Center a JFrame or JDialog at Screen - Centralizando um JFrame ou um JDialog na tela

You just have to put the pack(); before!
-
Basta colocar o pack(); antes!

//...
this.pack();
//pack() before!
Toolkit tk = Toolkit.getDefaultToolkit();
Dimension screenSize = tk.getScreenSize();
setLocation((screenSize.width - this.getWidth()) / 2, (screenSize.height - this.getHeight()) / 2);
//...

ASCII Table (7 bits: in Binary, Hex, Decimal & Character). Tabela ASCII (7 bits: em Binário, Hexadecimal, Decimal e Caracter)

Binary	Hex	Decimal	Character
0000_0000	00	0	NULL
0000_0001	01	1	ctrl A
0000_0010	02	2	ctrl B
0000_0011	03	3	ctrl C
0000_0100	04	4	ctrl D
0000_0101	05	5	ctrl E
0000_0110	06	6	ctrl F
0000_0111	07	7	bell
0000_1000	08	8	backspace
0000_1001	09	9	tab
0000_1010	0A	10	line feed
0000_1011	0B	11	ctrl K
0000_1100	0C	12	form feed
0000_1101	0D	13	return
0000_1110	0E	14	ctrl N
0000_1111	0F	15	ctrl O
0001_0000	10	16	ctrl P
0001_0001	11	17	ctrl Q
0001_0010	12	18	ctrl R
0001_0011	13	19	ctrl S
0001_0100	14	20	ctrl T
0001_0101	15	21	ctrl U
0001_0110	16	22	ctrl V
0001_0111	17	23	ctrl W
0001_1000	18	24	ctrl X
0001_1001	19	25	ctrl Y
0001_1010	1A	26	ctrl Z
0001_1011	1B	27	ctrl [
0001_1100	1C	28	ctrl \
0001_1101	1D	29	Esc
0001_1110	1E	30	ctrl ^
0001_1111	1F	31	ctrl _
0010_0000	20	32	space
0010_0001	21	33	!
0010_0010	22	34	"
0010_0011	23	35	#
0010_0100	24	36	$
0010_0101	25	37	%
0010_0110	26	38	&
0010_0111	27	39	'
0010_1000	28	40	(
0010_1001	29	41	)
0010_1010	2A	42	*
0010_1011	2B	43	+
0010_1100	2C	44	,
0010_1101	2D	45	-
0010_1110	2E	46	.
0010_1111	2F	47	/
0011_0000	30	48	0
0011_0001	31	49	1
0011_0010	32	50	2
0011_0011	33	51	3
0011_0100	34	52	4
0011_0101	35	53	5
0011_0110	36	54	6
0011_0111	37	55	7
0011_1000	38	56	8
0011_1001	39	57	9
0011_1010	3A	58	:
0011_1011	3B	59	;
0011_1100	3C	60	<
0011_1101	3D	61	=
0011_1110	3E	62	>
0011_1111	3F	63	?
0100_0000	40	64	@
0100_0001	41	65	A
0100_0010	42	66	B
0100_0011	43	67	C
0100_0100	44	68	D
0100_0101	45	69	E
0100_0110	46	70	F
0100_0111	47	71	G
0100_1000	48	72	H
0100_1001	49	73	I
0100_1010	4A	74	J
0100_1011	4B	75	K
0100_1100	4C	76	L
0100_1101	4D	77	M
0100_1110	4E	78	N
0100_1111	4F	79	O
0101_0000	50	80	P
0101_0001	51	81	Q
0101_0010	52	82	R
0101_0011	53	83	S
0101_0100	54	84	T
0101_0101	55	85	U
0101_0110	56	86	V
0101_0111	57	87	W
0101_1000	58	88	X
0101_1001	59	89	Y
0101_1010	5A	90	Z
0101_1011	5B	91	[
0101_1100	5C	92	\
0101_1101	5D	93	]
0101_1110	5E	94	^
0101_1111	5F	95	_
0110_0000	60	96	`
0110_0001	61	97	a
0110_0010	62	98	b
0110_0011	63	99	c
0110_0100	64	100	d
0110_0101	65	101	e
0110_0110	66	102	f
0110_0111	67	103	g
0110_1000	68	104	h
0110_1001	69	105	i
0110_1010	6A	106	j
0110_1011	6B	107	k
0110_1100	6C	108	l
0110_1101	6D	109	m
0110_1110	6E	110	n
0110_1111	6F	111	o
0111_0000	70	112	p
0111_0001	71	113	q
0111_0010	72	114	r
0111_0011	73	115	s
0111_0100	74	116	t
0111_0101	75	117	u
0111_0110	76	118	v
0111_0111	77	119	w
0111_1000	78	120	x
0111_1001	79	121	y
0111_1010	7A	122	z
0111_1011	7B	123	{
0111_1100	7C	124	|
0111_1101	7D	125	}
0111_1110	7E	126	~
0111_1111	7F	127	¯

It's very useful! :)