Sum

Task

1. You need to create a Sum class which will sum integers from command line arguments and output the sum to console.

2. Examples:

   java Sum 1 2 3

   Expected output: 6.

   java Sum 1 2 -3

   Expected output: 0.

   java Sum "1 2 3"

   Expected output: 6.

   java Sum "1 2" " 3"

   Expected output: 6.

   java Sum " "

   Expected output: 0.

3. Arguments can be:

   • digits,
   • signes + and -,
   • space symbols

4. You can assume that int type is sufficient for in-between calculations and result.

5. Before doing the task make sure to read docs for classes String and Integer.

6. For debugging use System.err, because it will be ingnored by the testing program.

Solution

After reading about String, Integer, System.err we now know about some usefull methods:

• Integer.parseInt(String s, int radix) which parses the string argument as a signed integer in the radix specified by the second argument. So it basically converts a number from the String data type to int.
• Character.isWhitespace(char ch) which checks if a character ch is a space symbol or not.

Now let’s start coding. Firstly let’s define the structure of our program. I will be putting the name of the file at the first line comment in a file and its path if its nessesary.

// Sum.java

public class Sum {
    public static void main(String[] args) {
        // ...
    }
}

Okay that’s done. What do we do next? Let’s look at String[] args argument to our main method. It represents an array of command line arguments which we need to sum. So know we made our task a little bit easier. Now we can just say that we need to find sum of elements of array args.

How are we going to do it though? First let’s understand what we can do with this array. Let’s modify our class a little bit.

// Sum.java

public class Sum {
    public static void main(String[] args) {
        System.out.println(args.length);
    }
}

We’ve added System.out.println(args.length) which takes the field length from our args object and prints it to the console.

Let’s try it. First compile our class using

$ javac Sum.java

And then we can do some manual testing.

$ java Sum 1 2 3
3

We got 3 as an output as expected. We gave our program 3 command line arguments: 1, 2 and 3.

Here are all of the examples

$ java Sum 1 2 -3
3

$ java Sum "1 2 3"
1

$ java Sum "1 2" " 3"
2

$ java Sum " "
1

Now let’s try not obly to count our arguments but to list them as well. Let’s modify our program a little bit more.

// Sum.java

public class Sum {
    public static void main(String[] args) {
        System.out.println("number of arguments: " + args.length);
        
        for (String argument : args) {
            System.out.println(argument);
        }
    }
}

Here I used for loop to do printing for every String element in args.

Let’s try this with our examples. And don’t forget to recompile using javac Sum.java.

$ java Sum 1 2 3
number of arguments: 3
1
2
3

$ java Sum 1 2 -3
number of arguments: 3
1
2
-3

$ java Sum "1 2 3"
number of arguments: 1
1 2 3

$ java Sum "1 2" " 3"
number of arguments: 2
1 2
 3

$ java Sum " "
number of arguments: 1
 

Okay, now that we know how to iterate (or do something for every element), we can calculate the sum of all the numbers in the args array. To do that we need to create a new variable sum of integer type and assign it the value of 0. Then for every number in args we will add it to sum, and so by the end of array we will have the sum of all numbers stored in variable sum. This is what the code will look like

// Sum.java

public class Sum {
    public static void main(String[] args) {
        System.out.println("number of arguments: " + args.length);
        
        int sum = 0;
        for (String argument : args) {
            sum = sum + argument;
            System.out.println(argument);
            System.out.println(sum);
        }
        
        System.out.println(sum);
    }
}

Seems ok. But let’s test it.

$ javac Sum.java
Sum.java:9: error: incompatible types: String cannot be converted to int
            sum = sum + argument;
                      ^
1 error

We got a compilation error that says String cannot be converted to int. It means that when we try to add 2 variables sum and argument their types don’t match. The type of sum is integer and string for argument. It seems pretty logical because what do we expect when adding for example 1 and apple?..

We can do so using Integer.parseInt() method.

It takes a String s and returns an int, which a string s represents. For example this code will work as expected

// ParseIntExample.java

public class ParseIntExample {

    public static void main(String[] args) {
        String s = "123";
        int sum = 321;
        //! sum = sum + s;
        sum = sum + Integer.parseInt(s);
        System.out.println(sum);
    }
}

The commented out line (//!) would have given us an exception.