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// we need to import the Scanner, if we
// don't import it, we cannot use it.
import java.util.Scanner;

class jamesprogram {

	public static void main(String args[]){
		
		// make variable that will hold the input from
		// the keyboard. So whatever we input into
		// the system, will be stored in the variable 
		// froggy
		Scanner froggy = new Scanner(System.in);
		
		// declare 3 double variables
		double firstnum,secondnum,answer;
		
		System.out.println("Enter first number: ");
		// nextDouble() is a method that stores whatever
		// number is typed in into firstnum
		firstnum = froggy.nextDouble();
		System.out.println("Enter second number: ");

		// nextDouble() is a method that stores whatever
		// number is typed in into secondnum
		secondnum = froggy.nextDouble();
		
		answer = firstnum+secondnum;
		
		System.out.println(answer);
	}
}

This outputs:

Enter first number:
10.3
Enter second number:
2.9
13.200000000000001