we add to the list Objects. The ArrayList list receives objects.

Henk

Level 19 , Pretoria, South-Africa

25 April 2019, 06:18

what on earth are you talking about in the above section ? sheesh.... How about explaining in "English" what the reason is you have int a = 5 or Integer a = 5 ? I guess this section I'll have to read up on seperately from this course, I don't get the lingo.

Alex Vypirailenko Java Developer at Toshiba Global Comme

25 April 2019, 15:19

Each primitive type has a "wrapper" class designed for it, which is written with a capital first letter, just like any other class. As an example, we can assign number 5 to both "int a" and to "Integer b", but you can do a lot more with the variable b, because the Integer class has a few methods (you can see them here) which can be used to manipulate the data. However, by using a wrapper class you will be using a lot more memory and is not an efficient use of resources, unless there's a real need for this in the project/program you are writing the code for. More information on the wrapper classes can be found here.

+31

Henk

Level 19 , Pretoria, South-Africa

27 April 2019, 05:41

Thank you Alex, for taking the time to answer my question, it makes sense :)