Java-Read pairs of large numbers from file and represent them with linked list, get the sum and product of each pair

Question

data file looks like below

12,234,123
123,452,622
(here is a empty line)
1,000,010
20,000

result supposes to looks like below

12234123

123452622

sum: 125686745

product:1510334562...

1000010

20000

sum: 1020010

product: 20000200000

can not use any utility library such as java.math.BigDecimal javautil.LinkedList etc.

1. I dont know how to display each number on one row

my result is like

12

234

123

but I need to display it on one row as

12234123

2. the problem of the space, there is a empty line between each pair of numbers I only can get first pair of numbers since my while statement works only when line is not null.

Shown on below:

while((Line = File.readLine())!= null)

I dont know how to modify it.

for the method of sum and the method of product. hope someone can give me some clues.

here is the code. (blank line problem solved)

public static void main (String[] args){
    if (args.length == 0)
        System.out.println("No file specified.");
    else{
        FileReader theFile;
        BufferedReader inFile;
        String num1,num2;
        try{
            theFile = new FileReader(args[0]);
            inFile = new BufferedReader(theFile);
            while((num1 = inFile.readLine())!= null){
                if(num1.length()==0){   
                }
                else{
                num2 = inFile.readLine();
                num1.replace(",", "");      
                num2.replace(",", "");
                System.out.println(num1);
                System.out.println(num2);
                }
            }
        }
            catch (Exception e) {
            System.out.println(e);
        }
    }
}

Answer 1

Well, I'll address your two points, but you should read up on I/O here.

1) I don't know how to display each number on one row.

while ((line = file.readLine()) != null)

Since you're consuming your input line by line, you'll read the whole number in already. The String.replace(",", "") method will take care of your commas. As for your sum and product, think like an adder, using the String representations to do the addition and multiplication the same way you do by hand.

2) There is a empty line between each pair of numbers. I only can get the first pair of numbers since my while statement works only when line is not null.

The null check is for EOF, or end of file. According to the Javadocs for BufferedReader, your blank line is really equivalent to "" + "\n", so you'll actually read a blank line. So just check if line.equals("") and do nothing.

EDIT: Addressing Updated Post

1. You need avoid resource leaks and create a File to pass to the FileReader. Take another look at the appropriate constructors.
2. I've refactored to deal with the fact you have basically multiple test cases
3. You shouldn't split on the comma delimiter, because you want a single line of output, correct?
4. Finally, for your logic, do some research on bignums, how BigInteger is implemented, and try to create your own simple implementations.

Here's all the fixes for your I/O implemented. You should read up more on the topic, but this will free you to focus on the bignum algorithms, which I suspect is the point of this exercise.

private static String add(String num1, String num2) {
    // implement
}

private static String multiply(String num1, String num2) {
    // implement
}

try (
    BufferedReader reader = new BufferedReader(new FileReader(new File(args[0])));
) {
    String num1, num2;
    while ((num1 = reader.readLine()) != null) {
        num2 = reader.readLine();       // assumes your input is formatted correctly
        num1.replaceAll(",", "");       // "delete" commas
        num2.replaceAll(",", "");
        System.out.printf("%s\n", num1);
        System.out.printf("%s\n", num2);
        System.out.printf("sum: %s\n",     add(num1, num2));
        System.out.printf("product: %s\n", multiply(num1, num2)); 
        reader.readLine();              // consume blank line
    }
} catch (IOException e) {
    System.err.println(e.getMessage()); // error output
}