What is List in Java (Android)?

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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>();

http://docs.oracle.com/javase/tutorial/java/generics/index.html — Brian Roach, Dec 29 '11 at 07:11
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. — , Dec 29 '11 at 07:20
http://stackoverflow.com/questions/1844770/what-does-list-mean-in-java-generics , http://stackoverflow.com/questions/2218216/in-java-what-does-it-mean-when-a-type-is-followed-by-angle-brackets-as-in-list — , Dec 29 '11 at 07:22

score 17 · Accepted Answer · answered Dec 29 '11 at 07:12

17

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).

answered Dec 29 '11 at 07:12

Buhake Sindi

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Can we simply say that it stores objects of Statements which in nested class in Statement?' – Chatar Veer Suthar Dec 29 '11 at 07:14
Thanks for reply, let me read documentation and make some examples, I am upvoting and accepting answer :-) – Chatar Veer Suthar Dec 29 '11 at 07:18

score 2 · Answer 2 · answered Dec 29 '11 at 07:09

2

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

answered Dec 29 '11 at 07:09

nmjohn

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score 2 · Answer 3 · edited Jun 20 '20 at 09:12

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.

score 1 · Answer 4 · answered Dec 29 '11 at 07:13

1

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

answered Dec 29 '11 at 07:13

Vaandu

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What is List in Java (Android)?

4 Answers4

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