Convert String to double?

I have three java source files Dog.java, Animal.java, and DogApp.java

Java Code:

public class Dog extends Animal
{
private String breed;
private String sound;

        public Dog()
        {
        super();
        breed = "";
        sound = "";
        }
        public Dog(String n, String w, String l, String c, String b, String s)
        {
        super(n,w,l,c);
        name = n;
        weight = w;
        length = l;
        color = c;
        breed = b;
        sound = s;
        }
        public String getName()
        {
        return name;
        }
        public String getWeight()
        {
        return weight;
        }
        public String getLength()
        {
        return length;
        }
        public String getColor()
        {
        return color;
        }
        public String getBreed()
        {
        return breed;
        }
        public String getSound()
        {
        return sound;
        }
}

Java Code:

import java.util.Scanner;
public class DogApp
{
        public static void main(String [] args)
        {
        String name, weight, breed, length, sound, color;
        Scanner input = new Scanner(System.in);
        System.out.print("Please name your dog: ");
        name = input.next();
        System.out.print("What color is your dog? (One color only): ");
        color = input.next();
        System.out.print("What breed is your dog? (One breed only): ");
        breed = input.next();
        System.out.print("What sound does your dog make?: ");
        sound = input.next();
        System.out.print("What is the length of your dog?: ");
        length = input.next();
        System.out.print("How much does your dog weigh?: ");
        weight = input.next();
        Dog a_dog = new Dog(name, weight, length, color, breed, sound);
        System.out.println("\nThe name of the dog is " + a_dog.getName() + ".");
        System.out.println("The color of the dog is " + a_dog.getColor() + ".");
        System.out.println("The breed of the dog is " + a_dog.getBreed() + ".");
        System.out.println("The sound of the dog is " + a_dog.getSound() + ".");
        System.out.println("The length of the dog is " + a_dog.getLength() + ".");
        System.out.println("The weight of the dog is " + a_dog.getWeight() + ".");
        }

Java Code:

public class Animal extends Object
{
protected String name;
protected String weight;
protected String length;
protected String color;

        public Animal()
        {
        name = "";
        weight = "";
        length = "";
        color = "";
        }
public Animal(String n, String w, String l, String c)
{
name = n;
weight = w;
length = l;
color = c;
}
}

Everything compiles fine, my answers however must be in double format. What is the simplest way to convert these strings to doubles?