Merge strings of ints

I am stuck ... can anyone tell me how can i do this:

Here is the required input and output:

input: [4, 2, +, 8, +, 2, 5, multiply sign, 2]

output: [42,+,8,+,25,multiply sign,2]

This was my last try and it output nothing :

public static List<String> unifyNumbers(List<String> data) {
		List<String> temp = new ArrayList<String>();
		
		for (int i = 0; i < data.size(); i++) {
			String num = "";
			try {
				num += Integer.parseInt(data.get(i));
				for (int i2 = i; i < data.size(); i++) {
					try {
					
						num += Integer.parseInt(data.get(i2));
					} catch (NumberFormatException e) {
						e.printStackTrace();
						i = i2 - 1;
						temp.add(num);
						num = "";

					}

				}
			} catch (NumberFormatException e) {
				e.printStackTrace();
				temp.add(data.get(i));

			}
		}
		return temp;
	}

import java.util.*;
import java.lang.*;
import java.io.*;
class Sample
{
public static void main (String[] args) throws java.lang.Exception
{
List<String> sampleData = new ArrayList<String>();
sampleData.add("1");
sampleData.add("2");
sampleData.add("*");
sampleData.add("8");
sampleData.add("+");
sampleData.add("2");
sampleData.add("5");
List<String> resultList = unifyNumbers(sampleData);
System.out.println(resultList);
}
public static List<String> unifyNumbers(List<String> data) {
String concatedNum = "";
List<String> resultList = new ArrayList<String>();
for(String arrVal: data) {
if(isInteger(arrVal)) {
concatedNum += arrVal;
} else {
resultList.add(concatedNum);
resultList.add(arrVal);
concatedNum = "";
}
}
if(!concatedNum.isEmpty()) {
resultList.add(concatedNum);
}
return resultList;
}
public static boolean isInteger( String input ) {
try {
Integer.parseInt( input );
return true;
}
catch( Exception e ) {
return false;
}
}
}

The i2 variable doesn't iterate, which is why you are not getting any output. That loop isn't necessary anyway. Putting the test to see if a string is a number in its own function simplifies things a little.

Here is a revised solution, and a link to a place you can test it.
http://tpcg.io/l2xbRASM

public static Boolean isNumeric(String value) {
try {
Integer.parseInt(value);
return true;
} catch(NumberFormatException e) {
return false;
}
}
public static List<String> unifyNumbers(List<String> data) {
List<String> temp = new ArrayList<String>();
String num = "";
for (int i = 0; i < data.size(); i++) {
// HelloWorld is the name of the class I am testing this in.
if(HelloWorld.isNumeric(data.get(i))) {
num += Integer.parseInt(data.get(i));    
} else {
temp.add(num);
// this line adds the arithmetic operator to the resulting output
temp.add(data.get(i)); 
num = "";
}
}
// add number that remains to the array
if(num != "") 
temp.add(num);
return temp;
}

I hope this adds to the answers other people have given.

It seems like Gowtham Nagarajan already gave you a better way to do it. I'd advise you to use a modified version of his code, one that checks whether your String is an Integer without a try/catch block (see this question for a discussion on why your current approach isn't really good practice).

I'll still show you what I did, just to help you understand what went wrong with your first approch and how to fix something like that.

I changed your code to make it work like you want it to. All my check prints that I used to find problems are still in there, to give you an idea how to find problems in the execution. Checks like these make it easier to follow the "flow" of your program and to find out what went wrong. You can take a piece of paper and write down every step your program goes through, the checks will show you if everything works like you want it to or if there's unwanted behavior.

 public static List<String> unifyNumbers(List<String> data) {
List<String> temp = new ArrayList<String>();
for (int i = 0; i < data.size(); i++) {
String num = "";
try {
num = "" + Integer.parseInt(data.get(i));
System.out.println("Check 1");
for (int i2 = i + 1; i2 < data.size(); i2++) {
try {
num += Integer.parseInt(data.get(i2));
System.out.println("Check 2");
} catch (NumberFormatException e) {
System.out.println("Error inside loop");
e.printStackTrace();
i = i2 - 1;
System.out.println("To add:" + num);
temp.add(num);
num = "";
break;
}
}
} catch (NumberFormatException e) {
System.out.println("Error outside loop");
e.printStackTrace();
temp.add(data.get(i));
}
if (!num.equals("")) {
System.out.println(num);
temp.add(num);
}
}
return temp;
}

When using the print statements, I found out that your original code never left the first inside loop. The print statement at the end System.out.println(num); printed something like 44444444. Hints like this one can help you make the code work.

There were a lot of small mistakes that were reasonably easy to find this way. I suggest you try that next time. Good Luck!