Why is setSize method necessary to show scroll on Composite in SWT?

Question

I'm working on to create UI with JAVA SWT.

One thing makes me confused with using ScrolledComposite. Why my code does not show a scroll without setting the MinSize of Composite?

Can anyone explain what setMinSize does in ScrolledComposite?

Why can't we use "setSize" insetead of "setMinSize"?

Below is my code.

import org.eclipse.swt.SWT;
import org.eclipse.swt.custom.ScrolledComposite;
import org.eclipse.swt.graphics.Color;
import org.eclipse.swt.layout.GridData;
import org.eclipse.swt.layout.GridLayout;
import org.eclipse.swt.widgets.Button;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;


public class MyPractice {

public static void main(String[] args) {

    Display display = new Display();
    Shell shell = new Shell(display);
    shell.setLayout(new GridLayout());

    final ScrolledComposite sc = new ScrolledComposite(shell, SWT.BORDER | SWT.H_SCROLL | SWT.V_SCROLL);
    sc.setLayoutData(new GridData(SWT.FILL, SWT.FILL, true, true, 1, 1));

    Composite c = new Composite(sc, SWT.NONE);
    c.setLayout(new GridLayout(7, true));
    c.setBackground(new Color(display, 0,255,0));

    for (int i = 0; i < 200; i++) {
      Button b = new Button(c, SWT.PUSH);
      b.setText("Button " + i);
    }

    sc.setContent(c);
    sc.setExpandHorizontal(true);
    sc.setExpandVertical(true);


    //*****Why is this code necessary to get Scroll on UI?
    sc.setMinSize(c.computeSize(SWT.DEFAULT, SWT.DEFAULT));

    //Why can't we use "setSize" insetead of setMinSize? setSize does not show scroll. 
    //sc.setSize(c.computeSize(SWT.DEFAULT, SWT.DEFAULT));


    shell.setSize(300, 500);
    shell.open();
    while (!shell.isDisposed()) {
      if (!display.readAndDispatch())
        display.sleep();
    }
    display.dispose();

}
}

Answer 1

setSize() and setMinSize() are two very different functions. Control#setSize() sets the actual size of a Control whereas ScrolledComposite#setMinSize() is specific to ScrolledComposite and tells it at which size to start showing the scroll bars:

Specify the minimum width and height at which the ScrolledComposite will begin scrolling the content with the horizontal scroll bar. This value is only relevant if setExpandHorizontal(true) and setExpandVertical(true) have been set.

The minimum size of the scrolled composite will be based on the size of the child.