Question

How to convert ArrayList to HashMap in Java

Answer 1

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;

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

        // Create and populate an ArrayList
        ArrayList<String> list = new ArrayList<>();
        list.add("aaa");
        list.add("bbb");
        list.add("cccc");
        list.add("ddddddd");

        // Convert ArrayList to HashMap (index → value)
        Map<Integer, String> map = new HashMap<>();

        for (int i = 0; i < list.size(); i++) {
            map.put(i, list.get(i));
        }

        System.out.println("ArrayList: " + list);
        System.out.println("HashMap:   " + map);
    }
}



/*
run:

ArrayList: [aaa, bbb, cccc, ddddddd]
HashMap:   {0=aaa, 1=bbb, 2=cccc, 3=ddddddd}

*/

 

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Answer 2

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

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

       ArrayList<String> list = new ArrayList<>(List.of("Java", "C", "Python", "PHP")); 
       
       Map<String, Integer> map = new HashMap<>(); 
       
       for (String s : list) { 
           map.put(s, s.length()); 
       }

        System.out.println("ArrayList: " + list);
        System.out.println("HashMap:   " + map);
    }
}



/*
run:

ArrayList: [Java, C, Python, PHP]
HashMap:   {Java=4, C=1, PHP=3, Python=6}

*/

 

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Answer 3

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

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

        ArrayList<String> list = new ArrayList<>(List.of("1=One", "2=Two", "3=Three"));
        Map<String, String> map = new HashMap<>();
        
        for (String entry : list) {
            String[] parts = entry.split("=");
            map.put(parts[0], parts[1]);
        }


        System.out.println("ArrayList: " + list);
        System.out.println("HashMap:   " + map);
    }
}



/*
run:

ArrayList: [1=One, 2=Two, 3=Three]
HashMap:   {1=One, 2=Two, 3=Three}

*/

 

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