data structures - stack and queues using doubly linked list in java -

i have problems code. read file in code , build 1 stack , 1 queues structure. code wasn't run correctly.

this node class used double linkedlist

public class node { string data; node next; node prev;  public node(string data,node next, node prev){      this.next=next;     this.data=data;     this.prev=prev;  } public node(){  }  public string getdata(){      return data; }  public void setdata(string data){      this.data=data; }  public node getnext(){      return next; }  public void setnext(node next){     this.next=next; }  public node getprev(){      return prev; }  public void setprev(node prev){     this.prev=prev; }      } 

** stack class. **

public class stack {  node head = null; node tail = null;  int size=0;        public int getsize() {      return size;        }     public boolean isempty()        {     return head == null;     }       public void push(string data) {       tail = head;     head = new node(data,null,null);     head.data=data;     head.next= tail;     head.prev = null;       if(tail != null) {         tail.prev=head;     }      size++;    }    public void pop() {     if (!isempty()) {         head = head.next;   // delete first node         size--;     } else {         system.out.println("İs empty");     }  }     public void top() {      node tmp = head;     while (tmp != null) {         system.out.println(tmp.getdata());         tmp = tmp.getnext();     } }   } 

this queues class

    public class oueues {       node head ;     node tail;    int size=0;  public oueues(){     this.head=null;     this.tail=null; }   public boolean isempty() {     return head == tail; }        public int getsize()      {            return size;    }        public void insert(string data){      node tmp = new node(data,null,null);      tmp.data=data;      tmp.next=null;       if(head==null){          head=tail=tmp;          head.prev=null;        }      else{          tail.next=tmp;          tmp.prev=tail;          tail=tmp;        }      }    public string remove(){     if(head.next==tail)        return null;// list empty    node tmp=head.next;    head.next=tmp.next;    tmp.next.prev=head;    list();    return tmp.data;      }    public void list(){       system.out.println("queues");       if(size==0){           system.out.println("İs empty");        }      node tmp=head;      while(tmp !=tail.getnext()){          system.out.println(tmp.getveri()+" ");        tmp= tmp.getnext();      }       system.out.println();   }          } 

this queues class

    import java.io.bufferedreader;     import java.io.file;     import java.io.filereader;     import java.io.filewriter;     import java.io.ioexception;     import java.util.scanner;      public class ogrenci {      public static void main(string[] args) throws ioexception {      scanner s = new scanner(system.in);     stack y = new stack();     oueues k = new oueues();     filewriter fwy;     filewriter fwk;      file stack = new file("stack.txt");      if (!stack.exists()) {         stack.createnewfile();     } else {         system.out.println("already exists ");     }      bufferedreader reader = null;     reader = new bufferedreader(new filereader(stack));     string line = reader.readline();      while (line != null) {          y.push(line = reader.readline());         system.out.println(line);     }      file queue = new file("queue.txt");      if (!queue.exists()) {         queue.createnewfile();     } else {         system.out.println("already exists ");     }      bufferedreader read = null;     read = new bufferedreader(new filereader(queue));     string lines = read.readline();      while (lines != null) {          lines = read.readline();         k.insert(lines);         system.out.println(lines);     }      int choice;        system.out.println("1. stack out- queue add");       system.out.println("2. stack add- queue out");       system.out.println("3. stack , queue ");       system.out.println("4. file writer");       choice = s.nextint();      switch (choice) {         case 1:              k.insert(s.next());             k.list();             y.pop();              break;         case 2:           y.push(s.next());             y.top();           k.remove();               break;         case 3:              y.top();             k.list();             break;         case 4:              fwy = new filewriter(stack);             node no = y.head;             while (no.next != null) {                 fwy.write("\n" + no.data);                 no = no.next;             }             fwy.flush();             fwy.close();              fwk = new filewriter(queue);             node noo = k.head;             while (noo.next != null) {                 fwk.write("\n" + noo.data);                 noo = noo.next;             }              fwk.flush();             fwk.close();             break;          }       } 

ok, have couple of problems. i'm going point out few , let work fix rest because looks assignment , don't want homework :).

first, when read file careful not ignore first element:

string line = reader.readline();      while (line != null)     {         system.out.println("read stack: " + line);          // read 1 element         y.push(line);         line = reader.readline();     } 

notice unlike solution first push y.push(line) don't forget add whatever read line. same goes queue file:

string lines = read.readline();      while (lines != null)     {         system.out.println("read queue: " + lines);         // read 1 line         k.insert(lines);         lines = read.readline();     } 

just add if it's not null , read next line. missing on first element file.

another problem queues class (which way misspelled should replace o q). 1 not working because you forgot increment , decrement size when insert or remove.

public void insert(string data){      node tmp = new node(data,null,null);     tmp.data=data;     tmp.next=null;      if(head==null){         head=tail=tmp;         head.prev=null;     }     else{         tail.next=tmp;         tmp.prev=tail;         tail=tmp;     }     size++; } 

notice @ end of insert i'm increasing size list method doesn't throw nullpointerexception every time call it. same goes remove method:

public string remove(){      if(head == null)         return null;// list empty     node tmp=head.next;     head.next=tmp.next;     tmp.next.prev=head;     size--;     list();     return tmp.data; } 

please notice check before (if(head.next==tail)) throwing nullpointerexception because @ beginning head null cannot access next member. i've made small improvement list method return earlier:

public void list(){     system.out.println("queues");     if(size==0){         system.out.println("İs empty");         return;     }      node tmp=head;     while(tmp != tail.getnext()){         system.out.println(tmp.getdata() + " ");         tmp= tmp.getnext();     }     system.out.println(); } 

notice return if queue empty, otherwise attempt tail.getnext() throw nullpointerexception.

some important thoughts code in general please avoid weird naming. why queues? there 1 should queue. please avoid weird variable names. code not you, chances else might need read , gets hard know s, y, k, fwy , fwk. why not naming them this:

scanner scanner = new scanner(system.in); stack stack = new stack(); queues queue = new queues(); filewriter stackfilewriter; filewriter queuefilewriter; 

and same goes methods . why push, pop , top methods start upper-case letter ? if don't agree standard java naming convention fine @ least consistent :).

try suggested improvements , see how program it's working. i'm sure there more problems it. if can't figure them out leave comment , you. luck!