Question

How to concatenate multiple lists in Java

Answer 1

import java.util.List;
import java.util.Arrays;
import java.util.ArrayList;

public class MyClass {
    public static<T> List<T> ConcatenateLists(List<T>... lists) {
        List<T> result = new ArrayList<>();
     
        for (List<T> l: lists) {
            result.addAll(l);
        }
     
        return result;
    }
    public static void main(String args[]) {
        List<String> list1 = Arrays.asList("java", "c");
        List<String> list2 = Arrays.asList("c++", "rust");
        List<String> list3 = Arrays.asList("c#", "python");
        List<String> list4 = Arrays.asList("php", "javascript");

        List<String> result = ConcatenateLists(list1, list2, list3, list4);

        System.out.println(result);
    }
}




/*
run:

[java, c, c++, rust, c#, python, php, javascript]

*/

 

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Answer 2

import java.util.List;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.stream.Stream;
import java.util.stream.Collectors;

public class MyClass {
    public static<T> List<T> ConcatenateLists(List<T>... lists) {
        return Stream.of(lists)
                .flatMap(x -> x.stream())
                .collect(Collectors.toList());
    }
    public static void main(String args[]) {
        List<String> list1 = Arrays.asList("java", "c");
        List<String> list2 = Arrays.asList("c++", "rust");
        List<String> list3 = Arrays.asList("c#", "python");
        List<String> list4 = Arrays.asList("php", "javascript");

        List<String> result = ConcatenateLists(list1, list2, list3, list4);

        System.out.println(result);
    }
}




/*
run:

[java, c, c++, rust, c#, python, php, javascript]

*/

 

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Answer 3

import java.util.List;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.stream.Stream;

public class MyClass {
    public static<T> List<T> ConcatenateLists(List<T>... lists) {
        List<T> result = new ArrayList<>();
        
        Stream.of(lists).forEach(result::addAll);
 
        return result;
    }
    public static void main(String args[]) {
        List<String> list1 = Arrays.asList("java", "c");
        List<String> list2 = Arrays.asList("c++", "rust");
        List<String> list3 = Arrays.asList("c#", "python");
        List<String> list4 = Arrays.asList("php", "javascript");

        List<String> result = ConcatenateLists(list1, list2, list3, list4);

        System.out.println(result);
    }
}




/*
run:

[java, c, c++, rust, c#, python, php, javascript]

*/

 

70+ SQL courses for beginners and professionals