ScrollPane没有Layout, 不能setLayout

类声明 继承自Container

public class ScrollPane extends Container implements Accessible {

Layout被设为了null

 /**
     * Create a new scrollpane container.
     * @param scrollbarDisplayPolicy policy for when scrollbars should be shown
     * @throws IllegalArgumentException if the specified scrollbar
     *     display policy is invalid
     * @throws HeadlessException if GraphicsEnvironment.isHeadless()
     *     returns true
     * @see java.awt.GraphicsEnvironment#isHeadless
     */
    @ConstructorProperties({"scrollbarDisplayPolicy"})
    public ScrollPane(int scrollbarDisplayPolicy) throws HeadlessException {
        GraphicsEnvironment.checkHeadless();
        this.layoutMgr = null; //////被设为null
        this.width = 100;
        this.height = 100;
        switch (scrollbarDisplayPolicy) {
            case SCROLLBARS_NEVER:
            case SCROLLBARS_AS_NEEDED:
            case SCROLLBARS_ALWAYS:
                this.scrollbarDisplayPolicy = scrollbarDisplayPolicy;
                break;
            default:
                throw new IllegalArgumentException("illegal scrollbar display policy");
        }

        vAdjustable = new ScrollPaneAdjustable(this, new PeerFixer(this),
                                               Adjustable.VERTICAL);
        hAdjustable = new ScrollPaneAdjustable(this, new PeerFixer(this),
                                               Adjustable.HORIZONTAL);
        setWheelScrollingEnabled(defaultWheelScroll);
    }

this.layoutMgr = null; //////被设为null

不能setLayout

    /**
     * Sets the layout manager for this container.  This method is
     * overridden to prevent the layout mgr from being set.
     * @param mgr the specified layout manager
     */
    public final void setLayout(LayoutManager mgr) {
        throw new AWTError("ScrollPane controls layout");
    }

始终抛异常