Q:

Java: Difference between the setPreferredSize() and setSize() methods in components

What is the main difference between setSize() and setPreferredSize(). Sometimes I used setSize(), sometimes setPreferredSize(), sometimes one does what I want, sometimes the other.
What call should I use for JFrames and JPanels?

A:

Usage depends on whether the component's parent has a layout manager or not.

setSize() -- use when a parent layout manager does not exist;
setPreferredSize() (also its related setMinimumSize and setMaximumSize) -- use when a parent layout manager exists.

The setSize() method probably won't do anything if the component's parent is using a layout manager; the places this will typically have an effect would be on top-level components (JFrames and JWindows) and things that are inside of scrolled panes.  You also must call setSize() if you've got components inside a parent without a layout manager.
Generally, setPreferredSize() will lay out the components as expected if a layout manager is present; most layout managers work by getting the preferred (as well as minimum and maximum) sizes of their components, then using setSize() and setLocation() to position those components according to the layout's rules.
For example, a BorderLayout tries to make the bounds of its "north" region equal to the preferred size of its north component---they may end up larger or smaller than that, depending on the size of the JFrame, the size of the other components in the layout, and so on.

A:

setSize() or setBounds() can be used when no layout manager is being used.
However, if you are using a layout manager you can provide hints to the layout manager using the setXXXSize() methods like setPreferredSize() and setMinimumSize() etc. 
And be sure that the component's container uses a layout manager that respects the requested size. The FlowLayout, GridBagLayout, and SpringLayout managers use the component's preferred size (the latter two depending on the constraints you set), but BorderLayout and GridLayout usually don't.If you specify new size hints for a component that's already visible, you need to invoke the revalidate method on it to make sure that its containment hierarchy is laid out again. Then invoke the repaint method.

A:

setSize will resize the component to the specified size.
setPreferredSize sets the preferred size.  The component may not actually be this size depending on the size of the container it's in, or if the user re-sized the component manually.

