Java Collections Framework:ArrayList-7(ArrayList Conversions)

 

How to convert ArrayList to string array in java


.

Method 1: Manual way of conversion using ArrayList get() method

This is a manual way of copying all the ArrayList<String> elements to the String Array[]. In this example we have copied the whole list to array in three steps a) First we obtained the ArrayList size using size() method b) Fetched each element of the list using get() method and finally c) Assigned each element to corresponding array element using assignment = operator.

package beginnersbook.com;
import java.util.*;

public class ArrayListTOArray {
	public static void main(String[] args) {

		/*ArrayList declaration and initialization*/
		ArrayList<String> arrlist= new ArrayList<String>();
		arrlist.add("String1");
		arrlist.add("String2");
		arrlist.add("String3");
		arrlist.add("String4");

		/*ArrayList to Array Conversion */
		String array[] = new String[arrlist.size()];              
		for(int j =0;j<arrlist.size();j++){
		  array[j] = arrlist.get(j);
		}

		/*Displaying Array elements*/
		for(String k: array)
		{
			System.out.println(k);
		}
	}
}

Output:

String1
String2
String3
String4

Method2: Conversion using toArray() method

In the above example we have manually copied each element of the array list to the array. However there is a method toArray() which can convert the ArrayList of string type to the array of Strings. More about toArray() here.



package beginnersbook.com;
import java.util.*;

public class Example {
	public static void main(String[] args) {

		/*ArrayList declaration and initialization*/
		ArrayList<String> friendsnames= new ArrayList<String>();
		friendsnames.add("Ankur");
		friendsnames.add("Ajeet");
		friendsnames.add("Harsh");
		friendsnames.add("John");

		/*ArrayList to Array Conversion */
		String frnames[]=friendsnames.toArray(new String[friendsnames.size()]);

		/*Displaying Array elements*/
		for(String k: frnames)
		{
			System.out.println(k);
		}
	}
}

Output:

Ankur
Ajeet
Harsh
John
How to convert an array to ArrayList in java
Here we are sharing three different ways to convert an Array to ArrayList. Basically we are converting an String Array to ArrayList of String type.
String array[] to ArrayList<String>
Method 1: Conversion using Arrays.asList()
Syntax:
ArrayList<T> arraylist= new ArrayList<T>(Arrays.asList(arrayname));


Example:
In this example we are using Arrays.asList method to convert the Array to ArrayList.
import java.util.*;

public class ArrayToArrayList {
     public static void main(String[] args) {

	  /* Array Declaration and initialization*/
	  String citynames[]={"Agra", "Mysore", "Chandigarh", "Bhopal"};

	  /*Array to ArrayList conversion*/
	  ArrayList<String> citylist= new ArrayList<String>(Arrays.asList(citynames));

	  /*Adding new elements to the converted List*/
	  citylist.add("New City2");
	  citylist.add("New City3");

	  /*Final ArrayList content display using for*/
	  for (String str: citylist)
	  {
		System.out.println(str);
       	  }
      }
}
Output:
Agra
Mysore
Chandigarh
Bhopal
New City2
New City3
Method 2: Collections.addAll method
Collections.addAll method all the array elements to the specified collection. This is how Collections.addAll method is being called. It does the same as Arrays.asList method however it is much faster than it so performance wise this is a best way to get the array converted to ArrayList.
String array[]={new Item(1), new Item(2), new Item(3), new Item(4)};
ArrayList<T> arraylist = new ArrayList<T>();
Collections.addAll(arraylist, array);
OR

Collections.addAll(arraylist, new Item(1), new Item(2), new Item(3), new Item(4));

Example
import java.util.*;

public class Example2 {
	public static void main(String[] args) {

	    /* Array Declaration and initialization*/
	    String array[]={"Hi", "Hello", "Howdy", "Bye"};

	    /*ArrayList declaration*/
	    ArrayList<String> arraylist= new ArrayList<String>();

	    /*Conversion*/
	    Collections.addAll(arraylist, array);

	    /*Adding new elements to the converted List*/
	    arraylist.add("String1");
	    arraylist.add("String2");

	    /*Display array list*/
	    for (String str: arraylist)
	    {
	 	System.out.println(str);
	    }
	}
}
Output
Hi
Hello
Howdy
Bye
String1
String2
Method 3: Manual way of doing things
We can also add all the array’s element to the array list manually. Below example shows the logic of manual conversion.
package beginnersbook.com;

import java.util.*;

public class Details {
	public static void main(String[] args) {

	    /*ArrayList declaration*/
	    ArrayList<String> arraylist= new ArrayList<String>();

	    /*Initialized Array*/
	    String array[] = {"Text1","Text2","Text3","Text4"};   

	    /*array.length returns the current number of 
	     * elements present in array*/
	    for(int i =0;i<array.length;i++)
            {

	         /* We are adding each array's element to the ArrayList*/
		 arraylist.add(array[i]);
	    }

	    /*ArrayList content*/
	    for(String str: arraylist)
	    {
	         System.out.println(str);
	    }
      }
}
Output:
Text1
Text2
Text3
Text4

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