Building a Linked List in Java

Linked List In the Procedural Paradigm a linked list consisted of: –A pointer to the head of the list –Nodes (in dynamic memory i.e. the heap) containing data and additional next pointers –Modules that would perform a variety of functions as needed: add (in order), traverse, find, delete, etc. In the Object Oriented Paradigm a linked list will consist of a class which contains: –The head pointer (globally scoped in the class) –Methods to perform the function listed above We will also need a class to hold the nodes. We’ll start with that...

Node class Node { StudentRecord data; Node next; public Node(StudentRecord data) { setData(data); setNext(null); } // Constructor public void setData(StudentRecord data) { this.data = data; // !!!!!!!!!!!!!!!!!!!!!!!!!! } public void setNext(Node next) { this.next = next; } public StudentRecord getData() { return data; } public Node getNext() { return next; } public String toString() { if(data == null) return "Node: null"; else return "Node: " + data.toString(); } Comments omitted for clarity!!! Comments omitted for clarity!!!

Are we done? What about the test main???

Remember the main(e) public static void main(String args[]) { StudentRecord sr = new StudentRecord("Bob", 3.5, ); Node n1 = new Node(null); System.out.println("Empty node test\n" + n1); // Load up and print n1.setData(sr); System.out.println("Bob: "+n1); sr = new StudentRecord("Mary", 3.7, ); Node n2 = new Node(sr); n1.setNext(n2); System.out.println("Bob: "+n1); System.out.println("Mary: "+n2); } // main } // Node

Let’s see what’s happening

Tracing StudentRecord sr = new StudentRecord("Bob", 3.5, ); name = "Bob" gpa = 3.5 ssn = sr

Tracing Node n1 = new Node(null); name = "Bob" gpa = 3.5 ssn = sr n1 data next

Tracing n1.setData(sr); name = "Bob" gpa = 3.5 ssn = sr n1 data next

Tracing sr = new StudentRecord("Mary", 3.7, ); name = "Bob" gpa = 3.5 ssn = sr n1 data next name = "Mary" gpa = 3.7 ssn =

Tracing Node n2 = new Node(sr); name = "Bob" gpa = 3.5 ssn = sr n1 data next name = "Mary" gpa = 3.7 ssn = n2 data next

Tracing n1.setNext(n2); name = "Bob" gpa = 3.5 ssn = sr n1 data next name = "Mary" gpa = 3.7 ssn = n2 data next

Key to Understanding Java Understand that the "variables" that you may think of as objects (not primitives) are ALWAYS references (like pointers) to objects which live in the heap. The objects live and die immobile in the heap All the apparent movement of objects is just the moving, copying, setting-to-null of references

Review We have created and tested two classes –class StudentRecord –class Node We now construct the class LinkedList We’ll start with the fields and simple accessors/modifiers Then we’ll write the add, traverse, find and delete methods (plus helpers if necessary)

LinkedList class LinkedList { private Node head; public LinkedList() { setHead(null); } // constructor private void setHead(Node head) { this.head = head; } // setHead private Node getHead() { return head; } // getHead

LinkedList (add method) // Purpose: add in order by SSN // Postcon: list will contain one additional item public void add(StudentRecord sr) { if(getHead() == null || getHead().getData().getSsn() > sr.getSsn() ) { Node temp = new Node(sr); temp.setNext(getHead()); setHead(temp); } else { add(getHead(), sr); } } // add

LinkedList (add helper method) // Purpose: add helper private void add(Node cur, StudentRecord sr) { if( cur.getNext() == null || cur.getNext().getData().getSsn() > sr.getSsn() ) { Node temp = new Node(sr); temp.setNext(cur.getNext()); cur.setNext(temp); } else { add(cur.getNext(), sr); } } // add

LinkedList (traverse ) // Purpose: traverse list // Postcon: no change to list public void traverse() { traverse(getHead()); } // traverse // Purpose: traverse helper private void traverse(Node cur) { if(cur != null) { System.out.println(cur); traverse(cur.getNext()); } } // traverse

LinkedList (traverse ) // Purpose: print out string passed in as parameter // then traverse list (useful for // debugging) // Postcon: No change to list public void traverse(String s) { System.out.println(s); traverse(); } // traverse

LinkedList (traverse iteratively ) // Purpose: traverse iteratively (just shown for // comparison // Postcon: No change to list public void traverseI() { Node cur = getHead(); while(cur != null) { System.out.println(cur); cur = cur.getNext(); } } // traverseI // Purpose: traverse iteratively with title public void traverseI(String s) { System.out.println(s); traverseI(); } // traverseI

LinkedList (find ) // Purpose: Locate record by SSN // Postcon: No change to list public StudentRecord find(int targSsn) { return find(getHead(), targSsn); } // find // Purpose: Find helper private StudentRecord find(Node cur, int targSsn) { if(cur == null) return null; else if(cur.getData().getSsn() == targSsn) { return cur.getData(); } else { return find(cur.getNext(), targSsn); } } // find

LinkedList (deleteFirst) occurence // Purpose: delete first occurence of record with // matching SSN // Postcon: If SSN found record will be removed thus // list will be one shorter public void deleteFirst(int targSsn) { if(getHead() != null) { if(getHead().getData().getSsn() == targSsn) { setHead(getHead().getNext()); } else { deleteFirst(getHead(), targSsn); } } // deleteFirst

LinkedList (deleteFirst) occurence // Purpose: delete first occurence helper private void deleteFirst(Node cur, int targSsn) { if(cur.getNext() != null) { if(cur.getNext().getData().getSsn() == targSsn) { cur.setNext(cur.getNext().getNext()); } else { deleteFirst(cur.getNext(), targSsn); } } // deleteFirst

LinkedList (deleteAll) occurences // Purpose: delete all occurences matching a target // SSN // Postcon: All matching occurences will be // eliminated public void deleteAll(int targSsn) { // (Extra Credit!!!) }

LinkedList (main) public static void main(String args[]) { LinkedList ell = new LinkedList(); ell.traverse("Empty list traversal"); ell.add(new StudentRecord("Adam", 3.0, 333)); ell.add(new StudentRecord("Bozo", 2.0, 222)); ell.add(new StudentRecord("Carl", 1.0, 444)); ell.traverseI("Should be "); ell.add(new StudentRecord("Doug", 0.0, 111)); ell.traverse("Should be "); ell.deleteFirst(222); ell.traverseI("Should be "); ell.deleteFirst(999); ell.deleteFirst(333); ell.deleteFirst(111); ell.deleteFirst(444); ell.traverse("Empty list???"); } // main } // LinkedList

Application /* Demo application to allow user to add, find, list, * and delete student records */ class Application { // Purpose: print menu and get user choice // Postcon: returns choice as int public static int menuChoice() { System.out.println("Enter 1 to add"); System.out.println("Enter 2 to find"); System.out.println("Enter 3 to list"); System.out.println("Enter 4 to delete"); System.out.println("Enter 5 to quit"); return IOGadget.readInt("Choice"); }

Application // Purpose: get information to fill student record // Postcon: returns filled StudentRecord public static StudentRecord getSR() { String name = IOGadget.readLine("Name"); double gpa = IOGadget.readDouble("GPA"); int ssn = IOGadget.readInt("SSN"); return new StudentRecord(name, gpa, ssn); }

Application // print menu and fulfill request public static void menuloop() { LinkedList list = new LinkedList(); int choice; do { choice = menuChoice(); if(choice == 1) list.add(getSR()); else if(choice == 2) { StudentRecord sr = list.find(IOGadget.readInt("SSN")); if(sr == null) System.out.println("Not found"); else System.out.println(sr); }

Application else if(choice == 3) list.traverse("Student List"); else if(choice == 4) { int ssn = IOGadget.readInt("SSN?"); list.deleteFirst(ssn); } else if(choice == 5) System.out.println("Exiting"); else System.out.println("Illegal choice"); } while(choice != 5); } // menuloop

Application public static void main(String args[]) { menuloop(); } } // class Application

Diagram class Application menuloop { LinkedList list } main { menuloop() } class LinkedList LLNode head methods… LinkedList object head class LLNode StuRec data LLNode next methods… LLNode object StuRec data LLNode next LLNode object StuRec data LLNode next Instance of