What is List<?> in Java (Android)? [duplicate]

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What is Type<Type> called?
What does List<?> mean in java generics?

package com.xyz.pckgeName; import java.util.ArrayList; import java.util.List; public class Statement { // public String public String status; public String user_id; public String name; public String available_balance; public String current_balance; public String credit_card_type; public String bank_id; public List<Statements> statements = new ArrayList<Statement.Statements>(); public class Statements { public String month; public String account_id; public String user_id; public String id; public List<Transaction> transactions = new ArrayList<Transaction>(); } }

Can anyone explain me what these two statements mean

public List<Statements> statements = new ArrayList<Statement.Statements>(); public List<Transaction> transactions = new ArrayList<Transaction>();

I added the "generics" tag for you. I know you didn't know about it, but I did. Hopefully it will lead to some trivially similar related questions. — user166390
– user166390, Commented Dec 29, 2011 at 7:20
stackoverflow.com/questions/1844770/… , stackoverflow.com/questions/2218216/… — user166390
– user166390, Commented Dec 29, 2011 at 7:22

Buhake Sindi · Accepted Answer · 2011-12-29 07:12:44Z

This is Generics in java

List<?> is essentially translated as a "List of unknowns", i.e., a list of unknown types. The ? is known as a Wildcard (which essentially means unknown).

public List<Statements> statements = new ArrayList<Statement.Statements>();

This essentially creates a List that only accepts Statement.Statements. Anything outside of Statement.Statements that you want to add to statements will create a compilation error. The same applies to public List<Transaction> transactions = new ArrayList<Transaction>();. This means that the List is bounded to a type Statement.Statements (on statements variable).

Can we simply say that it stores objects of Statements which in nested class in Statement?'
Thanks for reply, let me read documentation and make some examples, I am upvoting and accepting answer :-)

nmjohn · Accepted Answer · 2011-12-29 07:09:15Z

It is the use of generics. You are declaring a List of either Statement objects or Transaction objects.

Check out wikipedia for more info

http://en.wikipedia.org/wiki/Generics_in_Java

Community · Accepted Answer · 2020-06-20 09:12:55Z

You should read about Generics to understand this . List is a raw type which could be the list of Objects such as Strings,Wrappers and user defined objects .

public List<Statements> statements = new ArrayList<Statement.Statements>();

The above code says that statements is the reference to the ArrayList of Statement Objects . It does specifies that list would contain Statements objects which is inner class to the Statement class.

List<?> is used to say the list of unknown type.

Vaandu · Accepted Answer · 2011-12-29 07:13:46Z

The statements List can hold only Statements object. It is called Generics. Check this for sample programs.

http://www.java2s.com/Code/Java/Language-Basics/Asimplegenericclass.htm

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