Java 11 - Convert Collection to Array
How do you turn a collection into an array? Here what I'll do to convert a list of String to an array in Java 8:
CollectionToArray.java
package com.dariawan.jdk8;
import java.util.Arrays;
import java.util.List;
public class ListToArray {
public static void main(String[] args) {
List<String> list = Arrays.asList("Doc", "Grumpy", "Happy",
"Sleepy", "Dopey", "Bashful", "Sneezy");
System.out.println("List to Array example in Java 8:");
Object[] objects = list.toArray();
System.out.println(Arrays.toString(objects));
String[] array = list.toArray(new String[list.size()]);
System.out.println(Arrays.toString(array));
}
}
With output:
List to Array example in Java 8: [Doc, Grumpy, Happy, Sleepy, Dopey, Bashful, Sneezy] [Doc, Grumpy, Happy, Sleepy, Dopey, Bashful, Sneezy]
list.toArray() convert the list into Object[] instead of String[], so normally I'll not do it. So it's left to second approach list.toArray(new String[list.size()]) which also not optimal (because computing the size of a collection can be "expensive"). Isn’t there a better way to do this?
In Java 11, a new default method toArray(IntFunction) has been added to the java.util.Collection interface:
default <T> T[] toArray(IntFunction<T[]> generator)
Returns an array containing all of the elements in this collection, using the provided generator function to allocate the returned array.
API Note: This method acts as a bridge between array-based and collection-based APIs. It allows creation of an array of a particular runtime type. Use toArray() to create an array whose runtime type is Object[], or use toArray(T[]) to reuse an existing array.
Suppose x is a collection known to contain only strings. The following code can be used to dump the collection into a newly allocated array of String:
String[] y = x.toArray(String[]::new);
Let's take this new method in Java 11 example:
ListToArray.java
package com.dariawan.jdk11;
import java.util.Arrays;
import java.util.List;
public class ListToArray {
public static void main(String[] args) {
List<String> list = List.of("Doc", "Grumpy", "Happy",
"Sleepy", "Dopey", "Bashful", "Sneezy");
System.out.println("List to Array example in Java 11:");
// old method
String[] array1 = list.toArray(new String[list.size()]);
System.out.println(Arrays.toString(array1));
// new method
String[] array2 = list.toArray(String[]::new);
System.out.println(Arrays.toString(array2));
}
}
With output:
List to Array example in Java 11: [Doc, Grumpy, Happy, Sleepy, Dopey, Bashful, Sneezy] [Doc, Grumpy, Happy, Sleepy, Dopey, Bashful, Sneezy]
Here another example with HashSet/LinkedHashSet:
CollectionToArray.java
package com.dariawan.jdk11;
import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
import lombok.ToString;
public class CollectionToArray {
public static void main(String[] args) {
List<Integer> list = List.of(1, 2, 3, 4, 5);
Integer[] array1 = list.toArray(Integer[]::new);
System.out.println(Arrays.toString(array1));
Set<Integer> hset1 = new LinkedHashSet<>(list);
hset1.remove(1);
Integer[] array2 = hset1.toArray(Integer[]::new);
System.out.println(Arrays.toString(array2));
Set<Country> hset2 = new HashSet<>();
hset2.add(new Country("ID", "Indonesia"));
hset2.add(new Country("SG", "Singapore"));
hset2.add(new Country("MY", "Malaysia"));
Country[] array3 = hset2.toArray(Country[]::new);
System.out.println(Arrays.toString(array3));
}
@ToString
static class Country {
String code;
String name;
Country(String code, String name) {
this.code = code;
this.name = name;
}
}
}
The ouput is:
[1, 2, 3, 4, 5] [2, 3, 4, 5] [CollectionToArray.Country(code=ID, name=Indonesia), CollectionToArray.Country(code=SG, name=Singapore), CollectionToArray.Country(code=MY, name=Malaysia)]
Desson AriawanProgrammer