I have a class that implements getter and setter methods and related code as follows.
ArrayList<String> viewArray = new ArrayList<String>();
public ArrayList<String> getView() {
return viewArray;
}
from my activity, I am trying to get acces to stored array like:
ArrayList<String> al = new ArrayList<String>();
al = parsedExampleDataSet.getView();
But "al" receives no data. However, when getView() is executed, viewArray is filled properly. What am I missing? Thank you.
Others have make some good comments but I thought I'd take you through the code as I see it.
public class SomeClass {
// this is local to this class only
ArrayList<String> viewArray = new ArrayList<String>();
public void process() {
// i'm guessing there is some sort of processing method that is called
}
public ArrayList<String> getView() {
return viewArray;
}
}
Here's your activity class annotated with some details about the value of a1:
public class YourActivity {
ArrayList<String> al = new ArrayList<String>();
public void someMethod() {
// here a1 will be the same blank List you initialized it with
// unless someMethod() has been called before or a1 modified elsewhere
al = parsedExampleDataSet.getView();
// after the call to getView, a1 is now a reference to SomeClass.viewArray
// the ArrayList that a1 was initialized with is then garbage collected
}
}
Please edit your question to explain more what you are having problems with.
Related
I have a superclass called "Items" and a sub class called "PickUpHealth1". I create an array of Items similar to this code:
ArrayList<Items> itemsArray = new ArrayList<Items>();
Items h1 = new PickUpHealth1(x,y);
itemsArray.add(h1);
The subclass has it's own methods. I go through the array itemsArray and when a certain event occurs I want to initiate methods in the subclass PickUpHealth1. I know that the superclass doesn't know about the methods in the subclass but I don't want to create a separate array for each subclass if possible. Is there anyway I can reference the methods in the subclass through itemsArray? Since h1 is initialized as an object of PickUpHealth1 I would think there should be a way to do this, but I can't figure it out. Is there a way? Or am I going about it all wrong? Thanks.
So PickUpHealth1 extends Item and you have a List<Item> items
As you've noted Items can only do Item things, so you can't go trying to use PickUpHealth1 methods when you are accessing it as an Item through the items list.
If Items don't have anything in common then don't extend from it.
If they are all "useable" then give them a common method.
For example:
abstract class Item {
public abstract void useItem();
}
class PickUpHealth extends Item {
private int healAmount;
public PickUpHealth(int healAmount) {
this.healAmount = healAmount;
}
#Override
public void useItem() {
player.addHealth(healAmount);
}
}
public static void main(String[] args) {
List<Item> items = Arrays.asList(new PickUpHealth(10));
Item item = items.get(0);
item.useItem();
}
I am a rather large-scale noob. I have come across the answer a multitude of times, but, it was just not in a format I could understand. Right now, I am attempting to send a List full of values, to a separate class. I tried each of the resources but to no avail. Apologies if this is an inappropriate question here, a simple ask-with-no-code.
You need a method or a constructor on the class the receives the ArrayList that accepts ArrayLists.
Pseudocode:
public class Class1 {
//Constructor
public Class1(){}
//Methods
public ArrayList createArray(){
//Your code here
}
}
public class Class2 {
//Constructor
public Class2(ArrayList myArray){
//Your code here
}
}
public class Test{
Class1 c1 = new Class1;
ArrayList array = c1.createArray();
Class2 c2 = new Class2(array);
}
class list_receiver{ //class in which you want to send your list
public void print_array_list(ArrayList<String> l ){
System.out.println(l);
}
}
class list_sender{//class from where you are sending your list
public static void main(String args[]){
list_receiver r = new list_receiver();
ArrayList<String> list=new ArrayList<String>();
list.add("A");
list.add("B");
r.print_array_list(list);
}
}
I am trying to up create and update an ArrayList by passing an argument, so that I will end up with a list of say 10 names; however, the current function doesn't seem to be working - any ideas pls?
public String addClient(String name) {
ArrayList<String> myList = new ArrayList<String>();
myList.add(name);
return myList;
}
You are creating a new ArrayList every time you call it. This means that every time you call this method you create a brand new Collection and only store the one client in it. You need to keep a reference of a single collection around and keep adding to that. You can do that by passing in the array you want to add it to:
public List<String> addClient(String name, List<String> array) {
array.add(name);
return array;
}
This doesn't seem like a useful function, so I'm guessing this is within a class. So this might be the approach you want:
/**
* Class is not Thread Safe
*/
public class ClientList {
private final ArrayList<string> clients;
public ClientList() {
this.clients = new ArrayList<>();
}
public void addClient(String client) {
this.clients.add(client);
}
public List<String> getClients() {
// Note: Never give a reference to the internal objects of the class
// as that means someone outside this class can own a reference to it
// and can update the object without you knowing (by not going
// through this class)
Collections.unmodifiableList(this.clients);
}
}
This is what you need to do:
ArrayList<String> myList = new ArrayList<String>();
public void addClient(String name) {
myList.add(name);
}
If you create a list inside the method, it will only have one value, and will go away once method execution finishes (unless it's returned). Have a look at different scopes here. You should create a list at a class level and add the elements into it.
Also, method does not need to return anything, so it's better to change the type to void.
The problem with your approach is that everytime you call the method addClient a new ArrayList will be created.
I think this will work for you :
static ArrayList<String> myList;
public static void main(String[] args) {
myList = new ArrayList<>();
}
public void addClient(String name){
myList.add(name);
}
I want to add everything from ArrayList in class Taster in another ArrayList in class Tastatura. It's calling no suitable method error and I don't really know what to do and how to do it . I would be really thankfull if someone quickly explained me how this work and write my code correctly.
This is main class:
public class Tastatura {
public static void main(String[] args) {
Taster slovo = new Taster();
ArrayList<String> collection1 = new ArrayList<String>();
collection1.addAll(slovo); //<--- error here
}
}
another class:
public class Taster {
public Taster(){
ArrayList<String> Slova = new ArrayList<String>();
Slova.add("q");
Slova.add("w"); }}
The problem is by collection1.addAll(slovo); you are adding an object to a collection.
The addAll method requires a Collection as argument.
You class should look like this :
public class Taster {
private ArrayList<String> Slova;
public Taster() {
Slova = new ArrayList<String>();
Slova.add("q");
Slova.add("w");
}
public ArrayList<String> getList() {
return Slova;
}
}
And in you main method :
collection1.addAll(slovo.getList());
OR
You don't need to change the Taster class :
Just change your addAll to :
collection1.addAll(slovo.Slova);
try this :
collection1.addAll(slovo.Slova) ;
and change your Taster class to this :
public class Taster {
public ArrayList<String> Slova = new ArrayList<String>();
public Taster(){
Slova.add("q");
Slova.add("w");
}
}
You are trying to add class Taster into method, which expects String or something like that (ArrayList<String> in this case)
:)
Nafas code should work..
Other way is to write your own "add" method in your Taster class (which takes actual ArrayList and adds in it every item from ArrayList given in param, and returns that result)
I am working on a project, and I was taught to instantiate variables in constructors. I'm having some trouble doing this with an ArrayList thought. Can you suggest some best practices, do I need to define the ArrayList with the instance variables or can I do it in the constructor. Thanks for your suggestions! I have an example of what I'm talking about below:
//imports
import java.util.*;
import java.lang.*;
public class ArrayListConstructorDemo
{
//instance variables/attributes
String string;
List<String> list;// for example does this line need to say List<String> list = new ArrayList<String>();
//constructors
public ArrayListConstructorDemo()
{
String string = "null";
List<String> list = new ArrayList<String>();//is there anyway I can do this here instead of 6 lines up?
}//end default constructor
public ArrayListConstructorDemo(String string,List<String> list)
{
this.string = string;
this.list = list;
}//end generic constructor
//observers/getters/accessors
public String getString(){return string;}//end method getString()
public List<String> getList(){return list;}//end method getList()
//transformers/setters/mutators
public void setTable(String string){this.string = string;}
public void setValues(String list)
{
// for(String s : test)
// {
list.add(this.list);
// }
}
public String toString()
{
return "this is a generic toString method for the class ArrayListConstructorDemo";
}//end toString
public static void main(String[] args)
{
ArrayListConstructorDemo alcd = new ArrayListConstructorDemo();
System.out.println(alcd.list.size());
//test Lists in general
List<String> bleh = new ArrayList<String>();
bleh.add("b1");
System.out.println(bleh.get(0));
}//end method main()
}//end class ArrayListConstructorDemo
Change
List<String> list = new ArrayList<String>();
to
list = new ArrayList<String>();
If you want to just declare it in the constructor you can have the code:
ArrayList<String> name = new ArrayList<String>();
Otherwise you can declare it as a field, and then initialize it in the constructor.
private ArrayList<String> name;
And then in the constructor:
name = new ArrayList<String>();
Making it a field would be useful, as you would then be able to create accessor/mutator methods in order to retrieve and use the List from different classes, without having to declare it public (which is rarely a good thing).
If you want to declare it in the constructor, then you (most likely) want to declare the outer field, so you want:
list = new ArrayList<String>();
Currently you are shadowing the List<String> list class variable, meaning that you are creating a new instance of list, rather than that you are initializing the list instance variable.
I personally prefer initializing it at declaration time though, so what you previously had. I prefer this to make the code more concise and you most likely won't end up forgetting to initialize it if you teach yourself that habbit.
Generally the practice is to declare before the constructor, and initialize in the constructor.
Here's an example:
class myClass
ArrayList<String> strings
public myClass()
{
strings=new ArrayList<String>();
}
How can you do this ??
public void setValues(String list) {
// for(String s : test)
// {
list.add(this.list);
// }
}
There is no method like add() to manipulate Strings, Instead you would have done this :
public void setValues(List<String> list) {
// for(String s : test)
// {
list.add(this.list);
// }
}
And regarding declaring ArrayList in the constructors you can do like this :
String string;
List<String> list;// for example does this line need to say List<String>
// list = new ArrayList<String>();
// constructors
public ArrayListConstructorDemo() {
string = "null";
list = new ArrayList<String>();// is there anyway I can do this here
// instead of 6 lines up?
}// end default constructor
java offers you also an Initializing Fields
http://docs.oracle.com/javase/tutorial/java/javaOO/initial.html see Initializing Instance Members