PIC20A Lect5

PIC20A Lecture 5

How to find the number of arguments?

args.length is the number of arguemnts.
In fact, if variable array is an array, then array.length is the length of the array (you will learn array later).

public class ArgTest {
   public static void main(String[] args) {
       System.out.println("There are " + args.length + " arguments.");
   }
}

Sample output

Z:\>java ArgTest abc def
There are 2 arguments.

Z:\>java ArgTest 12 23 44 55
There are 4 arguments.

Z:\>java ArgTest "Charles Li" "Amy Johnson" "Adam Smith"
There are 3 arguments.

Z:\>java ArgTest
There are 0 arguments.

How to access arguments?

args[i] is the i-th argument.

public class ArgTest2 {
    public static void main(String[] args) {
        for(int i = 0; i < args.length; i++) {
            System.out.println("argument " + i + " : " + args[i]);
        }
    }
}

Sample output

Z:\>java ArgTest2 abc def
argument 0 : abc
argument 1 : def

Z:\>java ArgTest2 12 23 44 55
argument 0 : 12
argument 1 : 23
argument 2 : 44
argument 3 : 55

Z:\>java ArgTest2 "Charles Li" "Amy Johnson" "Adam Smith"
argument 0 : Charles Li
argument 1 : Amy Johnson
argument 2 : Adam Smith

How to change String to other primitive types?

args[i] is of type String
Sometime you want to change a String to a double or an int
Double.parseDouble(args[i]) returns the double represented by args[i].
Integer.parseInt(args[i]) returns the integer represented by args[i].

public class Compare {
    public static void main(String[] args) {
        if(args.length != 2) { // if the number of arguments is not 2
            System.out.println("Please use exactly 2 arguments.");
        } else {   
            // return the double represented by args[0] 
            double num1 = Double.parseDouble(args[0]); 
            double num2 = Double.parseDouble(args[1]);
            if(num1 > num2) {
                System.out.println("The first argument is bigger.");
            } else if(num1 < num2) {
                System.out.println("The second argument is bigger.");
            } else {
                System.out.println("The two arguments are equal.");
            }
        }
    }
}

Sample Output

Z:\>java Compare 2.3 5.79
The second argument is bigger.

Z:\>java Compare 3.1 -0.9
The first argument is bigger.

Z:\>java Compare
Please use exactly 2 arguments.

Z:\>java Compare 1.2 5.9 4.21
Please use exactly 2 arguments.

Z:\>java Compare 2.11 2.11
The two arguments are equal.

Question What is the output of java Compare 1.2 b.c ?