Question

JAVA LANG PLEASE:

I have follwed these below guidelines but when i run my queue test it is not executing but my stack is working fine, can you fix it please!

MyQueue.java

Implement a queue using the MyStack.java implementation as your data structure.  In other words, your instance variable to hold the queue items will be a MyStack class.

• enqueue(String item): inserts item into the queue
• dequeue(): returns and deletes the first element in the queue
• isEmpty(): returns true or false depending on if the queue is empty
• printQueue(): prints the items in the queue to console
• MyQueue(String[] list): constructor which creates the queue with the items in list in the queue: So if list had {“a”, “b”, “c”} the queue would look like: “a” first, then “b”, then “c.”

MyTest.java

• You can have other methods in this class but will be graded to have a:
• Main method which tests every method you implemented above in MyStack and MyQueue.
• As you test the methods, print to console (1) what you are testing, (2) what value you are expecting, (3) whether your test passed or failed.
• Test for edge cases and clearly state in your comment/print out which edge case you’re testing.

MyStack.java code:

5 public class MyStack {
6 public static class Node {
7 String data;
8 Node next;
9 }
10
11 private Node head;
12
13 public MyStack(String[] list) {
14 head = null;
15 for (int i = 0; i < list.length; i++) {
16 push(list[i]);
17 }
18 }
19
20
21 public void push(String item) //push a given value onto the stack
22 {
23 Node node = new Node();
24 node.data = item;
25
26 //checking for edge cases in case empty
27 if(head == null){
28 head = node;
29 }
30
31 else{
32 node.next = head;
33 head = node;
34 }
35 }
36
37 public boolean isEmpty() //returns true or false depending on if the stack is empty or not
38 {
39 return head == null;
40 }
41
42 /* public String peek() {
43 return head.data;
44 //System.out.println();
45 }*/
46
47 /* public String pop()
48 {
49 if(head !=null){
50 result= head.data;
51 head= head.next;
52 }
53 return result; */
54
55 public String pop() //return and removes the last value on the stack
56 {
57 if(head ==null){
58 System.out.printf("empty");
59 }
60
61 if (head!= null){
62 String t = head.data;
63 }
64
65 return head.data;
66
67 }
68
69 public void printStack() {
70 String result = "";
71 Node temp = head;
72 while (temp != null) {
73 result += temp.data + " ";
74 temp = temp.next;
75 }
76 System.out.println(result);
77 }
78 }
79
80 class MyStackTest {
81
82 public static void main(String[] args) {
83 MyStack stack = new MyStack(new String[] {"a","d", "b", "c"});
84 System.out.println("The elements of the list are: ");
85 stack.printStack();
86 System.out.println("pop an element:");
87 System.out.println(stack.pop());//checking pop method
88 System.out.println("pushing an element:");
89 stack.push("e");
90 System.out.println("now the new list is:");
91 stack.printStack();
92 System.out.println("now pop last element:");
93 System.out.println(stack.pop());
94 }
95
96 }

MyQueue.java code

1 public class MyQueue {
2
3 // Primary stack will always be used for dequeue operations
4 // Secondary stack will be used as helper.
5 private MyStack primary, secondary;
6
7 public MyQueue(String[] list) {
8 primary = new MyStack(new String[] {});
9 secondary = new MyStack(list);
10
11 while(!secondary.isEmpty()) {
12 primary.push(secondary.pop());
13 }
14 }
15
16 public void enqueue(String item) {
17 // first make primary empty
18 while(!primary.isEmpty()) {
19 secondary.push(primary.pop());
20 }
21 primary.push(item);
22
23 // again put items back from secondary
24 while(!secondary.isEmpty()) {
25 primary.push(secondary.pop());
26 }
27 }
28 public String dequeue() {
29
30 if(isEmpty()) {
31 return null;
32 }
33
34 return primary.pop();
35 }
36
37 public boolean isEmpty() {
38 return primary.isEmpty();
39 }
40
41 public void printQueue() {
42 primary.printStack();
43 }
44
45 public static void main(String[] args) {
46 MyQueue mq = new MyQueue(new String[] {"a", "b", "c"});
47 mq.printQueue();
48 System.out.println(mq.dequeue());
49 mq.enqueue("d");
50 System.out.println(mq.dequeue());
51 System.out.println(mq.dequeue());
52 System.out.println(mq.dequeue());
53 }
54
55 }

Answer 1

MyQueue.java

import java.util.Arrays;

public class MyQueue {

    // Primary stack will always be used for dequeue operations
    // Secondary stack will be used as helper.
    private MyStack primary, secondary;

    public MyQueue(String[] list) {
        primary = new MyStack(new String[]{});
        secondary = new MyStack(list);
        //System.out.println( secondary.isEmpty() );

        while (!secondary.isEmpty()) {
            primary.push(secondary.pop());
        }
        //System.out.println( primary.isEmpty() );
    }

    public void enqueue(String item) {
        // first make primary empty
        while (!primary.isEmpty()) {
            secondary.push(primary.pop());
        }
        primary.push(item);

        // again put items back from secondary
        while (!secondary.isEmpty()) {
            primary.push(secondary.pop());
        }
    }

    public String dequeue() {

        if (isEmpty()) {
            return null;
        }

        return primary.pop();
    }

    public boolean isEmpty() {
        return primary.isEmpty();
    }

    public void printQueue() {
        primary.printStack();
    }

    public static void main(String[] args) {
        String[] testArray = new String[]{"a","b","c"};
        MyQueue mq = new MyQueue(testArray);
        mq.printQueue();
        System.out.println(mq.dequeue());
        mq.enqueue("d");
        System.out.println(mq.dequeue());
        System.out.println(mq.dequeue());
        System.out.println(mq.dequeue());
    }

}

MyStack.java

class MyStack {
    public static class Node {
        String data;
        Node next;
    }

    private Node head;

    MyStack(String[] list) {
        head = null;
        for (String aList : list) {
            push(aList);
        }
    }


    void push(String item) //push a given value onto the stack
    {
        Node node = new Node();
        node.data = item;

        //checking for edge cases in case empty
        if (head == null) {
            head = node;
        } else {
            node.next = head;
            head = node;
        }
    }

    boolean isEmpty() //returns true or false depending on if the stack is empty or not
    {
        return head == null;
    }

    String pop() //return and removes the last value on the stack
    {
        if (head == null) {
            System.out.print("empty");
        }

        /** here is the change which you were missing 
         *  you were not changing the head therefore it went's into infinite loop
         *  also you were sending head.data instead of String variable t
         * 
          */ 
        String t = "";
        if (head != null) {
            t = head.data;
            head = head.next;
        }
        return t;

    }

    void printStack() {
        StringBuilder result = new StringBuilder();
        Node temp = head;
        while (temp != null) {
            result.append(temp.data).append(" ");
            temp = temp.next;
        }
        System.out.println(result);
    }
}

class MyStackTest {

    public static void main(String[] args) {
        MyStack stack = new MyStack(new String[]{"a", "d", "b", "c"});
        System.out.println("The elements of the list are: ");
        stack.printStack();
        System.out.println("pop an element:");
        System.out.println(stack.pop());//checking pop method
        System.out.println("pushing an element:");
        stack.push("e");
        System.out.println("now the new list is:");
        stack.printStack();
        System.out.println("now pop last element:");
        System.out.println(stack.pop());
    }

}

//Look at String pop() method in at line 42

//small thing which you were missing

//Here's the output

a b c
a
b
c
d