// assertions/Queue.java

// (c)2016 MindView LLC: see Copyright.txt

// We make no guarantees that this code is fit for any purpose.

// Visit http://mindviewinc.com/Books/OnJava/ for more book information.

// Demonstration of Design by Contract (DbC)

// combined with white-box unit testing

// (Install libraries from www.junit.org)

import java.util.*;

import org.junit.Test;

import static org.junit.Assert.assertEquals;

import static org.junit.Assert.assertTrue;

import static org.junit.Assert.assertFalse;

public class Queue {

private Object[] data;

private int

in = 0, // Next available storage space

out = 0; // Next gettable object

// Has it wrapped around the circular queue?

private boolean wrapped = false;

public static class

QueueException extends RuntimeException {

public QueueException(String why) { super(why); }

}

public Queue(int size) {

data = new Object[size];

// Must be true after construction:

assert invariant();

}

public boolean empty() {

return !wrapped && in == out;

}

public boolean full() {

return wrapped && in == out;

}

public void put(Object item) {

precondition(item != null, "put() null item");

precondition(!full(), "put() into full Queue");

assert invariant();

data[in++] = item;

if(in >= data.length) {

in = 0;

wrapped = true;

}

assert invariant();

}

public Object get() {

precondition(!empty(), "get() from empty Queue");

assert invariant();

Object returnVal = data[out];

data[out] = null;

out++;

if(out >= data.length) {

out = 0;

wrapped = false;

}

assert postcondition(

returnVal != null, "Null item in Queue");

assert invariant();

return returnVal;

}

// Design-by-contract support methods:

private static void

precondition(boolean cond, String msg) {

if(!cond) throw new QueueException(msg);

}

private static boolean

postcondition(boolean cond, String msg) {

if(!cond) throw new QueueException(msg);

return true;

}

private boolean invariant() {

// Guarantee that no null values are in the

// region of 'data' that holds objects:

for(int i = out; i != in; i = (i + 1) % data.length)

if(data[i] == null)

throw new QueueException("null in queue");

// Guarantee that only null values are outside the

// region of 'data' that holds objects:

if(full()) return true;

for(int i = in; i != out; i = (i + 1) % data.length)

if(data[i] != null)

throw new QueueException(

"non-null outside of queue range: " + dump());

return true;

}

private String dump() {

return "in = " + in +

", out = " + out +

", full() = " + full() +

", empty() = " + empty() +

", queue = " + Arrays.asList(data);

}

// JUnit testing.

// As an inner class, this has access to privates:

public static class WhiteBoxTest {

private Queue queue = new Queue(10);

private int i = 0;

public WhiteBoxTest() {

while(i < 5) // Preload with some data

queue.put("" + i++);

}

// Support methods:

private void showFullness() {

assertTrue(queue.full());

assertFalse(queue.empty());

// Dump is private, white-box testing allows access:

System.out.println(queue.dump());

}

private void showEmptiness() {

assertFalse(queue.full());

assertTrue(queue.empty());

System.out.println(queue.dump());

}

@Test

public void full() {

System.out.println("testFull");

System.out.println(queue.dump());

System.out.println(queue.get());

System.out.println(queue.get());

while(!queue.full())

queue.put("" + i++);

String msg = "";

try {

queue.put("");

} catch(QueueException e) {

msg = e.getMessage();

System.out.println(msg);

}

assertEquals(msg, "put() into full Queue");

showFullness();

}

@Test

public void empty() {

System.out.println("testEmpty");

while(!queue.empty())

System.out.println(queue.get());

String msg = "";

try {

queue.get();

} catch(QueueException e) {

msg = e.getMessage();

System.out.println(msg);

}

assertEquals(msg, "get() from empty Queue");

showEmptiness();

}

@Test

public void nullPut() {

System.out.println("testNullPut");

String msg = "";

try {

queue.put(null);

} catch(QueueException e) {

msg = e.getMessage();

System.out.println(msg);

}

assertEquals(msg, "put() null item");

}

@Test

public void circularity() {

System.out.println("testCircularity");

while(!queue.full())

queue.put("" + i++);

showFullness();

// White-box testing accesses private field:

assertTrue(queue.wrapped);

while(!queue.empty())

System.out.println(queue.get());

showEmptiness();

while(!queue.full())

queue.put("" + i++);

showFullness();

while(!queue.empty())

System.out.println(queue.get());

showEmptiness();

}

}

public static void main(String[] args) {

org.junit.runner.JUnitCore.runClasses(

Queue.WhiteBoxTest.class);

}

}

/* Output:

testNullPut

put() null item

testCircularity

in = 0, out = 0, full() = true, empty() = false, queue =

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

0

1

2

3

4

5

6

7

8

9

in = 0, out = 0, full() = false, empty() = true, queue =

[null, null, null, null, null, null, null, null, null,

null]

in = 0, out = 0, full() = true, empty() = false, queue =

[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

10

11

12

13

14

15

16

17

18

19

in = 0, out = 0, full() = false, empty() = true, queue =

[null, null, null, null, null, null, null, null, null,

null]

testFull

in = 5, out = 0, full() = false, empty() = false, queue =

[0, 1, 2, 3, 4, null, null, null, null, null]

0

1

put() into full Queue

in = 2, out = 2, full() = true, empty() = false, queue =

[10, 11, 2, 3, 4, 5, 6, 7, 8, 9]

testEmpty

0

1

2

3

4

get() from empty Queue

in = 5, out = 5, full() = false, empty() = true, queue =

[null, null, null, null, null, null, null, null, null,

null]

*/