Question

Is it contraindicated to use several presetted dimension objects to set the preferred size of non resizable components on the screen.
E.g.: two textfields both should be 80×20 px so:

Dimension d = new Dimension(80, 20);
tf1.setPreferredSize(d);
tf2.setPreferredSize(d);

Answer 1

Yes, it is “safe”. Quote from Java Performance Tuning:

[…] This same Dimension object can be reused for multiple components. […]

Depends on what you want though. Note that the implementation in Component does not copy the content of the argument, but stores the reference:

public void setPreferredSize(Dimension preferredSize) {
    ...
    this.prefSize = preferredSize;
    ...
}

so changing d will affect the dimension object stored in both tf1 and tf2.

(What I’m saying is that the code below may not do what you would expect.)

Dimension d = new Dimension(80, 20);
tf1.setPreferredSize(d);

d.width += 1;               // <-- will affect also tf1.
tf2.setPreferredSize(d);