Date: 2012nov2
Update: 2025sep19
Language: java
Keywords: add, attach, join
Q. Java: Best way to concatenate two ArrayList<>s into another ArrayList<>
A. Our function called concat() does it as shown in this full example:
import java.util.ArrayList;
import java.util.Arrays;
class Demo {
static ArrayList<String> concat(ArrayList<String> a, ArrayList<String> b) {
ArrayList<String> out = new ArrayList<String>();
out.addAll(a);
out.addAll(b);
return out;
}
public static void main(String []args) {
ArrayList<String> list1 = new ArrayList<>(Arrays.asList("red", "green"));
ArrayList<String> list2 = new ArrayList<>(Arrays.asList("cold", "hot"));
var list3 = concat(list1, list2);
System.out.println("list3=" + list3);
}
}
Output:
list3=[red, green, cold, hot]