CODEWITHSUNDEEP
java
Hien Le
Hien Le

Reputation: 301

How to run java class with custom variables from terminal

My scenario:

I have a Main.java file that simply does System.out.println("Hello"). I run it by first, compiling with javac Main.java and then excecuting the command java Main.

Now what I want is that instead of printing "Hello", it will print whatever the user wants, but I don't want to change the source code whenever I want a different output. So I replaced the System.out.println("Hello") with System.out.println(${MESSAGE}). But this gives error "Cannot resolve symbol MESSAGE".

Ultimately, I want a Main.class file and run with something like java Main -env MESSAGE=whateverIPutHere and it should print out whateverIPutHere.

Is it possible?

Upvotes: 1

Views: 2620

Answers (5)

Rostislav Matl
Rostislav Matl

Reputation: 4553

You can either read it from args as mentioned above, or, if you know how to add a library to your project, try args4j. You'll get way cleaner code as you can use it to separate commandline argument processing to a dedicated class.

Upvotes: 1

Nandakumar
Nandakumar

Reputation: 11

In this example, we are printing all the arguments passed from the command-line. For this purpose, we have traversed the array using for loop. The arguments passed in command line is captured by args argument.

class test{  
        public static void main(String args[]){  

            for(int i=0;i<args.length;i++)  
                System.out.println(args[i]);  

        }  
}

compile by > javac test.java

run by > java test sonoo jaiswal 1 3 abc

Output:

sonoo

jaiswal

1

3

abc

Upvotes: 1

Almas Abdrazak
Almas Abdrazak

Reputation: 3632

You can use system properties

public final class Test {
    public static void main(String[] args) {
        System.out.println(System.getProperty("port") + " port");
    }
}

And then compile and run

javac Test.java
java -Dport=8080 Test

Output is : 8080 port

Upvotes: 3

GhostCat
GhostCat

Reputation: 140631

Now what I want is that instead of printing "Hello", it will print whatever the user wants, but I don't want to change the source code.

Simply not possible without changing code. 

System.out.println("Hello")

Prints that string. End of story. And:

System.out.println(${MESSAGE})

is simply not valid Java. If you want to read an environment variable, see here how to do that.

But then: that is really a detour here. You can simply pass arguments on the command line:

java Main "some string" "and another one"

and then retrieve those two strings via the String args[] parameter that your main method receives!

The real answer here: you learn a new language by researching how that language works. You don't assume how syntax might look like, based on experiences from other languages. Meaning: $ENV_VAR is a "shell language" concept. Your idea: "maybe Java has the same" is a very inefficient strategy to go about this.

Upvotes: 2

Patric
Patric

Reputation: 1637

You can use the input arguments: 

    public static void main(String[] args) {
        System.out.println(args[0]);
    }

And then call it like this: java Main whateverIPutHere

Simple as that! args is an array containing all the arguments that you pass in the command line.

Upvotes: 1

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