{"id":2273,"date":"2018-02-17T23:43:11","date_gmt":"2018-02-18T04:43:11","guid":{"rendered":"https:\/\/www.manualjava.net\/?p=2273"},"modified":"2018-02-20T15:57:03","modified_gmt":"2018-02-20T20:57:03","slug":"iteradores-de-una-coleccion","status":"publish","type":"post","link":"https:\/\/www.manualjava.net\/?p=2273","title":{"rendered":"Iteradores de una colecci\u00f3n"},"content":{"rendered":"

\"\"<\/p>\n

Un iterador permite acceder a cada elemento de una colecci\u00f3n sin necesidad de tener que conocer la estructura de esta. Hist\u00f3ricamente, Java 1.0 incorpora el iterador Enumeration<\/strong>, posteriormente Java 2 desarrolla dos nuevos iteradores:\u00a0Iterador y ListIterador<\/strong>.\u00a0<\/span><\/p>\n

Enumeration<\/strong><\/span><\/h2>\n

La interfaz Enumeration declara m\u00e9todos que recorren una colecci\u00f3n. Mediante este tipo de iterador se puede acceder a cada elemento de una colecci\u00f3n, pero no permite modificar la colecci\u00f3n, es de solo lectura. Enumeration forma parte del paquete java.util. Su declaraci\u00f3n es la siguiente:<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0public interface Enumeration{<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 boolean hasMoreElements();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Object nextElement();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0}<\/span><\/p>\n

nextElement() devuelve el siguiente elemento. Levanta la excepci\u00f3n de tipo\u00a0NoSuchElementException\u00a0<\/strong>si no hay m\u00e1s elementos, es decir si ya se ha recorrido toda la colecci\u00f3n. La primera llamada devuelve el primer elemento.<\/span><\/p>\n

hasMoreElements() devuelve true si no se ha accedido a todos los elementos de la colecci\u00f3n. Normalmente se dise\u00f1a un bucle, controlado por este m\u00e9todo, para acceder a cada elemento de la colecci\u00f3n.<\/span><\/p>\n

Las colecciones hist\u00f3ricas:\u00a0Vector, Stack, Dictionary, HashTable\u00a0<\/strong>disponen del m\u00e9todo elements() que devuelve un Enumeration, a partir del cual se puede recorrer la colecci\u00f3n. La declaraci\u00f3n es la siguiente:<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0Enumeration elements()<\/span><\/p>\n

El esquema que se sigue para acceder a cada elemento de una colecci\u00f3n consta de los siguientes pasos:<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 1.\u00a0 Declarar una variable Enumeration.<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0Enumeration enumera;<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 2.\u00a0 Llamar al m\u00e9todo elements() de la colecci\u00f3n.<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0enumera = coleccion.elements()<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 3.\u00a0 Dise\u00f1ar el bucle que obtiene y procesa cada elemento.<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0while(enumera.hasMoreElements()){<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0elemento = (TipoElemento)enumera.nextElement();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<proceso de elemento><\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0}<\/span><\/p>\n

Ejemplo<\/strong><\/span><\/h3>\n

Se crea una pila de diferentes objetos, posteriormente, se recorre con un iterador Enumeration y con m\u00e9todos de Stack. La pila se llena de objetos String, Integer y Double, sin un orden establecido. Para recorrer la pila se crea un enumerador y un bucle hasta que no queden m\u00e1s elementos sin visitar. Tambi\u00e9n se recorre aplicando la operaci\u00f3n pop() y controlando que no est\u00e9 vac\u00eda.<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 import java.util.*;<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 public class EnumeradorPila {<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0public static void main(String[] args) {<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 final int N=8;<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Stack pila = new Stack();<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 String []palabra = <\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0{\u00abUrbion\u00bb, \u00abMagina\u00bb, \u00abAbantos\u00bb, \u00abPeralte\u00bb, \u00abCitores\u00bb};<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 for(int i=0;i<N;i++){<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0int n;<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0n = (int)(Math.random()*N*2);<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0if(n<palabra.length)<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 pila.push(palabra[n]);<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0else if(n<N+2)<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 pila.push(new Double(Math.pow(n,2)));<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0else<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 pila.push(new Integer(n*3));<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 }<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \/\/ crea un enumerador de la pila <\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Enumeration enumera = pila.elements();<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \/\/ bucle para recorrer la pila<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 System.out.println(\u00abElementos de la pila\u00bb<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0+ \u00aben el orden establecido por el enumerador: \u00ab);<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 while(enumera.hasMoreElements()){<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Object q;<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 q = enumera.nextElement();<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 System.out.print(q + \u00bb \u00ab);<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 }<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \/\/ bucle para recorrer la pila<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 System.out.println(\u00ab\\nElementos de la pila en orden LIFO: \u00ab);<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 while(!pila.empty()){<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Object q;<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 q = pila.pop();<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 System.out.print(q + \u00bb \u00ab);<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0}<\/span>
\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0}<\/span>
\n \u00a0 \u00a0 \u00a0 }<\/span><\/p>\n

Iterator<\/strong><\/span><\/h2>\n

Java 2 desarrolla nuevas colecciones y el iterador com\u00fan Iterator.\u00a0Todo objeto colecci\u00f3n se puede recorrer con este iterador. Todas las colecciones tienen el m\u00e9todo iterator() que devuelve un objeto Iterator.<\/span><\/span><\/p>\n

\u00a0<\/span> \u00a0 \u00a0\u00a0Iterator iterador;<\/span>
\n\u00a0 \u00a0 \u00a0 iterador = Coleccion.iterator();<\/span><\/p>\n

La interfaz Iterator permite no solo acceder a los elementos, sino tambi\u00e9n eliminarlos. Pertenece al paquete java.util; la declaraci\u00f3n es la siguiente:<\/span><\/p>\n

\u00a0 \u00a0 \u00a0\u00a0 public interface Iterator{<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 boolean hasNext();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Object next();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 void remove();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0}<\/span><\/p>\n

hasNext() devuelve true si quedan elementos no visitados, es el equivalente a hasMoreElements().<\/span><\/p>\n

next() la primera llamada devuelve el primer elemento, seg\u00fan el orden establecido por el iterador.<\/span><\/p>\n

remove() elimina de la colecci\u00f3n el elemento obtenido por la \u00faltima llamada a next(). Solo se puede llamar una vez despu\u00e9s de next(), en caso contrario, o bien si no ha habido una llamada previa a next(), levanta la excepci\u00f3n IllegalStateException. Normalmente las colecciones implementan este m\u00e9todo en un bloque sincronizado.<\/span><\/p>\n

Norma<\/strong><\/span><\/h3>\n

Se recomienda utilizar Iterator en lugar de Enumeration usada en colecciones hist\u00f3ricas. Los m\u00e9todos de acceso son m\u00e1s sencillos de recordar y adem\u00e1s declara el m\u00e9todo remove().<\/span><\/p>\n

Ejemplo<\/span><\/strong><\/h3>\n

Dada una lista de puntos del plano se quiere eliminar de la lista aquellos cuya coordenada x est\u00e9 fuera del rango [2,12]. Se da por declarada la clase Punto con el m\u00e9todo int getX(), y que la lista de puntos ya est\u00e1 creada. La lista es del tipo ArrayList. El fragmento de c\u00f3digo escrito declara la lista y el iterador; realiza un bucle controlado por hasNext() para acceder a cada elemento y si la coordenada x no est\u00e1 en el rango [2,12] lo elimina llamando al m\u00e9todo remove().<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0class Punto(. . .)<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0List lista = new ArrayList();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0Iterator iterador;<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0\/\/ se llena la lista de objetos Punto<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0iterador = lista.iterator();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0while(iterador.hasNext()){<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0Punto q;<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0q = (Punto)iterador.next();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0if(q.getX()<2 || q.getX()>12){<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0System.out.println(\u00abPunto: \u00bb +q+ \u00bb se elimina\u00bb);<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0iterador.remove();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0}<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0}<\/span><\/p>\n

La colecci\u00f3n guarda elementos de cualquier tipo (Object), esto exige realizar una conversaci\u00f3n al tipo concreto (Punto) con el que se trabaja. Con Java 1.5 se puede parametrizar el tipo de los elementos, de tal forma que no es necesario realizar la conversi\u00f3n y, adem\u00e1s, el compilador verifica el tipo de los elementos a\u00f1adidos. A continuaci\u00f3n se escribe el c\u00f3digo con esta caracter\u00edstica.<\/span><\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0List<Punto> lista = new ArrayList<Punto>();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0Iterator<Punto> iterador;<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0iterador = lista.iterator();<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0while(iterador.hasNext()){<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0Punto q;<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0q = iterador.next();\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0\/\/ no es necesario un cast<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0. . .<\/span>
\n\u00a0 \u00a0 \u00a0 \u00a0}<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

Un iterador permite acceder a cada elemento de una colecci\u00f3n sin necesidad de tener que conocer la estructura de esta. Hist\u00f3ricamente, Java 1.0 incorpora el iterador Enumeration, posteriormente Java 2 desarrolla dos nuevos iteradores:\u00a0Iterador y ListIterador.\u00a0<\/p>

Seguir leyendo<\/a><\/p>\n","protected":false},"author":1,"featured_media":2275,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[17],"tags":[],"_links":{"self":[{"href":"https:\/\/www.manualjava.net\/index.php?rest_route=\/wp\/v2\/posts\/2273"}],"collection":[{"href":"https:\/\/www.manualjava.net\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.manualjava.net\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.manualjava.net\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.manualjava.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2273"}],"version-history":[{"count":37,"href":"https:\/\/www.manualjava.net\/index.php?rest_route=\/wp\/v2\/posts\/2273\/revisions"}],"predecessor-version":[{"id":2333,"href":"https:\/\/www.manualjava.net\/index.php?rest_route=\/wp\/v2\/posts\/2273\/revisions\/2333"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.manualjava.net\/index.php?rest_route=\/wp\/v2\/media\/2275"}],"wp:attachment":[{"href":"https:\/\/www.manualjava.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2273"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.manualjava.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2273"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.manualjava.net\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2273"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}