Command Line Arguments In Java

Before we dive in to Command Line Arguments In Java , let’s first explore various way of accepting inputs from the user . In Java there are 3 ways to accept input from user :

→ Through Command Line Arguments

→ Using Scanner classes

→ Using GUI Components

In this post we will cover command line arguments and the other two methods will be covered in future Posts.

Command Line Arguments In Java

» Using command line arguments we can accept input when we are about to execute the program .

» This is where the arguments of method main() comes in action.

» Let us take an example to understand this…

 
Case 1:

 class Test
{
 public static void main(String [ ] args)
 {
  System.out.println(“Hello ”+args[0]);
  }
}

 
Case 2:

 
Case 3:

 
Case 4:

 
Case 5:

 
Case 6 ( Passing Integers )

class AddNos
{
 public static void main(String [ ] args)
 {
    System.out.println("First number is "+args[0]);
    System.out.println("Second number is "+args[1]);
    System.out.println("Their sum is "+args[0]+args[1]);
 }
}

Output

C:\Program Files\Java\jdk1.8.0_66\bin>javac AddNos.java
C:\Program Files\Java\jdk1.8.0_66\bin>java AddNos 10 20
First number is 10
Second number is 20
Their sum is 1020
C:\Program Files\Java\jdk1.8.0_66\bin>

Why was the output 1020 ?

» Because anything which we pass from “Command prompt” is by default treated as a String by java.

» Now since java is considering the values 10 and 20 as “10” and “20” , so the operator + concatenated them instead of adding them mathematically.

» To solve this problem we have to convert the values “10” and “20” from String to int and this is done using special classes in java called “Wrapper Classes”

Wrapper Classes

» In java , corresponding to 8 primitive data types we have 8 predefined classes also , called “Wrapper Classes”.

» These classes are available in the package java.lang and have their names similar to the name of data type.

For ex: Integer, Character, Float, Boolean etc

Notice that the first letter in wrapper class name is in uppercase while in case of data type name it is in lowercase.

For ex: byte and Byte, long and Long and so on.

Uses Of Wrapper Classes

Wrapper classes are mainly used for two purposes:

» To represent primitive data types as objects.

» To convert String form of a primitive value to it’s original form, for ex: “10” to 10

Representing primitives as objects

Consider the following statement:

int a=10; // variable a

Here a is a variable initialized to 10

But if we want we can convert it into an object by using the Wrapper Class Integer, as shown below:

Integer obj=a; // a converted to object

Converting String To Primitive

» Another importance of wrapper classes is that they contain special methods which perform conversion from String to primitive data type.

» These methods have their name as parseXXX where XXX is the name of primitive type.

» Also they are static in nature, so they can be directly called by their class name.

For Example :- Integer.parseInt(“……”);

List Of Wrapper Classes

Addition Of Number Using Wrapper Classes

class AddNos
{
public satic void main(String args[])
{
int a,b,c;
a=Integer.parseInt(args[0]);
b=Integer.parseInt(args[1]);
c=a+b;
Syetem.out.println(“First number is ”+a);
Syetem.out.println(“Second  number is ”+b);
Syetem.out.println(“Their sum is ”+c);
}
}

Output

C:\Program Files\Java\jdk1.8.0_66\bin>javac AddNos.java
C:\Program Files\Java\jdk1.8.0_66\bin>java AddNos 10 20
First number is 10
Second  number is 20
Their sum is 30
C:\Program Files\Java\jdk1.8.0_66\bin>

Guess the output ?

class AddNos
{
public satic void main(String args[])
{
int a,b,c;
a=Integer.parseInt(args[0]);
b=Integer.parseInt(args[1]);
c=a+b;
Syetem.out.println(“First number is ”+a);
Syetem.out.println(“Second  number is ”+b);
Syetem.out.println(“Their sum is ”+c);
}
}

During the execution if we pass integer value as a first input and String as a second input

Running:

bin> java AddNos 10 Varanasi

Output:

C:\Program Files\Java\jdk1.8.0_66\bin>javac AddNos.java
C:\Program Files\Java\jdk1.8.0_66\bin>java AddNos 10 Varanasi
Exception in thread "main" java.lang.NumberFormatException: For input string: "Varanasi"
        at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
        at java.lang.Integer.parseInt(Integer.java:580)
        at java.lang.Integer.parseInt(Integer.java:615)
        at AddNos.main(AddNos.java:7)
C:\Program Files\Java\jdk1.8.0_66\bin>

Why did Exception occur ?

» Because the method parseInt( ) can only work with Strings containing digits.

» If any String contains non integer values then the method parseInt( ) will throw Exception.

» Even if we pass 20.5 , then also it will throw “NumberFormatException”

Output:

C:\Program Files\Java\jdk1.8.0_66\bin>java AddNos 10 20.5
Exception in thread "main" java.lang.NumberFormatException: For input string: "20.5"
        at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
        at java.lang.Integer.parseInt(Integer.java:580)
        at java.lang.Integer.parseInt(Integer.java:615)
        at AddNos.main(AddNos.java:7)
C:\Program Files\Java\jdk1.8.0_66\bin>

How to accept decimal values?

» So if we want to accept decimal values , then we must use the method parseFloat( ) or parseDouble( ).

» They accept decimal/integer both kinds of values and throw Exception only if the given values is non-numeric like “bhopal”, “10a” etc

Command Line Arguments In Java from Oracle’s Official Website