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Asked: 2020-04-15 02:43:30 +0800 CST 2020-04-15 02:43:30 +0800 CST

How to loop through the collection of an <object> and a <object)>

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If I want to traverse collections in a HashMap: HashMap<Integer, Elemento>) I would traverse it with a foreach()also if I implement a Iterator()I don't understand very well the two ways to traverse it. I do not find the difference or their functions of each one to go through what I want to go through.

Element.java

package stackoverflowejemplo;

public class Elemento {

private int numeros;
private String nombre;

public Elemento(int numeros,  String nombre){
    this.numeros = numeros;
    this.nombre = nombre;
}

public int getNumeros() {
    return numeros;
}

public void setNumeros(int numeros) {
    this.numeros = numeros;
}

public String getNombre() {
    return nombre;
}

public void setNombre(String nombre) {
    this.nombre = nombre;
}
public String toString(){
    return getNombre() +" - "+getNumeros() ;
    }
    }

Map.java

import java.util.HashMap; import java.util.Iterator; import java.util.Map.Entry;

 public class Mapa {

      public static void main(String[] args) {

       HashMap<Integer, Elemento> map  = new HashMap<>();
       map.put(1,new Elemento(2,"hola"));

       //foreach    

    } }
java

2 Answers

  1. First of all, in Java it doesn't exist for-each()like in other languages. What exists in Java since Java 5 is enhanced for-loop. What this statement does, behind the scenes, is use an iterator to iterate through the collection (in the case of arrays, you don't need to use an iterator).

    The following code that uses enhanced for-loop:

    public void imprimir(List<String> lista) {
    for (String string : lista) {
        System.out.println(string);
    }
}

    Java then translates it to:

    public void imprimir(List<String> lista) {
    for (Iterator<String> it = lista.iterator(); it.hasNext(); ) {
        String string = it.next();
        System.out.println(string);
    }
}

    If you want to create a class that can work with the enhanced for-loop, it is not enough for your class/interface to provide a method that allows you to obtain a Iterator, it must implement the interface Iterable. Here's an example (very simplified):

    public class MiListaSimple implements Iterable {
    private static class Node {
        int dato;
        Node sig;
    }
    private Node cabeza;

    public void agregar(int dato) {
        Node nuevo = new Node();
        nuevo.dato = dato;
        nuevo.sig = cabeza;
        cabeza = nuevo;
    }

    private class MiIterador implements Iterator<Integer> {
        private Node actual;
        public MiIterador(Node actual) {
            this.actual = actual;
        }
        @Override
        public boolean hasNext() {
            return actual.sig != null;
        }

        @Override
        public Integer next() {
            Integer resultado = actual.dato;
            actual = actual.sig;
            return resultado;
        }
    }

    @Override
    public Iterator<Integer> iterator() {
        return new MiIterador(cabeza);
    }
}

    With the above class, you can use the following code:

    MiListaSimple lista = new MiListaSimple();
lista.agregar(3);
lista.agregar(2);
lista.agregar(1);
for(Integer dato : lista) {
    System.out.println(dato);
}

    Departure:

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  2. Best Answer

    I ensure that the elements I print have a method toString(). It then foreachensures that if you have a method toString()on the class Elemento.javain which, you will loop through the object valorwith the help of the llave(Key).

    map.keySet().stream().forEach((llave) -> {//expresión lambda
            System.out.println(map.get(llave));
        });

    or this way:

    for (String llave : map.keySet()) {//foreach clásico
    System.out.println(map.get(llave));
    }

    Then an iterator I can take a look here to understand how it works and the lambda expression is clearly described here .

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