what means smaller than and bigger than "" sign after some class in declare?

Question

I wonder what really means these signs and their data types inside them? I saw a lot of code like this:

ArrayAdapter<String> adapter =  new ArrayAdapter<String> ( this, android.R.layout.simple_list_item_1, names );

There are a lot of code in java and android and i think c# that using this type of declaring objects. I want to know what is these declaring types and what is the usage of theme? Thanks in advance.

Start reading here: [The Java Tutorials. Lesson: Generics (Updated)](http://docs.oracle.com/javase/tutorial/java/generics/) — Luiggi Mendoza, Oct 26 '13 at 20:11
please read about Generic in java. introduced in Java 1.5 http://docs.oracle.com/javase/tutorial/java/generics/index.html — Ashu, Oct 26 '13 at 20:20

Levenal · Accepted Answer · 2013-10-26T20:27:28.470

2

They declare the type of object that the code (in this case ArrayAdapter) will be holding. Generics in java allows for some code to hold potentially any type of object, but in general if you can specify what type you want the code to hold you should. Take an ArrayList for Example:

Arraylist<String> myList = new ArrayList<String>() //Holds Strings
Arraylist<Integer> myList = new ArrayList<Integer>() //Holds ints
Arraylist<aCustomClass> myList = new ArrayList<aCustomClass>() //Holds objects of a class I made called aCustomClass
Arraylist myList = new ArrayList()  //Is set to recieve any type of object

If you do not specify what type of object you want, then it is expecting a "raw type" which is considered unsafe/bad practice. To learn more search for Generics in Java

edited Oct 26 '13 at 20:27

answered Oct 26 '13 at 20:20

Levenal

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how can i create a class that apply generic like this? – Majid Daeinejad Oct 26 '13 at 20:23
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`Arraylist` is not valid. – Alexis C. Oct 26 '13 at 20:26
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`Arraylist<> myList` is not valid too. – Alexis C. Oct 26 '13 at 20:28
@user2777005 please edit you answer and correct them. – Majid Daeinejad Oct 26 '13 at 20:30
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Note that while `ArrayList<> myList` does not work, now that we use Java 7, we can do `ArrayList myList = new ArrayList<>();` or even `ArrayList myList = new ArrayList<>();` Might want to look at http://stackoverflow.com/questions/4166966/what-is-the-point-of-the-diamond-operator-in-java-7/4167148#4167148 – Justin Oct 26 '13 at 20:32
what means ArrayList? @Quincunx – Majid Daeinejad Oct 26 '13 at 20:33
ArrayList of any type (I was too lazy to type `ArrayList` etc) – Justin Oct 26 '13 at 20:34

score 0 · Answer 2 · answered Oct 26 '13 at 20:23

<> is used to type a generic class. A common example would be lists or maps .

So the source code of a list looks like this:

public class MyList<T> extends List<T>

You can then create methods like this:

public void add(T value)

To learn more about generics, you can check out the official guide here: http://docs.oracle.com/javase/tutorial/java/generics/ (java)

score 0 · Answer 3 · answered Oct 26 '13 at 20:25

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Suppose you want to display data in a list view here Array Adapter is used

Adapter inflate the layout for each row in ListView

here adapter is the object of ArrayAdapter class for inflating layout for each row. String is define List having String type data. this define the current state of object android.R.layout.simple_list_item_1 is system generated resource. name is define array of string which wil be deplay in the ListView

answered Oct 26 '13 at 20:25

Gajendra Rawat

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I was meaning all of this type of declaration not only Adapter. – Majid Daeinejad Oct 26 '13 at 20:27
you mean why are we using this line of code...??? – Gajendra Rawat Oct 26 '13 at 20:32

what means smaller than and bigger than "" sign after some class in declare?

3 Answers3