ArrayList problem

[Lars Jahr]

I've just started learning java, and I'm of course using the book Head First Java

This chapter's topic is ArrayLists but unfortunately I have run into some problems:

1. When I try to run the following code I get an error:

  class ArrayList {
    public static void main(String[] args) {
        ArrayList<String> myList = new ArrayList<String>();
        System.out.println(myList.size());
    }
}
  

The error I get is:

ArrayList.java:3: type ArrayList does not take parameters
ArrayList<String> myList = new ArrayList<String>();
^
ArrayList.java:3: type ArrayList does not take parameters
ArrayList<String> myList = new ArrayList<String>();
^
2 errors

What am I doing wrong here? I've more or less copied the code out of the book.

2. I also get an error when trying to compile this file:

 class ArrayList2 {
    public static void main(String[] args) {
        ArrayList myList = new ArrayList();
        SimpleObject b = new SimpleObject();
 
        myList.add(b);
    }
}
 
class SimpleObject {
    String a = "Hello";
}
   

Error:

ArrayList2.java:6: cannot find symbol
symbol : method add(SimpleObject)
location: class ArrayList
myList.add(b);
^

Why doesn't t his work?

Can anyone give me a hint or two?

[Henry Wong]

What am I doing wrong here? I've more or less copied the code out of the book.

The problem is the "more or less" part...

Basically, you named your class ArrayList, and then tried to use the ArrayList class. When compiling, the compiler is trying to use your ArrayList class, and not the one in the utils package.


The other issue is probably has the same cause. It is trying to use the ArrayList in the same package -- as I am guessing those two classes are in the same directory.

Henry

[Lars Jahr]

Aha, thank you! I'm really not used to java yet, so I just name my classes in the same way I would have done in python (I also forgot the import statement.. )

Well, your hint made my first problem disappear, however I still don't manage to use the add method.

I've rewritten my code to:

 import java.util.ArrayList;
 
class Test2 {
    public static void main(String[] args) {
    
        ArrayList<String> myList = new ArrayList<String>();
       
        SimpleObject b = new SimpleObject();
       
        myList.add(b);
        
        System.out.println(myList.size());
    }
}
 
class SimpleObject {
    String a = "Hello";
}
   

The error message I get now is:

Test2.java:10: cannot find symbol
symbol : method add(SimpleObject)
location: class java.util.ArrayList<java.lang.String>
myList.add(b);
^

Got another tip?

[Jesper de Jong]

You have an ArrayList of String objects (that's what the <String> means). You can only add String objects to the ArrayList, and you are trying to add a SimpleObject - that won't work.

Try making your ArrayList an ArrayList of SimpleObjects:

ArrayList<SimpleObject> list = new ArrayList<SimpleObject>();

[Lars Jahr]

Hehe, it wasn't harder Thank you!

[camilo lopes]

generics is an appeal java 1.5.