Java SystemsetIn Examples

In this tutorial, we will learn about the Java System.setIn() function, and learn how to use this function to set the standard input stream, with the help of examples.

setInInputStream in

System.setIn() reassigns the “standard” input stream with specified InputStream object.

By default, Console input is the standard input stream. But, using setIn(), you can set the standard input stream to a InputStream object or object of any subclass of InputStream.

Syntax

The syntax of setIn() function is 

setIn(InputStream in)

where

Parameter Description
in The new standard input stream for System.

Returns

The function returns void.

Example 1 setIn

In this example, we will set a FileInputStream object as the System standard input using setIn(). System can read the input from this input stream.

We will use System.in.readAllBytes() to read the data from this input stream.

The following file input.txt is used to create FileInputStream object which we have passed as argument to setIn() method.

input.txt

Java Program

import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.FileInputStream;  

public class Example {  
	public static void main(String[] args) {  
		try {
			System.setIn(new FileInputStream("input.txt"));
			String content = new String( System.in.readAllBytes() );
			System.out.println(content);
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}      
	}  
}

Output

Hello World!

Example 2 setIn Null InputStream

In this example, we will set a null value, for InputStream object, and set this as the System standard input using setIn(). Since the argument to setIn() is null, the method throws java.lang.NullPointerException.

Java Program

import java.io.FileNotFoundException;
import java.io.IOException;

public class Example {  
	public static void main(String[] args) {  
		try {
			System.setIn(null);
			String content = new String( System.in.readAllBytes() );
			System.out.println(content);
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}      
	}  
}

Output

Exception in thread "main" java.lang.NullPointerException
	at Example.main(Example.java:8)

Conclusion

In this Java Tutorial, we have learnt the syntax of Java System.setIn() function, and also learnt how to use this function with the help of examples.