creating a method that combines two ArrayLists in Java

I'm new to programming in Java and I am trying to write a method that combines the contents of two ArrayLists and then prints them. I wrote the array lists and they will print individually fine but I am unsure how to print the result.

The result should be [1, 4, 9, 16, 9, 7, 4, 9, 11]

package practice;

import java.util.ArrayList;

public class practice_1 {

	public static void main(String[] args) {
		/**10/3/2020
		 * Appends one array list to another
		 */
	
		ArrayList <Integer> partA = new ArrayList <Integer>();
			partA.add(1);
			partA.add(4);
			partA.add(9);
			partA.add(16);
		ArrayList <Integer> partB = new ArrayList <Integer>();
	    	partB.add(9);
	    	partB.add(7);
	    	partB.add(4);
	    	partB.add(9);
	    	partB.add(11);
	   	
	   	System.out.println("A: " + partA);
        System.out.println("B: " + partB);
		System.out.println("Append: " + ???); //How do I print the two ArrayLists together?
	}
	public static ArrayList<Integer> append(ArrayList <Integer> partA, ArrayList <Integer> partB)
	{
	ArrayList<Integer> append = new ArrayList<Integer>();
	

		for(int x = 0; x < 1; x++) 
			{
			append.add(partA.get(x));
			append.add(partB.get(x));
			}
		return append;
		}
	}

Use addAll()

    List <Integer> partAB = new ArrayList <Integer>();
partAB.addAll(partA);
partAB.addAll(partB);
System.out.println("Append: " + partAB);

See context:

public static void main(String[] args) throws Exception {
List <Integer> partA = Arrays.asList(1, 4, 9, 16);
List <Integer> partB = Arrays.asList(9, 7, 4, 9, 11);
System.out.println("A: " + partA);
System.out.println("B: " + partB);
List <Integer> partAB = new ArrayList <Integer>();
partAB.addAll(partA);
partAB.addAll(partB);
System.out.println("Append: " + partAB);
}

Output:

A: [1, 4, 9, 16]
B: [9, 7, 4, 9, 11]
Append: [1, 4, 9, 16, 9, 7, 4, 9, 11]

If your end result is supposed to be:

> The result should be [1, 4, 9, 16, 9, 7, 4, 9, 11]

The first half of the values is from the PartA list then the second half is all from the PartB list. So you need to first add all the PartA values first to your 'appended' list, then you'd add the PartB values.

With that in mind your append method needs to be changed to:

public static ArrayList<Integer> append(ArrayList <Integer> partA, ArrayList <Integer> partB)
{
ArrayList<Integer> append = new ArrayList<Integer>();
for(int x = 0; x < partA.size(); x++)  
{
append.add(partA.get(x));
}
for(int x = 0; x < partB.size(); x++)  
{
append.add(partB.get(x));
}
return append;
}
}

Then simply iterate over the appended array list and it'll print it out in the order you specified.

Updated:

simpler way is to do what the other post says and you could use the addAll method to add the values of the first two ArrayLists to the appended array list.

import java.util.ArrayList;
public class practice_1 {
public static void main(String[] args) {
/**10/3/2020
* Appends one array list to another
*/
ArrayList <Integer> partA = new ArrayList <Integer>();
partA.add(1);
partA.add(4);
partA.add(9);
partA.add(16);
ArrayList <Integer> partB = new ArrayList <Integer>();
partB.add(9);
partB.add(7);
partB.add(4);
partB.add(9);
partB.add(11);
System.out.println("A: " + partA);
System.out.println("B: " + partB);
partA.addAll(partB);
System.out.println("Append: " + partA); //How do I print the two ArrayLists together?
}
}

There are multiple ways to combine two lists.

Using Streams

 List<String> newList = Stream.of(listOne, listTwo).flatMap(Collection::stream).collect(Collectors.toList());

Using Streams concat

List<String> newList = Stream.concat(listOne.stream(), listTwo.stream()).collect(toList());

Using Apache commons-collections

List<String> newList = ListUtils.union(listOne, listTwo);

Finally to print you can use

System.out.println(newList)