92 lines
2.6 KiB
Java
92 lines
2.6 KiB
Java
// validating/CircularQueue.java
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// (c)2017 MindView LLC: see Copyright.txt
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// We make no guarantees that this code is fit for any purpose.
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// Visit http://OnJava8.com for more book information.
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// Demonstration of Design by Contract (DbC)
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package validating;
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import java.util.*;
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public class CircularQueue {
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private Object[] data;
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private int
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in = 0, // Next available storage space
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out = 0; // Next gettable object
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// Has it wrapped around the circular queue?
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private boolean wrapped = false;
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public CircularQueue(int size) {
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data = new Object[size];
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// Must be true after construction:
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assert invariant();
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}
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public boolean empty() {
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return !wrapped && in == out;
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}
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public boolean full() {
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return wrapped && in == out;
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}
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public boolean isWrapped() { return wrapped; }
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public void put(Object item) {
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precondition(item != null, "put() null item");
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precondition(!full(),
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"put() into full CircularQueue");
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assert invariant();
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data[in++] = item;
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if(in >= data.length) {
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in = 0;
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wrapped = true;
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}
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assert invariant();
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}
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public Object get() {
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precondition(!empty(),
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"get() from empty CircularQueue");
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assert invariant();
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Object returnVal = data[out];
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data[out] = null;
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out++;
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if(out >= data.length) {
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out = 0;
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wrapped = false;
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}
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assert postcondition(
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returnVal != null,
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"Null item in CircularQueue");
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assert invariant();
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return returnVal;
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}
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// Design-by-contract support methods:
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private static void
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precondition(boolean cond, String msg) {
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if(!cond) throw new CircularQueueException(msg);
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}
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private static boolean
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postcondition(boolean cond, String msg) {
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if(!cond) throw new CircularQueueException(msg);
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return true;
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}
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private boolean invariant() {
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// Guarantee that no null values are in the
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// region of 'data' that holds objects:
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for(int i = out; i != in; i = (i + 1) % data.length)
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if(data[i] == null)
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throw new CircularQueueException(
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"null in CircularQueue");
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// Guarantee that only null values are outside the
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// region of 'data' that holds objects:
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if(full()) return true;
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for(int i = in; i != out; i = (i + 1) % data.length)
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if(data[i] != null)
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throw new CircularQueueException(
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"non-null outside of CircularQueue range: "
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+ dump());
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return true;
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}
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public String dump() {
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return "in = " + in +
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", out = " + out +
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", full() = " + full() +
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", empty() = " + empty() +
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", CircularQueue = " + Arrays.asList(data);
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}
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}
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