Come posso iterare su un Set
/ HashSet
senza quanto segue?
Iterator iter = set.iterator();
while (iter.hasNext()) {
System.out.println(iter.next());
}
Come posso iterare su un Set
/ HashSet
senza quanto segue?
Iterator iter = set.iterator();
while (iter.hasNext()) {
System.out.println(iter.next());
}
Risposte:
È possibile utilizzare un ciclo for avanzato :
Set<String> set = new HashSet<String>();
//populate set
for (String s : set) {
System.out.println(s);
}
O con Java 8:
set.forEach(System.out::println);
Esistono almeno sei modi aggiuntivi per scorrere su un set. Mi sono conosciuti:
Metodo 1
// Obsolete Collection
Enumeration e = new Vector(movies).elements();
while (e.hasMoreElements()) {
System.out.println(e.nextElement());
}
Metodo 2
for (String movie : movies) {
System.out.println(movie);
}
Metodo 3
String[] movieArray = movies.toArray(new String[movies.size()]);
for (int i = 0; i < movieArray.length; i++) {
System.out.println(movieArray[i]);
}
Metodo 4
// Supported in Java 8 and above
movies.stream().forEach((movie) -> {
System.out.println(movie);
});
Metodo 5
// Supported in Java 8 and above
movies.stream().forEach(movie -> System.out.println(movie));
Metodo 6
// Supported in Java 8 and above
movies.stream().forEach(System.out::println);
Questo è quello HashSet
che ho usato per i miei esempi:
Set<String> movies = new HashSet<>();
movies.add("Avatar");
movies.add("The Lord of the Rings");
movies.add("Titanic");
stream()
.
Per dimostrare, considera il seguente set, che contiene diversi oggetti Person:
Set<Person> people = new HashSet<Person>();
people.add(new Person("Tharindu", 10));
people.add(new Person("Martin", 20));
people.add(new Person("Fowler", 30));
Classe modello persona
public class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
//TODO - getters,setters ,overridden toString & compareTo methods
}
for(Person p:people){ System.out.println(p.getName()); }
people.forEach(p -> System.out.println(p.getName()));
default void forEach(Consumer<? super T> action)
Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of iteration (if an iteration order is specified). Exceptions thrown by the action are relayed to the caller. Implementation Requirements:
The default implementation behaves as if:
for (T t : this)
action.accept(t);
Parameters: action - The action to be performed for each element
Throws: NullPointerException - if the specified action is null
Since: 1.8
È possibile utilizzare il funzionamento funzionale per un codice più accurato
Set<String> set = new HashSet<String>();
set.forEach((s) -> {
System.out.println(s);
});
Ecco alcuni suggerimenti su come iterare un Set insieme alle loro esibizioni:
public class IterateSet {
public static void main(String[] args) {
//example Set
Set<String> set = new HashSet<>();
set.add("Jack");
set.add("John");
set.add("Joe");
set.add("Josh");
long startTime = System.nanoTime();
long endTime = System.nanoTime();
//using iterator
System.out.println("Using Iterator");
startTime = System.nanoTime();
Iterator<String> setIterator = set.iterator();
while(setIterator.hasNext()){
System.out.println(setIterator.next());
}
endTime = System.nanoTime();
long durationIterator = (endTime - startTime);
//using lambda
System.out.println("Using Lambda");
startTime = System.nanoTime();
set.forEach((s) -> System.out.println(s));
endTime = System.nanoTime();
long durationLambda = (endTime - startTime);
//using Stream API
System.out.println("Using Stream API");
startTime = System.nanoTime();
set.stream().forEach((s) -> System.out.println(s));
endTime = System.nanoTime();
long durationStreamAPI = (endTime - startTime);
//using Split Iterator (not recommended)
System.out.println("Using Split Iterator");
startTime = System.nanoTime();
Spliterator<String> splitIterator = set.spliterator();
splitIterator.forEachRemaining((s) -> System.out.println(s));
endTime = System.nanoTime();
long durationSplitIterator = (endTime - startTime);
//time calculations
System.out.println("Iterator Duration:" + durationIterator);
System.out.println("Lamda Duration:" + durationLambda);
System.out.println("Stream API:" + durationStreamAPI);
System.out.println("Split Iterator:"+ durationSplitIterator);
}
}
Il codice si spiega da sé.
Il risultato delle durate sono:
Iterator Duration: 495287
Lambda Duration: 50207470
Stream Api: 2427392
Split Iterator: 567294
Possiamo vedere il Lambda
tempo più lungo mentre Iterator
è il più veloce.
Enumerazione(?):
Enumeration e = new Vector(set).elements();
while (e.hasMoreElements())
{
System.out.println(e.nextElement());
}
Un altro modo (java.util.Collections.enumeration ()):
for (Enumeration e1 = Collections.enumeration(set); e1.hasMoreElements();)
{
System.out.println(e1.nextElement());
}
Java 8:
set.forEach(element -> System.out.println(element));
o
set.stream().forEach((elem) -> {
System.out.println(elem);
});
Tuttavia ci sono già ottime risposte disponibili per questo. Ecco la mia risposta:
1. set.stream().forEach(System.out::println); // It simply uses stream to display set values
2. set.forEach(System.out::println); // It uses Enhanced forEach to display set values
Inoltre, se questo set è di tipo di classe personalizzato, ad esempio: cliente.
Set<Customer> setCust = new HashSet<>();
Customer c1 = new Customer(1, "Hena", 20);
Customer c2 = new Customer(2, "Meena", 24);
Customer c3 = new Customer(3, "Rahul", 30);
setCust.add(c1);
setCust.add(c2);
setCust.add(c3);
setCust.forEach((k) -> System.out.println(k.getId()+" "+k.getName()+" "+k.getAge()));
// Classe cliente:
class Customer{
private int id;
private String name;
private int age;
public Customer(int id,String name,int age){
this.id=id;
this.name=name;
this.age=age;
} // Getter, Setter methods are present.}