Song database null pointer -

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i'm creating simple music database userinterface.class, song.class, songdatabase.class , playlist.class. song object(artist, name, duration, filesize) input user meant stored in nearest dataslot in songdatabase , i'm receiving null pointer @ user input "filesize"

java.lang.nullpointerexception @ songdatabase.addsong(songdatabase.java:27) @ userinterface.userimport(userinterface.java:143) @ userinterface.main(userinterface.java:61)

i'm having trouble storing refrence of song object in playlist

class's here;

http://pastebin.com/k4hm08su

if @ great thanks.

the songdatabase;

public class songdatabase {

userinterface intface = new userinterface(); song songdata = new song(); private static song a; private static song b; private static song c; private static song d;   public songdatabase() {      = null;     b = null;     c = null;     d = null;   }   public void addsong(string dataartist, string dataname, double dataduration, int datafilesize)                                                      //add song database constructor according nearest free slot// {            if (a==null) {                      setdata(a.songartist, a.songname, a.songduration, a.songfilesize);                       a.songartist=dataartist;                     a.songname=dataname;                     a.songduration=dataduration;                     a.songfilesize=datafilesize;                  } else if (b==null) {                      setdata(b.songartist, b.songname, b.songduration, b.songfilesize);                     b.songartist=dataartist;                     b.songname=dataname;                     b.songduration=dataduration;                     b.songfilesize=datafilesize;                  } else if (c==null) {                      setdata(c.songartist, c.songname, c.songduration, c.songfilesize);                     c.songartist=dataartist;                     c.songname=dataname;                     c.songduration=dataduration;                     c.songfilesize=datafilesize;                  } else if (d==null) {                      setdata(d.songartist, d.songname, d.songduration, d.songfilesize);                     d.songartist=dataartist;                     d.songname=dataname;                     d.songduration=dataduration;                     d.songfilesize=datafilesize;                  } else {                     system.out.println("error: there no free space in database");                 }                  }     public void setdata( string setartist, string setname, double setduration, int setfilesize)                                                     //exchange of user input info. local variables// {     setartist = intface.artist;     setname = intface.name;     setduration = intface.duration;     setfilesize = intface.filesize; }  public static song areturn() {     return a;  }    public static string areturnartist() {     return a.songartist;  }  public static song breturn() {     return b;  }    public static string breturnartist() {     return b.songartist;  }  public static song creturn() {     return c;  }   public static string creturnartist() {     return c.songartist;  }  public static song dreturn() {     return d;  }  public static string dreturnartist() {     return d.songartist;  }  public string removesongdata() {      if(intface.removesong()==0) {         system.out.println(a.songname+" by: "+a.songartist+" has been deleted database..");         = null;     }else if(intface.removesong()==1) {         system.out.println(b.songname+" by: "+b.songartist+" has been deleted database..");         b = null;     }else if(intface.removesong()==2) {         system.out.println(c.songname+" by: "+c.songartist+" has been deleted database..");         c = null;     }else if(intface.removesong()==3) {         system.out.println(d.songname+" by: "+d.songartist+" has been deleted database..");         d = null;     }else{ system.out.println("error: check input choice");         }     return removesongdata();  }  public string dataslotcheck()                                                                                                                   //scan system , return song objects// {     string finalscan;      if (a==null&&b==null&&c==null&&d==null) {         finalscan=("there no music stored in database.");     }     else if (d==null&&c==null&&b==null) {         finalscan=("slot 0:"+a.songartist+":"+a.songname+":"+a.songduration+":"+a.songfilesize);     } else if (d==null&&c==null) {         finalscan= ("slot 0 :"+a.songartist+":"+a.songname+":"+a.songduration+":"+a.songfilesize+         "slot 1 :"+b.songartist+":"+b.songname+":"+b.songduration+":"+b.songfilesize);     } else if (d==null) {         finalscan= ("slot 0 :"+a.songartist+":"+a.songname+":"+a.songduration+":"+a.songfilesize+          "slot 1 :"+b.songartist+":"+b.songname+":"+b.songduration+":"+b.songfilesize+          "slot 2 :"+c.songartist+":"+c.songname+":"+c.songduration+":"+c.songfilesize);     } else {         finalscan= ("slot 0 :"+a.songartist+":"+a.songname+":"+a.songduration+":"+a.songfilesize+          "slot 1 :"+b.songartist+":"+b.songname+":"+b.songduration+":"+b.songfilesize+          "slot 2 :"+c.songartist+":"+c.songname+":"+c.songduration+":"+c.songfilesize+          "slot 3 :"+d.songartist+":"+d.songname+":"+d.songduration+":"+d.songfilesize);     }      return (finalscan);  }

}


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