The command line arguments of a Java program

Arguments (parameters) of the main method

Consider the main method in the following Java program:

public class Demo { public static void main(String[] args) { String[] name = {"John Doe", "Mary Jane", "Peter Pan"}; /* ---------------------------------------------- Print the String array name ---------------------------------------------- */ for ( int i = 0; i < name.length; i++ ) { System.out.println( name[i] ); } } }

Notes:

Notice the data type of the parameter variable args of the main method:
Notice that this data type is the same one as name:

In other words:

The data type of the parameter variable args of the main method is an array of String !!!
args[0] is the first element of this array.
The data type of a[0] is String
args[1] is the second element of this array.
The data type of a[1] is String
And so on.
args.length() is the length of the array.

$64,000 question:

Command arguments
- Previously discussed: (See: click here)

Passing command arguments to a Java program

Fact:

When we execute a Java program ProgramName using the following syntax:
the Java system will pass the words:

Graphically explained:

Java program that prints the elements in the parameter variable args:

public class Argument1 { public static void main(String[] args) { for ( int i = 0; i < args.length; i++ ) System.out.println( args[i] ); } }

Example Program: (Demo above code)

Prog file: click here

How to run the program:

Right click on link and save in a scratch directory
To compile: javac Argument1.java
To run: java Argument1

Sample outputs:

UNIX Prompt>> java Argument1 a b c a b c UNIX Prompt>> java Argument1 1 2 3 4 1 2 3 4

Explanation:

When the Java program java Argument1 is executed, the Java system first pass the command line arguments (a, b and c) to the main method

So, when the main method is executed, it's parameter variable args contains:

args[0] = "a" args{1] = "b" args[2] = "c" args.length = 3

The main method shows the values of its parameter variable by printing them out with a for-statement.

Using command arguments: a simple addition program --- part 1

Problem description:

Write a Java program that computes the sum of all its arguments

Example:

java Sum 1 2 will print 3 java Sum 4 5 8 will print 17 java Sum 2 3 4 5 will print 14

We use the running sum algorithm (see: click here )

Pseudo code:

sum = 0; for ( every element in args[i] ) do { sum = sum + args[0]; } Print sum;

Java program: a failed attempt

Question:

Can you spot the error in this program ???

Example Program: (Demo above code)

Prog file: click here

How to run the program:

Right click on link and save in a scratch directory

To compile: javac Sum1.java

Error message from the compiler:

Sum1.java:13: incompatible types found : java.lang.String required: int sum = sum + args[i]; ^ 1 error

What is the compiler trying to tell you:

public class Sum1 { public static void main(String[] args) { int sum; int i; sum = 0; // Initialize the running sum for ( i = 0; i < args.length; i++ ) { sum = sum + args[i]; // Add args[i] to running sum ^^^ ^^^ ^^^^^^^ int int String ** The Java compiler found: int + String ** The compiler requires: int + int } System.out.println( sum ); } }

Fixing the problem:

If we want to add, we must use data types that are "add-able"
A String typed value can not be added

Converting String to int

A numeric string can be converted into an integer using the parseInt method in the Integer class.
The parseInt method is invoked as follows:
The parseInt method returns an integer value of the numeric string

Example:

public class ParseInt { public static void main(String[] args) { String a = "12"; String b = "34"; System.out.println( a + b ); // a + b = "12" + "34" = "1234" // Because + = concatenate when used on strings System.out.println( Integer.parseInt(a) + Integer.parseInt(b) ); // Integer.parseInt(a) + Integer.parseInt(b) // = Integer.parseInt("12") + Integer.parseInt("34") // = 12 + 34 = 46 // Because + = ADD when used on integers } }

Output:

1234 46

Example Program: (Demo above code)
- Prog file: click here
How to run the program:

Using command arguments: a simple addition program --- Conclusion

Consider the failed attempt:

public class Sum1 { public static void main(String[] args) { int sum; int i; sum = 0; // Initialize the running sum for ( i = 0; i < args.length; i++ ) { sum = sum + args[i]; // Add args[i] to running sum ^^^^^^^ We need to convert the String to an integer } System.out.println( sum ); } }

Corrected program:

public class Sum2 { public static void main(String[] args) { int sum; int i; sum = 0; // Initialize the running sum for ( i = 0; i < args.length; i++ ) { sum = sum + Integer.parseInt(args[i]); // Add args[i] to running sum } System.out.println( sum ); } }

Example Program: (Demo above code)

Prog file: click here

How to run the program:

Right click on link and save in a scratch directory
To compile: javac Sum2.java
To run: java Sum2

Example outputs:

cheung@HOME2(17)> java Sum2 1 2 3 cheung@HOME2(18)> java Sum2 1 2 3 4 5 6 7 28 cheung@HOME2(20)> java Sum2 3 4 7 14

Postscript: converting String to double

A numeric string can be converted into an double using the parseDouble method in the Double class.
The parseDouble method is invoked as follows:
The parseDouble method returns an double precision value of the numeric string

Example:

public class ParseDouble { public static void main(String[] args) { String a = "12.34"; String b = "56.78"; System.out.println( a + b ); // a + b = "12.34" + "56.78" = "12.3456.78" // Because + = concatenate when used on strings System.out.println( Double.parseDouble(a) + Double.parseDouble(b) ); // Double.parseDouble(a) + Double.parseDouble(b) // = Double.parseDouble("12.34") + Double.parseDouble("56.78") // = 12.34 + 56.78 = 69.12 // Because + = ADD when used on doubles } }

Output:

12.3456.78 69.12

Example Program: (Demo above code)
- Prog file: click here
How to run the program:

Warning: passing bad parameters to methods

It is crucial that you pass correct parameters to methods

When you pass a incorrect parameter to a method, the result can range from:
to:

Warning:

If the numeric string does not contain an integer format number, the parseDouble method will cause the Java program to terminate.
If the numeric string does not contain a double format number, the parseDouble method will cause the Java program to terminate.

Example:

public class ParseInt2 { public static void main(String[] args) { String a = "12x"; // NOT an integer !!! String b = "34"; System.out.println( a + b ); System.out.println( Integer.parseInt(a) + Integer.parseInt(b) ); // CRASH !!! } }

Example Program: (Demo above code)

Prog file: click here

How to run the program:

Right click on link and save in a scratch directory
To compile: javac ParseInt2.java
To run: java ParseInt2

Output:

cheung@HOME2(26)> java ParseInt2 12x34 Exception in thread "main" java.lang.NumberFormatException: For input string: "12x" at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48) at java.lang.Integer.parseInt(Integer.java:456) at java.lang.Integer.parseInt(Integer.java:497) at ParseInt2.main(ParseInt2.java:12)