How to create a List of arrays using java

Question

I have a arrayList with values {a,b,a,c,d,b,a} I want to make a comparison of each element in the list and insert the pair of common indexes into a List of array or something using java

example output: [[0,2,6], [1,4]] explanation: a is at indexes 0,2,6 and b is at indexes 1,4 So far I have this:

 HashMap<Integer, Integer> hashMap = new HashMap<Integer, Integer>();
 List<String> name = new ArrayList<String>();
 letter.add("a");
 letter.add("b");
 letter.add("c");
 letter.add("b");
 letter.add("a");
 for (int i = 0; i < letter.size(); i++) {
 for (int j = 1; j < letter.size(); j++) {
 if (letters.get(i).equals(letters.get(j)) && i != j) {
 hashMap.put(i, j);
 }
 }
 }
 System.out.println(hashMap); //o/p: {0=4, 1=3, 3=1}
 List<int[]> myList = new ArrayList<int[]>();
 Iterator entries = hashMap.entrySet().iterator();
 while (entries.hasNext()) {
 Map.Entry entry = (Map.Entry) entries.next();
 Integer key = (Integer)entry.getKey();
 Integer value = (Integer)entry.getValue();
 myList.add(new int[] {key,hashMap.get(key)});
 }
 System.out.println(myList.toString()); 
 //O/P: [[I@380fb434, [I@668bc3d5, [I@3cda1055]

UPDATE: the idea was to get [[0,4],[1,3],[3,1]] as elements in myList but I am not able to get that. Any help is much appreciated! Thanks!

Based on the above array of indexes, I want to compare the elements in a different List B and C at those indexes - meaning compare elements at indexes 0,2,6 in List B and C and check if all three elements are equal. Same for elements at index 1,4

• B K.: stackoverflow.com/a/8321984/10747672
• Monis Majeed: use Comparator its in built in java, Kinldy have a look
• Adan Vivero: You make a new ArrayList like List<int[]> bList = new ArrayList<int[]>()
• user6348718: @AdanVivero I updated the questions. Please check and Thanks for the response

Answer 1

I think this will help you:

 ArrayList<int[]> arrayList =new ArrayList<>();
 int[] arrayItem={0,2,6};
 int[] arrayItem2={1,4};
 arrayList.add(arrayItem);
 arrayList.add(arrayItem2);

Answer 2

Homework done, check this:

package com.company;
import java.util.*;
public class Main {
 public static void main(String[] args) {
 // write your code here
 HashMap<Integer, Integer> hashMap = new HashMap<Integer, Integer>();
 List<String> letter = new ArrayList<String>();
 letter.add("a");
 letter.add("b");
 letter.add("c");
 letter.add("b");
 letter.add("a");
 for (int i = 0; i < letter.size(); i++) {
 for (int j = 1; j < letter.size(); j++) {
 if (letter.get(i).equals(letter.get(j)) && i != j) {
 hashMap.put(i, j);
 }
 }
 }
 System.out.println(hashMap); //o/p: {0=4, 1=3, 3=1}
 List<int[]> myList = new ArrayList<int[]>();
 Iterator entries = hashMap.entrySet().iterator();
 while (entries.hasNext()) {
 Map.Entry entry = (Map.Entry) entries.next();
 Integer key = (Integer)entry.getKey();
 Integer value = (Integer)entry.getValue();
 int[] intValues = new int[2];
 intValues[0] = key;
 intValues[1] = value;
 myList.add(intValues);
 }
 String toPrint = new String();
 toPrint = toPrint.concat("[");
 for(int k = 0; k < myList.size(); k++) {
 toPrint = toPrint.concat("[");
 for(int l = 0; l < myList.get(k).length; l++) {
 toPrint = toPrint.concat(String.valueOf(myList.get(k)[l]));
 if(l != (myList.get(k).length-1)){
 toPrint = toPrint.concat(",");
 }
 }
 toPrint = toPrint.concat("]");
 }
 toPrint = toPrint.concat("]");
 System.out.println(toPrint);
 //[[0,4][1,3][3,1]]
 }
}

If all you needed to do was print the values on the sceen this code will work for you.

• user6348718: I updated the questions. Please check and Thanks for the response

Answer 3

You can not use Map the way you used to check all indexes of String. You can try with ArrayList as below,

import java.util.*;
public class ListCharIndexes {
 public static void main(String[] args) {
 List<String> letter = Arrays.asList("a","b","a","c","d","b","a");
 //letter= Arrays.asList("a","b","c","b","a");
 List<List<Integer>> result=new ArrayList<>();
 Set<String> result1=new HashSet<>();
 for (int i = 0; i < letter.size(); i++) {
 if(result1.add(letter.get(i))){ //skip String if it is already processed
 List<Integer> indexes=indexOfAll(letter.get(i), letter);
 if(indexes.size()>1) //add only pairs
 result.add(indexes);
 }
 }
 System.out.println(result);
 }
 static List<Integer> indexOfAll(String obj, List<String> list) {
 final List<Integer> indexList = new ArrayList<>();
 for (int i = 0; i < list.size(); i++)
 if (obj.equals(list.get(i))) 
 indexList.add(i);
 return indexList;
 }
}

O/P:

[[0, 2, 6], [1, 5]]