Text renderer: materialize CharSequence values as String #130

jjava-jupyter/src/main/java/org/dflib/jjava/jupyter/kernel/display/common/Text.java

@@ -22,13 +22,17 @@ public static void registerAll(Renderer renderer) {
     }
 
     public static void renderCharSequence(CharSequence data, RenderContext context) {
-        context.renderIfRequested(JS, () -> data);
-        context.renderIfRequested(PLAIN, () -> data);
-        context.renderIfRequested(MARKDOWN, () -> data);
-        context.renderIfRequested(LATEX, () -> data);
-        context.renderIfRequested(HTML, () -> data);
-        context.renderIfRequested(CSS, () -> data);
-        context.renderIfRequested(SVG, () -> data);
-        context.renderIfRequested(JSON, () -> data);
+        // materialize as String: the stored value is later serialized by Gson, which
+        // handles Strings but turns other CharSequence implementations into a reflective
+        // field dump (or fails on JDK-internal types such as CharBuffer)
+        String text = String.valueOf(data);
+        context.renderIfRequested(JS, () -> text);
+        context.renderIfRequested(PLAIN, () -> text);
+        context.renderIfRequested(MARKDOWN, () -> text);
+        context.renderIfRequested(LATEX, () -> text);
+        context.renderIfRequested(HTML, () -> text);
+        context.renderIfRequested(CSS, () -> text);
+        context.renderIfRequested(SVG, () -> text);
+        context.renderIfRequested(JSON, () -> text);
     }
 }

jjava-jupyter/src/test/java/org/dflib/jjava/jupyter/kernel/display/common/TextTest.java

@@ -0,0 +1,92 @@
+package org.dflib.jjava.jupyter.kernel.display.common;
+
+import org.dflib.jjava.jupyter.kernel.display.DisplayData;
+import org.dflib.jjava.jupyter.kernel.display.Renderer;
+import org.dflib.jjava.jupyter.kernel.display.mime.MIMEType;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.nio.CharBuffer;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class TextTest {
+
+    private Renderer renderer;
+
+    @BeforeEach
+    public void setUp() {
+        this.renderer = new Renderer();
+        Text.registerAll(this.renderer);
+    }
+
+    @Test
+    public void rendersStringAsIs() {
+        DisplayData data = this.renderer.render("hello");
+
+        assertEquals("hello", data.getData(MIMEType.TEXT_PLAIN));
+    }
+
+    @Test
+    public void materializesStringBuilder() {
+        DisplayData data = this.renderer.render(new StringBuilder("hello"));
+
+        assertEquals("hello", data.getData(MIMEType.TEXT_PLAIN));
+    }
+
+    @Test
+    public void materializesCharBuffer() {
+        // CharBuffer is a CharSequence whose implementation classes Gson cannot
+        // serialize reflectively on modern JDKs (JsonIOException)
+        DisplayData data = this.renderer.render(CharBuffer.wrap("hello"));
+
+        assertEquals("hello", data.getData(MIMEType.TEXT_PLAIN));
+    }
+
+    @Test
+    public void materializesCustomCharSequence() {
+        DisplayData data = this.renderer.render(new CustomText("hello"));
+
+        assertEquals("hello", data.getData(MIMEType.TEXT_PLAIN));
+    }
+
+    @Test
+    public void materializesForAlternateMimeTypes() {
+        DisplayData data = this.renderer.renderAs(new CustomText("<b>hi</b>"), MIMEType.TEXT_HTML.toString());
+
+        assertEquals("<b>hi</b>", data.getData(MIMEType.TEXT_HTML));
+    }
+
+    /**
+     * Stand-in for library-provided CharSequence implementations (e.g.
+     * commons-text TextStringBuilder, Groovy GString) that Gson would
+     * otherwise serialize as a reflective field dump.
+     */
+    private static class CustomText implements CharSequence {
+        private final String value;
+
+        CustomText(String value) {
+            this.value = value;
+        }
+
+        @Override
+        public int length() {
+            return value.length();
+        }
+
+        @Override
+        public char charAt(int index) {
+            return value.charAt(index);
+        }
+
+        @Override
+        public CharSequence subSequence(int start, int end) {
+            return value.subSequence(start, end);
+        }
+
+        @Override
+        public String toString() {
+            return value;
+        }
+    }
+}