YAML Read casts exception

Question

I have an error while reading a file

Exception in thread "main" java.lang.ClassCastException: java.lang.String can not be cast to java.util.ArrayList.

Why does it not occur? I'm using YamlSnake lib

Person.java:

public class Person
{

    int id;
    String fname;
    String lname;
    int age;

    public Person(int id, String fname, String lname, int age)
    {

        this.id = id;
        this.fname = fname;
        this.lname = lname;
        this.age = age;
    }
}

yaml.java:

@Override
public ArrayList<Person> Read() {

    ArrayList<Person> pp = new ArrayList<Person>();
    try 
    {
        FileReader fr = new FileReader("d:\\Person.yml");
        BufferedReader br = new BufferedReader(fr);
        String str=br.readLine();
        Yaml yml = new Yaml();
        pp=(ArrayList<Person>) yml.load(str);
        fr.close();

    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    return pp;
}

Person.yaml:

-id:2 fname:uu lname:uu age:45

Answer 1

You have errors in your YAML syntax. In a block mapping, only one key is allowed per line. Also, there must be a space after both - and : for them to be recognized as YAML special characters. Try:

- id: 2
  fname: uu
  lname: uu
  age: 45

If you want to keep it in one line, you can use a flow map instead:

- {id: 2, fname: uu, lname: uu, age: 45 }

Now the second problem is type erasure. You cannot tell SnakeYaml to load the YAML sequence into an ArrayList<Person> because at runtime, even if you tell SnakeYaml you want to have an ArrayList, it is impossible to determine the generic parameter Person. However, SnakeYaml does provide a possibility to overcome that restriction - alas, it requires you to change your YAML.

Let this be your YAML:

persons:
- id: 2
  fname: uu
  lname: uu
  age:45

Notice that I placed a YAML mapping as root element, which only contains a key persons whose value is the original YAML.

Now, in Java, declare a class that matches this root element:

class Root {
    List<Person> persons;
}

As before, type erasure prevents SnakeYaml to automatically determine the generic parameter of the type of persons at runtime. But now, we can use a TypeDescription to carry that information. Here's the code:

@Override
public List<Person> Read() {
    Constructor c = new Constructor(Root.class);
    TypeDescription td = new TypeDescription(Root.class);
    td.putListPropertyType("persons", Person.class);
    c.addTypeDescription(td);
    Yaml yaml = new Yaml(c);

    try  {
        FileReader fr = new FileReader("d:\\Person.yml");
        Root r = yaml.loadAs(fr, Root.class);
        fr.close();
        return r.persons;
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    return Collections.<Person>emptyList();
}

Beware of bugs in above code, I didn't test it.