Demonstration of Design by Contract (DBC) combined with white-box unit testing
// : c15:JavaQueue.java
// Demonstration of Design by Contract (DBC) combined
// with white-box unit testing.
// {Depends: junit.jar}
// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// www.BruceEckel.com. See copyright notice in CopyRight.txt.
import junit.framework.*;
import java.util.*;
public class JavaQueue {
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 JavaQueue(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 extends TestCase {
private JavaQueue queue = new JavaQueue(10);
private int i = 0;
public WhiteBoxTest(String name) {
super(name);
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());
}
public void testFull() {
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();
}
public void testEmpty() {
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();
}
public void testNullPut() {
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");
}
public void testCircularity() {
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) {
junit.textui.TestRunner.run(JavaQueue.WhiteBoxTest.class);
}
} ///:~
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