Replace loop-based string concatenation with StringBuilder in path/polytree string rendering

src/main/java/clipper2/core/Path64.java

@@ -34,11 +34,11 @@ public Path64(Point64... path) {
 
 	@Override
 	public String toString() {
-		String s = "";
+		StringBuilder s = new StringBuilder();
 		for (Point64 p : this) {
-			s = s + p.toString() + " ";
+			s.append(p.toString()).append(" ");
 		}
-		return s;
+		return s.toString();
 	}
 
 }

src/main/java/clipper2/core/PathD.java

@@ -29,11 +29,11 @@ public PathD(List<PointD> path) {
 
 	@Override
 	public String toString() {
-		String s = "";
+		StringBuilder s = new StringBuilder();
 		for (PointD p : this) {
-			s = s + p.toString() + " ";
+			s.append(p.toString()).append(" ");
 		}
-		return s;
+		return s.toString();
 	}
 
 }

src/main/java/clipper2/core/Paths64.java

@@ -30,11 +30,11 @@ public Paths64(Path64... paths) {
 
 	@Override
 	public String toString() {
-		String s = "";
+		StringBuilder s = new StringBuilder();
 		for (Path64 p : this) {
-			s = s + p.toString() + "\n";
+			s.append(p.toString()).append("\n");
 		}
-		return s;
+		return s.toString();
 	}
 
 }

src/main/java/clipper2/core/PathsD.java

@@ -25,11 +25,11 @@ public PathsD(List<PathD> paths) {
 
 	@Override
 	public String toString() {
-		String s = "";
+		StringBuilder s = new StringBuilder();
 		for (PathD p : this) {
-			s = s + p.toString() + "\n";
+			s.append(p.toString()).append("\n");
 		}
-		return s;
+		return s.toString();
 	}
 
 }

src/main/java/clipper2/engine/PolyPathBase.java

@@ -63,26 +63,28 @@ public final void Clear() {
 
 	private String toStringInternal(int idx, int level) {
 		int count = children.size();
-		String result = "", padding = "", plural = "s";
+		StringBuilder result = new StringBuilder();
+		StringBuilder padding = new StringBuilder();
+		String plural = "s";
 		if (children.size() == 1) {
 			plural = "";
 		}
 		// Create padding by concatenating spaces
 		for (int i = 0; i < level * 2; i++) {
-			padding += " ";
+			padding.append(" ");
 		}
 
 		if ((level & 1) == 0) {
-			result += String.format("%s+- hole (%d) contains %d nested polygon%s.\n", padding, idx, children.size(), plural);
+			result.append(String.format("%s+- hole (%d) contains %d nested polygon%s.\n", padding, idx, children.size(), plural));
 		} else {
-			result += String.format("%s+- polygon (%d) contains %d hole%s.\n", padding, idx, children.size(), plural);
+			result.append(String.format("%s+- polygon (%d) contains %d hole%s.\n", padding, idx, children.size(), plural));
 		}
 		for (int i = 0; i < count; i++) {
 			if (children.get(i).getCount() > 0) {
-				result += children.get(i).toStringInternal(i, level + 1);
+				result.append(children.get(i).toStringInternal(i, level + 1));
 			}
 		}
-		return result;
+		return result.toString();
 	}
 
 	@Override
@@ -95,13 +97,13 @@ public String toString() {
 		if (children.size() == 1) {
 			plural = "";
 		}
-		String result = String.format("Polytree with %d polygon%s.\n", children.size(), plural);
+		StringBuilder result = new StringBuilder(String.format("Polytree with %d polygon%s.\n", children.size(), plural));
 		for (int i = 0; i < count; i++) {
 			if (children.get(i).getCount() > 0) {
-				result += children.get(i).toStringInternal(i, 1);
+				result.append(children.get(i).toStringInternal(i, 1));
 			}
 		}
-		return result + '\n';
+		return result.append('\n').toString();
 	}
 
-}
+}

src/test/java/clipper2/TestToStringOutput.java

@@ -0,0 +1,54 @@
+package clipper2;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+import clipper2.core.Path64;
+import clipper2.core.PathD;
+import clipper2.core.Paths64;
+import clipper2.core.PathsD;
+import clipper2.core.Point64;
+import clipper2.core.PointD;
+import clipper2.engine.PolyPathBase;
+import clipper2.engine.PolyTree64;
+
+class TestToStringOutput {
+
+	@Test
+	void path64ToStringMatchesExistingFormat() {
+		Path64 path = new Path64(new Point64(1, 2), new Point64(3, 4));
+		assertEquals("(1,2)  (3,4)  ", path.toString());
+	}
+
+	@Test
+	void pathDToStringMatchesExistingFormat() {
+		PathD path = new PathD(2);
+		path.add(new PointD(1.5, 2.5));
+		path.add(new PointD(3.5, 4.5));
+		assertEquals("(1.500000,2.500000)  (3.500000,4.500000)  ", path.toString());
+	}
+
+	@Test
+	void paths64ToStringMatchesExistingFormat() {
+		Paths64 paths = new Paths64(new Path64(new Point64(1, 2)));
+		assertEquals("(1,2)  \n", paths.toString());
+	}
+
+	@Test
+	void pathsDToStringMatchesExistingFormat() {
+		PathsD paths = new PathsD(1);
+		PathD path = new PathD(1);
+		path.add(new PointD(1.5, 2.5));
+		paths.add(path);
+		assertEquals("(1.500000,2.500000)  \n", paths.toString());
+	}
+
+	@Test
+	void polyPathBaseToStringMatchesExistingFormat() {
+		PolyTree64 tree = new PolyTree64();
+		PolyPathBase polygon = tree.AddChild(new Path64());
+		polygon.AddChild(new Path64());
+		assertEquals("Polytree with 1 polygon.\n  +- polygon (0) contains 1 hole.\n\n", tree.toString());
+	}
+}