我正在做week 3 Assignment of Princeton Algorithm Part I。最后一部分是我对类BruteCollinearPoints的编码。我无法通过以下测试。该测试表明，在我新建对象BruteCollinearPoints之后，它对输入数组points进行了突变，并发现更改了我的对象BruteCollinearPoints的实例变量。
我当然知道避免这种失败的直接方法是将points的每个元素复制到另一个arrayNew。然后对arrayNew执行操作。但是我想知道使我的代码错误的具体示例。由于结果将直接指向不可变的Point对象，因此如何对Point[] points进行突变才能更改结果？我在IDE中尝试了许多方法，但无法重现此错误。我真的希望有人可以向我展示可靠的示例来使此错误发生，并使我对不可变数据类型有更深入的了解。谢谢。
它给我一种如下感觉：
Item[] array = new Item[]{item0, item1, item2, item3} // Item is some immutable type.
Item[] array2 = new Item[2];
array2[0] = array[2];
array2[1] = array[1];
.....// Then doing some operation on ``array``
.....// Then my array2[0] array2[1] will change.
怎么可能？我的代码中应该有一些漏洞，但我看不到。我真的很想了解这个漏洞，而不是直接使用另一个绝对权利绕过它。
===========
Test 10: check that data type is immutable by testing whether each method
returns the same value, regardless of any intervening operations
* input8.txt
- failed after 11 operations involving BruteCollinearPoints
- first and last call to segments() returned different arrays
- sequence of operations was:
BruteCollinearPoints collinear = new
BruteCollinearPoints(points);
collinear.segments()
mutate array returned by last call to segments()
mutate array returned by last call to segments()
collinear.numberOfSegments() -> 2
mutate array returned by last call to segments()
collinear.numberOfSegments() -> 2
collinear.numberOfSegments() -> 2
mutate points[] array that was passed to constructor
mutate array returned by last call to segments()
collinear.segments()
- failed on trial 1 of 100
* equidistant.txt
- failed after 22 operations involving BruteCollinearPoints
- first and last call to segments() returned different arrays
- failed on trial 1 of 100
==> FAILED
=========================
import java.util.Arrays;
public class BruteCollinearPoints {
private final LineSegment[] lineResults;
private final int numOfSeg;
public BruteCollinearPoints(Point[] points) {
if (points == null || points.length == 0) {
throw new IllegalArgumentException();
}
for (Point p : points) {
if (p == null) {
throw new IllegalArgumentException();
}
}
for (int i = 0; i < points.length - 1; i++) {
for(int j = i + 1; j < points.length; j++ ) {
if (points[i].compareTo(points[j]) == 0) {
throw new IllegalArgumentException();
}
}
}
Point[][] results = new Point[points.length][4];
int tempNumOfSeg = 0;
for (int i = 0; i < points.length; i++) {
for (int j = i + 1; j < points.length; j++) {
double slopeij = points[i].slopeTo(points[j]);
for (int k = j + 1; k < points.length; k++) {
if (points[i].slopeTo(points[k]) == slopeij) {
for (int l = k + 1; l < points.length; l++) {
if (points[i].slopeTo(points[l]) == slopeij) {
results[tempNumOfSeg][0] = points[i];
results[tempNumOfSeg][1] = points[j];
results[tempNumOfSeg][2] = points[k];
results[tempNumOfSeg][3] = points[l];
Arrays.sort(results[tempNumOfSeg]);
tempNumOfSeg++;
}
}
}
}
}
}
LineSegment[] tempLineResults = new LineSegment[tempNumOfSeg];
for (int i = 0; i < tempNumOfSeg; i++) {
tempLineResults[i] = new LineSegment(results[i][0],results[i][3]);
}
lineResults = tempLineResults;
numOfSeg = tempNumOfSeg;
} // finds all line segments containing 4 points
public int numberOfSegments() {
return numOfSeg;
} // the number of line segments
public LineSegment[] segments() {
return lineResults;
} // the line segments
public static void main(String[] args) {
// read the n points from a file
}
}
解决方案
将此代码用作参考
import edu.princeton.cs.algs4.StdDraw;
import edu.princeton.cs.algs4.StdOut;
import edu.princeton.cs.algs4.In;
import edu.princeton.cs.algs4.MergeX;
public class BruteCollinearPoints {
private final LineSegment[] segments;
private int segmentsSize = 0;
/** finds all line segments containing 4 points */
public BruteCollinearPoints(Point[] points) {
if (points == null) {
throw new IllegalArgumentException("points can't be null");
}
for (Point p : points) {
if (p == null) {
throw new IllegalArgumentException("point can't be null");
}
}
Point[] p = new Point[points.length];
System.arraycopy(points, 0, p, 0, points.length);
segments = analyzeSegments(p);
}
private LineSegment[] analyzeSegments(Point[] points) {
LineSegment[] tmpSegments = new LineSegment[points.length * 4];
MergeX.sort(points);
for (int i = 0; i < points.length; i++) {
for (int j = i + 1; j < points.length; j++) {
for (int k = j + 1; k < points.length; k++) {
for (int m = k + 1; m < points.length; m++) {
Point p = points[i];
Point q = points[j];
Point r = points[k];
Point s = points[m];
if (Double.compare(p.slopeTo(q), p.slopeTo(r)) == 0 && Double.compare(p.slopeTo(q), p.slopeTo(s)) == 0) {
tmpSegments[segmentsSize++] = new LineSegment(p, s);
}
}
}
}
}
LineSegment[] resultSegments = new LineSegment[segmentsSize];
for (int i = 0; i < segmentsSize; i++) {
resultSegments[i] = tmpSegments[i];
}
return resultSegments;
}
/** the number of line segments */
public int numberOfSegments() {
return segmentsSize;
}
/** the line segments */
public LineSegment[] segments() {
LineSegment[] returnSegments = new LineSegment[segments.length];
System.arraycopy(segments, 0, returnSegments, 0, segments.length);
return returnSegments;
}
public static void main(String[] args) {
// read the n points from a file
In in = new In(args[0]);
int n = in.readInt();
Point[] points = new Point[n];
for (int i = 0; i < n; i++) {
String s = in.readString();
if ("null".equals(s)) {
points[i] = null;
} else {
int x = Integer.parseInt(s);
int y = in.readInt();
points[i] = new Point(x, y);
}
}
BruteCollinearPoints collinear = new BruteCollinearPoints(points);
// draw the points
StdDraw.enableDoubleBuffering();
StdDraw.setXscale(0, 32768);
StdDraw.setYscale(0, 32768);
for (Point p : points) {
p.draw();
}
StdDraw.show();
// print and draw the line segments
for (LineSegment segment : collinear.segments()) {
StdOut.println(segment);
segment.draw();
}
StdDraw.show();
}
}
顺便说一句，这是一门很棒的课程