cant find API method problems

[Ben Hultin]

I am running into problems about API methods which I included the import java.util for it. Seems it is still not found, either that or I have set up the usage incorrectly.

.\Mouse.java:40: cannot find symbol
symbol : method length()
location: class java.util.ArrayList<Mouse>
int mouse2 = random.nextInt(mice.length());
^


.\Mouse.java:39: cannot find symbol
symbol : variable random
location: class Mouse
int mouse1 = random.nextInt(mice.length());

My Mouse java file:

import java.util.*;
public class Mouse extends Mammal{
public Mouse (int a, double w) {
// mouse constructor: (default constructor)
// Randomly choose sex and assign to isMale as appropriate.
int sexSelected = (int) (Math.random() * 1);
if (sexSelected == 1) {
isMale = true;
} else {
isMale = false;
}
// Set age to 1.
age = 1;
// Set weight to 1.
weight = 1.0;
}
public void setSex(boolean newIsMale)
{
isMale = newIsMale;
}
public void grow () {
// Increase age of mouse by 1 and weight of mouse by 1% of current weight.
age += 1;
weight += (weight * 0.01);
} // end grow()
public void mate (ArrayList<Mouse> mice) {
// Randomly choose 2 mice objects from arraylist
int mouse1 = random.nextInt(mice.length());
int mouse2 = random.nextInt(mice.length());
boolean mouse1Sex = mice.getIsMale(mouse1);
boolean mouse2Sex = mice.getIsMale(mouse2);
if (mouse1Sex != mouse2Sex) { // 1 male, 1 female mouse, Both mice older than 1 day
// If successful mating, randomly create between 0-4 mice and append to arraylist received as argument.
int offspring = (int) (Math.random() * 4);
for (int i = 0; i <= offspring; i++) {
mice.add(new Mouse());
} // end for loop in mate()
} // end if statement in mate()
} // end mate()
} // end Mouse class

I appreciate the help

[Jesper de Jong]

You have two different errors here.

As you can see in the API documentation, class ArrayList does not have a method called length(). The method you should use to find out how many elements an ArrayList has is called size().

The second error happens because the variable named random is not in scope at the place you're trying to use it. In fact, you haven't declared a variable called random anywhere. You're probably missing a line:

Random random = new Random();

(where Random is from the java.util package).

[Ben Hultin]

Thanks a lot for the help. I am not sure if it was obvious but I was meaning for random to have the Math.random() property.

I got the syntax you see from a tutorial online that was suppose to serve this very purpose of randomly selecting an element from the Arraylist.

here is another idea I had:

selectedMouse = (int) Math.random.nextInt(mice.size());

Hopefully I am getting somewhere

[Jesper de Jong]

selectedMouse = (int) Math.random.nextInt(mice.size());

Math is a class. Class Math does not have a member variable called random, so this line of code will not compile. See the API documentation for class Math.

There are two ways to make random numbers with the standard Java classes: either use the Random class, or use the random() method (not a variable, or "property") of class Math. You seem to be mixing those two things up in the above line of code. (nextInt() is a method of class Random).

[David Newton]

Also, contrast this:

if (sexSelected == 1) {
isMale = true;
} else {
isMale = false;
}

with this:

isMale = (sexSelected == 1);

It's not a big deal, and obviously logically it's the same, but think about readability, and keeping things concise. Learning these little tricks now will be helpful in the long run. And everyone will think you're cool.

(I'm probably wrong about that last part, if my own life is any indication.)

Using a number to indicate gender is considered "magic"--there's nothing intrinsically male about the number 1 (well... I guess if we use our imagination maybe there is). In this case, male is 1, female is 0 (outraging feminists everywhere). It might make more sense, or at least be more clear, if there were constants:

private static int FEMALE = 0;
private static int MALE = 1;

That, in turn, leads me to think that it would be even better to just encapsulate the gender selection into its own method:

public int generateGender() { // I'd prefer "genderate", but people yell at me when I get clever.
return (int) (Math.random() * 1);
}
// ...
isMale = (generateGender() == MALE);

But this doesn't actually buy us very much, leading me to believe that the gender would be best expressed not as a boolean (isMale) but simply as a value, gender. Ideally this would be an enum, buying us all sorts of type safety and clarity. It would also allow the gender generation to be wrapped up in the Gender enum, perhaps as Gender.generateRandomGender().

Lastly, consider the following fragment:

// Set age to 1.
age = 1;
// Set weight to 1.
weight = 1.0;

Ask yourself how much information those comments really add. Since you've named your variables appropriately, you're basically saying the same thing twice, but not really increasing the signal-to-noise ratio.

I know, more than you wanted or needed :)