Fix style spans issue

richtextfx/src/main/java/org/fxmisc/richtext/model/Paragraph.java

@@ -230,7 +230,7 @@ public Paragraph<PS, SEG, S> restyle(int from, int to, S style) {
 
     public Paragraph<PS, SEG, S> restyle(int from, StyleSpans<? extends S> styleSpans) {
         int len = styleSpans.length();
-        if(styleSpans.equals(getStyleSpans(from, from + len))) {
+        if(styleSpans.equals(getStyleSpans(from, from + len)) || length() == 0) {
             return this;
         }
 

richtextfx/src/main/java/org/fxmisc/richtext/model/StyleSpansBuilder.java

@@ -40,6 +40,16 @@ public int getSpanCount() {
         public StyleSpan<S> getStyleSpan(int index) {
             return spans.get(index);
         }
+
+        @Override
+        public String toString() {
+            StringBuilder sb = new StringBuilder();
+            sb.append("StyleSpans(length=").append(length())
+                    .append(" spanCount=").append(getSpanCount())
+                    .append(" spans=").append(spans)
+                    .append(")");
+            return sb.toString();
+        }
     }
 
     static <S> StyleSpans<S> overlay(
@@ -276,6 +286,20 @@ public StyleSpan<S> getStyleSpan(int index) {
             return original.getStyleSpan(firstIdxInOrig + index);
         }
     }
+
+    @Override
+    public String toString() {
+        ArrayList<StyleSpan<S>> spans = new ArrayList<>(spanCount);
+        for (int i = 0; i < spanCount; i++) {
+            spans.add(getStyleSpan(i));
+        }
+        StringBuilder sb = new StringBuilder();
+        sb.append("SubSpans(length=").append(length)
+                .append(" spanCount=").append(getSpanCount())
+                .append(" spans=").append(spans)
+                .append(")");
+        return sb.toString();
+    }
 }
 
 

richtextfx/src/test/java/org/fxmisc/richtext/model/ParagraphTest.java

@@ -20,4 +20,19 @@ public void concatEmptyParagraphsTest() {
         assertEquals(Boolean.TRUE, p.getStyleAtPosition(0));
     }
 
+    // Relates to #345 and #505: calling `EditableStyledDocument::setStyleSpans` when the style spans
+    //  would style an empty paragraph would throw an exception
+    @Test
+    public void restylingEmptyParagraphViaStyleSpansWorks() {
+        TextOps<StyledText<Boolean>, Boolean> segOps = StyledText.textOps();
+        Paragraph<Void, StyledText<Boolean>, Boolean> p = new Paragraph<>(null, segOps, segOps.createEmpty());
+
+        StyleSpansBuilder<Boolean> builder = new StyleSpansBuilder<>();
+        builder.add(true, 2);
+        StyleSpans<Boolean> spans = builder.create();
+        Paragraph<Void, StyledText<Boolean>, Boolean> restyledP = p.restyle(0, spans);
+
+        assertEquals(p, restyledP);
+    }
+
 }