TOPCAT: truncate ellipsis in middle of string

topcat/src/main/uk/ac/starlink/topcat/CustomLabelUI.java

@@ -0,0 +1,103 @@
+package uk.ac.starlink.topcat;
+
+import java.awt.FontMetrics;
+import java.awt.Rectangle;
+import javax.swing.Icon;
+import javax.swing.JComponent;
+import javax.swing.JLabel;
+import javax.swing.plaf.basic.BasicLabelUI;
+
+/**
+ * An implementation of the (Basic)LabelUI for overriding JLabel formatting in
+ * the cells of JTables.
+ *
+ * This is used to change the location of the ellipsis when a string is
+ * rendered in a cell that cannot fit. By default, the ellipsis is always at
+ * the end of the string, but this class instead places the ellipsis in the
+ * middle of the string. See
+ *
+ *     https://docs.oracle.com/javase/8/docs/api/javax/swing/plaf/basic/BasicLabelUI.html
+ *
+ * @author   Fergus Baker
+ * @since    03 Feb 2026
+ */
+public class CustomLabelUI extends BasicLabelUI {
+    private static final String ELLIPSES = "...";
+    private static final int ELLIPSES_LEN = ELLIPSES.length();
+    private String displayedText;
+
+    /**
+     * Implicit constructor, called by Swing with a particular component in
+     * order to have something for drawing labels to the screen.
+     */
+    public static CustomLabelUI createUI(JComponent c) {
+        return new CustomLabelUI();
+    }
+
+    /**
+     * Returns the text displayed in the label.
+     *
+     * @return   String
+     */
+    public String getText() {
+        return displayedText;
+    }
+
+    private int findTruncationSplit(int maxWidth, FontMetrics fontMetrics,
+            String text) {
+        int textLength = text.length();
+        for (int i = 1; i <= textLength; i++) {
+            int subTextWidth = fontMetrics.stringWidth(
+                    text.substring( textLength - i )
+            );
+            if (subTextWidth > maxWidth) {
+                return textLength - i + 1;
+            }
+        }
+        return 0;
+    }
+
+    @Override
+    protected String layoutCL(JLabel label, FontMetrics fontMetrics,
+                              String text, Icon icon, Rectangle viewR,
+                              Rectangle iconR, Rectangle textR) {
+        /* Draw the text with the usual layout for a compound label. */
+        displayedText = super.layoutCL(label, fontMetrics, text, icon, viewR,
+                                       iconR, textR);
+
+        /* Sometimes the font metrics is null if the text is being displayed
+         * before the UI is ready. In such a case, there is nothing that this
+         * class needs to do. */
+        if (fontMetrics == null) {
+            return displayedText;
+        }
+
+        int displayedLength = displayedText.length();
+
+        /* Check if a truncation occurred, and if the text is long enough. If
+         * the text is too short, there is no point in truncating differently.
+         * */;
+        if (!displayedText.equals(text) && displayedLength > 2 * ELLIPSES_LEN) {
+            int splitLength = (displayedLength + ELLIPSES_LEN) / 2 - ELLIPSES_LEN;
+            String beforeEllipses = displayedText.substring(0, splitLength);
+            /* Determine the maximum width (in pixels) that the remaining text
+             * should fit into. */
+            int displayedTextWidth = fontMetrics.stringWidth(displayedText);
+            int maxWidth = (
+                    displayedTextWidth
+                    - fontMetrics.stringWidth(ELLIPSES)
+                    - fontMetrics.stringWidth(beforeEllipses)
+            );
+
+            int splitIndex = findTruncationSplit(
+                    maxWidth, fontMetrics, text
+            );
+
+            displayedText = (
+                    beforeEllipses + ELLIPSES + text.substring(splitIndex)
+            );
+        }
+
+        return displayedText;
+    }
+}

topcat/src/main/uk/ac/starlink/topcat/Driver.java

@@ -11,6 +11,7 @@
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import javax.swing.SwingUtilities;
+import javax.swing.UIManager;
 import org.astrogrid.samp.ErrInfo;
 import org.astrogrid.samp.JSamp;
 import org.astrogrid.samp.Response;
@@ -35,6 +36,7 @@
 import uk.ac.starlink.table.jdbc.TextModelsAuthenticator;
 import uk.ac.starlink.task.InvokeUtils;
 import uk.ac.starlink.topcat.interop.TopcatCommunicator;
+import uk.ac.starlink.topcat.CustomLabelUI;
 import uk.ac.starlink.ttools.Stilts;
 import uk.ac.starlink.util.gui.ErrorDialog;
 import uk.ac.starlink.util.DataSource;
@@ -126,6 +128,9 @@ private static void runMain( String[] args )
             // never mind
             cmdname = null;
         }
+        /* Register custom LabelUI for drawing cells in JTable. This has to
+         * happen before any of the UI is initialised. */
+        UIManager.put("LabelUI", CustomLabelUI.class.getName());
         Loader.tweakGuiForMac();
         Loader.setHttpAgent( TopcatUtils.getHttpUserAgent() );
         Loader.setDefaultProperty( "java.awt.Window.locationByPlatform",