OnJava8-Examples/assertions/Queue.java

// assertions/Queue.java
// 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];
    assert invariant(); // Must be true after construction
  }
  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]
*/