How to add the contents of 2 text files in java

Question

So I have 2 text files with sets of information containing names and numbers, is there a way I can separate the numbers, and add the numbers, while keeping the names in a new file?. Thanks

The first file looks like this

john-100
james-12
carlos-392

The second looks like this

john-4
james-38
carlos-8

So is there a way to get the final product to look like this in a new file?

john-104
james-50
carlos-400

Answer 1

I guess, there is definitely a way of doing that. But it doesn't seem correct to write code for you, so I just describe an algorythm, and it's up to you to implement it.

So you can first use https://docs.oracle.com/javase/8/docs/api/java/nio/file/Files.html#readAllLines-java.nio.file.Path- to read lines from file.

Then you can split each line per - and put the result into a Map<String, Integer using https://docs.oracle.com/javase/8/docs/api/java/util/Map.html#compute-K-java.util.function.BiFunction-

Then just iterate on the map entries and build another List<String> of the same structure with '%name%-%number%`.

And finally use https://docs.oracle.com/javase/8/docs/api/java/nio/file/Files.html#write-java.nio.file.Path-java.lang.Iterable-java.nio.charset.Charset-java.nio.file.OpenOption...- to write the List<String> to a new file.

Answer 2

The below code uses java's stream API . The code creates a stream of lines from each file. It reads the first file and creates a Map where the map key is the name that appears in the file and the map value is the number (that appears in the file next to the name). Then it reads the second file and merges the data in the second file into the existing map.

EDIT

In the original code (below), I forgot to handle the part where you want to write the resulting map to a file. I added it under the comment Added in edit . I create a List of strings from the map and write that list to a new file.

/* Required imports:
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Map;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.stream.Collectors;
 */
Path path1st = Paths.get("path-to-first-file");
Path path2nd = Paths.get("path-to-second-file");
Function<String, String> keyMapper = str -> {
    String key = null;
    if (str != null  &&  str.indexOf('-') >= 0) {
        String[] parts = str.split("-");
        if (parts.length == 2) {
            key = parts[0];
        }
    }
    return key;
};
Function<String, Integer> valueMapper = str -> {
    Integer value = null;
    if (str != null  &&  str.indexOf('-') >= 0) {
        String[] parts = str.split("-");
        if (parts.length == 2) {
            try {
                value = Integer.valueOf(parts[1]);
            }
            catch (NumberFormatException x) {
                // Ignore. 
            }
        }
    }
    return value == null ? Integer.valueOf(0) : value;
};
BinaryOperator<Integer> mergeFunction = (first, second) -> {
    int one = first == null ? 0 : first.intValue();
    int two = second == null ? 0 : second.intValue();
    return Integer.valueOf(one + two);
};
try {
    Map<String, Integer> theMap = Files.lines(path1st)
                                       .collect(Collectors.toMap(keyMapper, valueMapper));
    Map<String, Integer> totals = Files.lines(path2nd)
                                       .collect(Collectors.toMap(keyMapper, valueMapper, mergeFunction, () -> theMap));
    System.out.println(totals);
// Added in edit
    List<String> lines = totals.entrySet()
                               .stream()
                               .map(entry -> String.format("%s-%s", entry.getKey(), entry.getValue()))
                               .collect(Collectors.toList());
    Files.write(Paths.get("path-to-new-file"),
                lines,
                StandardOpenOption.CREATE,
                StandardOpenOption.WRITE);
//
}
catch (IOException x) {
    x.printStackTrace();
}

Running the above code, on the sample data you provided, outputs the following map contents:

{carlos=400, james=50, john=104}