Question

let’s say I have

class Dummy {
    String a, b;
    public Dummy(String a, String b) {
        this.a = a;
        this.b = b;
    }
    public String toString(){
        return a+b;
    }
    public String getA() {
        return a;
    }
    public String getB() {
        return b;
    }

}

i would like to have

List<Dummy> myList = new ArrayList<Dummy>() {{
     add(new Dummy("test", ""));
     add(new Dummy("boo", "o"));
}};

System.out.println( myList.toString());
System.out.println( myList.getAs());  //should return ["test", "boo"]
System.out.println( myList.getBs());//should return ["", "o"]

If you see what I mean

probably have to create class extending ArrayList<Dummy> ?

edit:

seems good like that

class Dummy {
    String a, b;
//...
    public static String toAString(List<Dummy> l){
       String s="";
       for (Dummy d : l){
           s+=d.a;
       }
       return s;
    }
}

edit2:

I will have just 2 Strings in Dummy, is it better doing a ArrayList<String []> rather ? in terms of memory

Answer 1

Define first-class functions corresponding to getA and getB. I am using Function from Guava for that.

class Dummy {
    String a, b;
    public Dummy(String a, String b) {
        this.a = a;
        this.b = b;
    }
    public String toString(){
        return a+b;
    }
    public String getA() {
        return a;
    }
    public String getB() {
        return b;
    }
    public static Function<Dummy, String> getA = new Function<Dummy, String>() {
      public String apply(Dummy d) {
        return d.a;
      }
    }
    public static Function<Dummy, String> getB = new Function<Dummy, String>() {
      public String apply(Dummy d) {
        return d.b;
      }
    }
}

This is how you can use it: (Iterables below is also from Guava).

List<Dummy> myList = new ArrayList<Dummy>() {{
     add(new Dummy("test", ""));
     add(new Dummy("boo", "o"));
}};

System.out.println( myList.toString());
System.out.println( Iterables.transform(myList, Dummy.getA)); // returns ["test", "boo"]
System.out.println( Iterables.transform(myList, Dummy.getB)); // returns ["", "o"]