Change the tab shape of JTabbedPane to trapezoid

• download example.jar
• download src.zip
• repository

Code

class IsoscelesTrapezoidTabbedPaneUI extends BasicTabbedPaneUI {
  private static final int ADJ2 = 3;
  private final Color selectedTabColor = UIManager.getColor("TabbedPane.selected");
  private final Color tabBackgroundColor = Color.LIGHT_GRAY;
  private final Color tabBorderColor = Color.GRAY;

  @Override protected void paintTabArea(Graphics g, int tabPlacement, int selectedIndex) { //NOPMD
    int tabCount = tabPane.getTabCount();

    Rectangle iconRect = new Rectangle(),
    textRect = new Rectangle();
    Rectangle clipRect = g.getClipBounds();

    for (int i = runCount - 1; i >= 0; i--) {
      int start = tabRuns[i];
      int next = tabRuns[(i == runCount - 1) ? 0 : i + 1];
      int end = next != 0 ? next - 1 : tabCount - 1; //NOPMD
      // for (int j = start; j <= end; j++) {
      // https://stackoverflow.com/questions/41566659/tabs-rendering-order-in-custom-jtabbedpane
      for (int j = end; j >= start; j--) {
        if (j != selectedIndex && rects[j].intersects(clipRect)) {
          paintTab(g, tabPlacement, rects, j, iconRect, textRect);
        }
      }
    }
    if (selectedIndex >= 0 && rects[selectedIndex].intersects(clipRect)) {
      paintTab(g, tabPlacement, rects, selectedIndex, iconRect, textRect);
    }
  }
  @Override protected void paintTabBorder(
      Graphics g, int tabPlacement, int tabIndex, int x, int y, int w, int h, boolean isSelected) {
    // Do nothing
  }
  @Override protected void paintFocusIndicator(
      Graphics g, int tabPlacement, Rectangle[] rects, int tabIndex,
      Rectangle iconRect, Rectangle textRect, boolean isSelected) {
    // Do nothing
  }
  @Override protected void paintContentBorderTopEdge(
      Graphics g, int tabPlacement, int selectedIndex, int x, int y, int w, int h) {
    super.paintContentBorderTopEdge(g, tabPlacement, selectedIndex, x, y, w, h);
    Rectangle selRect = getTabBounds(selectedIndex, calcRect);
    Graphics2D g2 = (Graphics2D) g.create();
    g2.setColor(selectedTabColor);
    g2.drawLine(selRect.x - ADJ2 + 1, y, selRect.x + selRect.width + ADJ2 - 1, y);
    g2.dispose();
  }
  @Override protected void paintTabBackground(
      Graphics g, int tabPlacement, int tabIndex, int x, int y, int w, int h, boolean isSelected) {
    Graphics2D g2 = (Graphics2D) g.create();
    g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
    int textShiftOffset = isSelected ? 0 : 1;

    Rectangle clipRect = g2.getClipBounds();
    clipRect.grow(ADJ2 + 1, 0);
    g2.setClip(clipRect);

    GeneralPath trapezoid = new GeneralPath();
    trapezoid.moveTo(x - ADJ2,     y + h);
    trapezoid.lineTo(x + ADJ2,     y + textShiftOffset);
    trapezoid.lineTo(x + w - ADJ2, y + textShiftOffset);
    trapezoid.lineTo(x + w + ADJ2, y + h);
    //trapezoid.closePath();

    g2.setColor(isSelected ? selectedTabColor : tabBackgroundColor);
    g2.fill(trapezoid);

    g2.setColor(tabBorderColor);
    g2.draw(trapezoid);

    g2.dispose();
  }
}

References

• java - Tabs Rendering Order in Custom JTabbedPane - Stack Overflow
• Change the tab shape of JTabbedPane to trapezoid(Japanese)