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ArrayList get by Object


ArrayList get by Object

By : user2952332
Date : November 19 2020, 12:41 AM
This might help you You can use list.indexOf(person) to get the index of the Person you are looking for.
code :
List<Person> list = ...
Person person = ...
int index = list.indexOf(person);
if (index >= 0) {
    Person p = list.get(index);
}


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In an ArrayList of Object, how can I convert Object into one of 3 instance of classes contained in an ArrayList?

In an ArrayList of Object, how can I convert Object into one of 3 instance of classes contained in an ArrayList?


By : Rogachev Michael
Date : March 29 2020, 07:55 AM
I wish this help you I think the best way to approach this problem would be using the Strategy design pattern.
Feline, Primate and Bird should implement an interface Animal. The Cage would then have a method public void addAnimal(Animal animal);
code :
public interface Animal {
    public String getName();
}
public interface EatingBehaviour {
    public void howManyTimes();
}

public class RealLionEatingBehaviour implements EatingBehaviour{
    @Override
    public void howManyTimes() {
        System.out.println("I eat once a day");
    }
}

public class ToyLionEatingBehaviour implements EatingBehaviour {
    @Override
    public void howManyTimes() {
        System.out.println("I never eat! I am a toy lion.");
    }
}

public abstract class Feline implements Animal{
    public abstract void scratchPost();
    private EatingBehaviour eatingBehaviour;
    public EatingBehaviour getEatingBehaviour() {
        return eatingBehaviour;
    }
    public void setEatingBehaviour(EatingBehaviour eatingBehaviour) {
        this.eatingBehaviour = eatingBehaviour;
    }
}

public class Lion extends Feline {
    private String name;
    public String getName() {
        return this.name; 
    }
    public void setName(String name) {
        this.name = name; 
    }
    Lion (String name) {
        this.name=name;
    }
    public void scratchPost(){
        System.out.println(getName() + " Lion Scratching Post!");
    }   
}

public class AnimalFactory {
    public static Animal getAnimalInstance(String type, String name){
        Animal animal=null;
        if ("lion".equalsIgnoreCase(type)) {
            animal = new Lion(name);
        }
        return animal;
    }
}

import java.util.ArrayList;
import java.util.List;

public class Cage {
    private List<Animal> animals = new ArrayList<Animal>();
    public void addAnimal(Animal animal) {
        animals.add(animal);
    }
    public void removeAnimal(int index){
        this.animals.remove(index);
    }
    public Animal getAnimal(int index){
        return this.animals.get(index);
    }   
}

public class Zoo {
    public static void main(String args[]) {
        Cage cage = new Cage();
        Animal animal = null;
        animal = AnimalFactory.getAnimalInstance("Lion", "Sweety");
        cage.addAnimal(animal);
        Animal animalFromCage = cage.getAnimal(0);
        if (animalFromCage instanceof Feline) {
            Feline feline = (Feline) animalFromCage;
            feline.setEatingBehaviour(new RealLionEatingBehaviour());
            feline.scratchPost();
            feline.getEatingBehaviour().howManyTimes();
            feline.setEatingBehaviour(new ToyLionEatingBehaviour());
            feline.getEatingBehaviour().howManyTimes();
        }
    }
} 
How can I convert ArrayList<Object> to ArrayList<String> or ArrayList<Timestamp>?

How can I convert ArrayList<Object> to ArrayList<String> or ArrayList<Timestamp>?


By : RumbleFly
Date : March 29 2020, 07:55 AM
around this issue The easiest way is to loop over it and convert each item into a new list of strings yourself.
code :
 ArrayList<Object> list = new ArrayList<Object>();
    list.add(1);
    list.add("Java");
    list.add(3.14);

    List<String> strings = new ArrayList<String>();
    for (Object object : list) {
        strings.add(object != null ? object.toString() : null);
    }
Why does adding ArrayList<object> inside another ArrayList<obj> replaces values of inner ArrayList<obj>

Why does adding ArrayList<object> inside another ArrayList<obj> replaces values of inner ArrayList<obj>


By : Daisy Fisher
Date : March 29 2020, 07:55 AM
should help you out When you add alInner into alOuter, you are not passing a copy. You are adding the same array list into alOuter multiple times. I can see that you attempted to clear the array list before adding it to alOuter again. However, this does not create a new array list. So as soon as you cleared alInner, alOuter's array lists will appear to have been changed. To fix this, you need to create new array lists using new:
code :
alInner = new ArrayList<>(); // do this in place of clear.
// this copies the stuff inside alInner to a new array list
alOuter.add(new ObjectName("some value",new ArrayList<>(alInner)));
Cannot convert ArrayList<ArrayList<Integer>> to ArrayList<ArrayList<Object>>

Cannot convert ArrayList<ArrayList<Integer>> to ArrayList<ArrayList<Object>>


By : user3149890
Date : March 29 2020, 07:55 AM
wish of those help There are 2 solutions:
1.Use generic (and btw should use List instead of ArrayList)
code :
public static <T> List<List<T>> foo(List<List<T>> parameter) {
    //do something
}

public void test() {
    List<List<Integer>> parameter;
    //do something with the parameter
    List<List<Integer>> product = foo(parameter);
}
public static List<? extends List<? extends Object>> foo(List<? extends List<? extends Object>> parameter) {
    //do something
}

public void test() {
    List<List<Integer>> parameter = new ArrayList<>();
    //do something with the parameter
    foo(parameter);
}
Increment count if the object exists in arraylist, else Add the object into arraylist

Increment count if the object exists in arraylist, else Add the object into arraylist


By : hejerdal
Date : March 29 2020, 07:55 AM
Any of those help I read words from a text file and then create a new Word object for each word and store the objects into an ArrayList. The text of the word is passed into the object as a parameter. I have overridden the equals(Object) and hashCode() method of the word class to check for the equality of objects based on text of a word instead of object memory location. I am trying to store all unique words in ArrayList as unique objects and increment the occurrence of the word object if the word repeats in the text file. , The problem is with this statement in your code;
code :
while (file.hasNext()) {

    Word w = new Word(fileWord);

    if (words.contains(w)) {
        w.increaseCount(); // Here's what goes wrong.
    } else {
        words.add(w);
    }
}
Scanner file = new Scanner(new File(textfile));
ArrayList<Word> words = new ArrayList<Word>();
while (file.hasNext()) {

    Word w = new Word(fileWord);

    if (words.contains(w)) {
        words.get(words.indexOf(w)).increaseCount(); // Note the change here.
    } else {
        w.increaseCount(); // This is for the first occurrence as 'count' is 0 initially.
        words.add(w);
    }
}
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