Casting interfaces in java

Question

I had two separate interfaces, one 'MultiLingual' for choosing language of text to return, and second 'Justification' to justify returned text. Now I need to join them, but I'm stuck at 'java.lang.ClassCastException' error. Class Book is not important here. Data.length and FormattedInt.width correspond to the width of the line. The code :

public interface MultiLingual {
    static int ENG = 0;
    static int PL = 1;
    String get(int lang);
    int setLength(int length);
}

class Book implements MultiLingual {

    private String title;
    private String publisher;
    private String author;
    private int jezyk;

    public Book(String t, String a, String p, int y){

        this(t, a, p, y, 1);
    }

    public Book(String t, String a, String p, int y, int lang){
        title = t;
        author = a;
        publisher = p;
        jezyk = lang;
    }

    public String get(int lang){

        jezyk = lang;
        return this.toString();
    }

    public int setLength(int i){     
        return 0;
    }

    @Override
    public String toString(){

        String dane;
        if (jezyk == ENG){
            dane = "Author: "+this.author+"\n"+
                    "Title: "+this.title+"\n"+
                    "Publisher: "+this.publisher+"\n";
        }
        else {
            dane = "Autor: "+this.author+"\n"+
                    "Tytul: "+this.title+"\n"+
                    "Wydawca: "+this.publisher+"\n";
        }
        return dane;
    }
}


class Data implements MultiLingual {
    private int day;
    private int month;
    private int year;
    private int jezyk;
    private int length;

    public Data(int d, int m, int y){
        this(d, m, y, 1);
    }

    public Data(int d, int m, int y, int lang){
        day = d;
        month = m;
        year = y;
        jezyk = lang;
    }

    public String get(int lang){
        jezyk = lang;
        return this.toString();
    }

    public int setLength(int i){
        length = i;
        return length;
    }

    @Override
    public String toString(){
        String dane="";
        String miesiac="";
        String dzien="";

        switch(month){
            case 1: miesiac="January";
            case 2: miesiac="February";
            case 3: miesiac="March";
            case 4: miesiac="April";
            case 5: miesiac="May";
            case 6: miesiac="June";
            case 7: miesiac="July";
            case 8: miesiac="August";
            case 9: miesiac="September";
            case 10: miesiac="October";
            case 11: miesiac="November";
            case 12: miesiac="December";
        }
        if(day==1){
            dzien="st";
        }
        if(day==2 || day==22){
            dzien="nd";
        }
        if(day==3 || day==23){
            dzien="rd";
        }
        else{
            dzien="th";
        }

        if(jezyk==ENG){
            dane =this.day+dzien+" of "+miesiac+" "+this.year;
        }
        else{
            dane = this.day+"."+this.month+"."+this.year;
        }
        return dane;
    }

}

interface Justification {
    static int RIGHT=1;
    static int LEFT=2;
    static int CENTER=3;

    String justify(int just);

}

class FormattedInt implements Justification {
    private int liczba;
    private int width;
    private int wyrownanie;

    public FormattedInt(int s, int i){
        liczba = s;
        width = i;
        wyrownanie = 2;
    }

    public String justify(int just){

        wyrownanie = just;
        String wynik="";
        String tekst = Integer.toString(liczba);
        int len = tekst.length();
        int space_left=width - len;
        int space_right = space_left;
        int space_center_left = (width - len)/2;
        int space_center_right = width - len - space_center_left -1;
        String puste="";

        if(wyrownanie == LEFT){

            for(int i=0; i<space_right; i++){
                puste = puste + " ";
            }
            wynik = tekst+puste;
        }
        else if(wyrownanie == RIGHT){
            for(int i=0; i<space_left; i++){
                puste = puste + " ";
            }
            wynik = puste+tekst;
        }
        else if(wyrownanie == CENTER){
            for(int i=0; i<space_center_left; i++){
                puste = puste + " ";
            }
            wynik = puste + tekst;
            puste = " ";
            for(int i=0; i< space_center_right; i++){
                puste = puste + " ";
            }
            wynik = wynik + puste;

        }
        return wynik;
    }
}

And the test code that shows this casting "(Justification)gatecrasher[1]" which gives me errors :

  MultiLingual gatecrasher[]={ new Data(3,12,1998),
                               new Data(10,6,1924,MultiLingual.ENG),
                               new Book("Sekret","Rhonda Byrne", "Nowa proza",2007),
                               new Book("Tuesdays with Morrie",
                                        "Mitch Albom", "Time Warner Books",2003,
                                        MultiLingual.ENG),
                             };

  gatecrasher[0].setLength(25);
  gatecrasher[1].setLength(25);

  Justification[] t={ new FormattedInt(345,25),
                      (Justification)gatecrasher[1],
                      (Justification)gatecrasher[0],
                      new FormattedInt(-7,25)
                    };

  System.out.println("         10        20        30");
  System.out.println("123456789 123456789 123456789");
  for(int i=0;i < t.length;i++)
    System.out.println(t[i].justify(Justification.RIGHT)+"<----\n");

Answer 1

Neither Data or book implement Justification and Multilingual does not inherit Justiifcation so it's expected that casts like (Justification)gatecrasher[1] will fail.

Answer 2

You should change your Justification class by adding a new method

String justify(Multilingual m);

Then you can justify the data or book by call the justify method with the instance of book or data.

Answer 3

If you cannot change what you have written, then you might want to consider

1. using the DynamicProxy API. It uses reflection so potentially you'll have to code more.

2. If you know how to map justify() to some algorithm do perform the justification with Multilingual then consider also using Facade pattern. It's similar to DynamicProxy but easier to use.