SpaceBoxing.java

import java.util.Scanner;

public class SpaceBoxing
{
    public static void main( String[] args )
    {
        Scanner keyboard = new Scanner(System.in);
        double weight;
        int whichplanet;
        
        System.out.print( "Please enter your current earth weight: " );
        weight = keyboard.nextDouble();
        
        System.out.println();
        System.out.println( "I have information for the following planets:" );
        System.out.println( "\t1. Venus   2. Mars    3. Jupiter" );
        System.out.println( "\t4. Saturn  5. Uranus  6. Neptune" );
        
        System.out.print( "Which planet are you visiting? " );
        whichplanet = keyboard.nextInt();
        
        if ( whichplanet == 1 )
        {
            System.out.println( "Your weight would be " + weight * 0.78 + " on that planet." );
        }
        if ( whichplanet == 2 )
        {
            System.out.println( "Your weight would be " + weight * 0.39 + " on that planet." );
        }
        if ( whichplanet == 3 )
        {
            System.out.println( "Your weight would be " + weight * 2.65 + " on that planet." );
        }
        if ( whichplanet == 4 )
        {
            System.out.println( "Your weight would be " + weight * 1.17 + " on that planet." );
        }
        if ( whichplanet == 5 )
        {
            System.out.println( "Your weight would be " + weight * 1.05 + " on that planet." );
        }
        if ( whichplanet == 6 )
        {
            System.out.println( "Your weight would be " + weight * 1.23 + " on that planet." );
        }
    }
}

Have you introduced associative arrays / maps by this point?