How do I merge 2 separate lists in java based on index?

Question

Say I have 2 lists.

I want to merge both list into 1 list(mergeList in code) and expected output is: [11.65148,48.2490,0 11.6515,48.2490,0 11.6516,48.2491,0 ].

Please suggest.

 List<Double> latitudeList = new ArrayList<Double>();
 List<Double> longitudeList = new ArrayList<Double>();
 List<Double> mergeList= new ArrayList<Double>();
 longitudeList.add(11.65148);
 longitudeList.add(11.6515);
 longitudeList.add(11.6516);
 latitudeList.add(48.2490);
 latitudeList.add(48.2490);
 latitudeList.add(48.2491);

• Vishal Aggarwal: Do you want to merge lists based on index instead of values?

Answer 1

Add the code below to have your lists merged and sorted.

 mergeList.addAll(latitudeList);
 mergeList.addAll(longitudeList);
 mergeList.sort(new Comparator<Double>() {
 @Override
 public int compare(Double o1, Double o2) {
 if(o1.doubleValue() < o2.doubleValue()){
 return -1;
 }else if(o1.doubleValue() > o2.doubleValue()){
 return 1;
 }
 return 0;
 }
 });

Now:

System.out.println(mergeList);

Prints out:

[1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0]

UPD:

 if(longitudeList.size() != latitudeList.size()){
 throw new IllegalStateException("longitudeList and latitudeList have to be of the same size");
 }
 for(int i = 0; i < longitudeList.size(); i++){
 mergeList.add(longitudeList.get(i));
 mergeList.add(latitudeList.get(i));
 }
 System.out.println(mergeList);

Will print out:

[11.65148, 48.249, 11.6515, 48.249, 11.6516, 48.2491]

UPD 2:

Better to override toString() method for your mergeList like it is shown below:

 List<Double> mergeList= new ArrayList<Double>(){
 @Override
 public String toString() {
 StringBuilder stringBuilder = new StringBuilder();
 for(int i = 1; i <= size(); i++){
 stringBuilder.append(get(i - 1));
 if(i % 3 == 0){
 stringBuilder.append(" ");
 }else{
 stringBuilder.append(",");
 }
 }
 stringBuilder.deleteCharAt(stringBuilder.length() - 1);
 return stringBuilder.toString();
 }
 };

Then just add hardcoded altitude when you build up merged list:

 for(int i = 0; i < longitudeList.size(); i++){
 mergeList.add(longitudeList.get(i));
 mergeList.add(latitudeList.get(i));
 mergeList.add(0d);
 }
 System.out.println(mergeList);

So the output would be:

11.65148,48.249,0.0 11.6515,48.249,0.0 11.6516,48.2491,0.0

• Nikhil Vernekar: Hi Alexey, Thanks for your quick help. But i cannot use compare. The actual lists with which i am working contain longitude and latitude values. So list1 has longitude values and list2 has latitude values. I need list3 with merged values as [ longitude,latitude,longitude,latitude,...etc]. Appreciate your help.
• Alexey R.: I gave you the answer that solves the issue stated in your example. Rework your example.
• Nikhil Vernekar: I have updated the example. You can take a look now.
• Alexey R.: Check updated answer
• Nikhil Vernekar: Hi Alexey. I have updated the expected a little. Can you please suggest how can i get such output?