展望2011
2007-01-04 | java的初始化(2)【转载】
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/ 个人分类:编程基础
1.初始化的顺序
对类而言,初始化的顺序是由变量在类的定义里面的顺序决定的,变量的定义可能会分散在类定义的各个地方,并且与方法的定义相互交错,但是变量的初始化会先于任何方法,甚至是构造函数的调用。
class Tag {
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v8nb+r0 Tag(int marker) {51Testing软件测试网0vDT#n'AeV
System.out.println("Tag(" + marker + ")");51Testing软件测试网*XbJ!vzaDIC
}
S0y`j"u3P,X|0}51Testing软件测试网s^&`D
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class Card {51Testing软件测试网8lQdc @m9bO0v
Tag t1 = new Tag(1); // Before constructor
p0d0H9J-\pD&he0 Card() {51Testing软件测试网C3b8S-Lg.kk
// Indicate we're in the constructor:
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Q+pSz+jD2h0 System.out.println("Card()");
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qS0 t3 = new Tag(33); // Reinitialize t351Testing软件测试网fIBz7a
}
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~9c%X0 Tag t2 = new Tag(2); // After constructor51Testing软件测试网+h@r_8i|qF
void f() {
3XW4Ar5_7UF [ j%}0 System.out.println("f()");
n'nQh `:[*DM0 }
"d^0^E'x*\e3_+N0 Tag t3 = new Tag(3); // At end51Testing软件测试网!jR%lN-w
}
rKI&U5th0public class Flower {51Testing软件测试网2C0G X/M"ja,e&z_
public static void main(String[] args) {
:KTX"ml7wMA0 Card t = new Card();
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O@
]~0 t.f(); // Shows that construction is done51Testing软件测试网e E%D@ W
}51Testing软件测试网!M-W!i0jz0Q
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}
输出结果如下:
Tag(1)
Tag(2)
Tag(3)
Card()
Tag(33)
f()
2.静态数据的初始化
无论创建多少对象,static数据只能有一份。它的初始化不是在定义的时候进行的。
class Bowl {51Testing软件测试网7c4^4g!U&s_|'Mz*z?/}
Bowl(int marker) {51Testing软件测试网Qm:P;PwMN GK
System.out.println("Bowl(" + marker + ")");51Testing软件测试网G7d`3M)J'}
}
g.X b-B&}m P;^i0 void f(int marker) {
5IhN.G|e"Fz0 System.out.println("f(" + marker + ")");
Ou6p*o:P^^0 }
y-RtbB3[*Cz7NXg4k0}
JX `S^0class Table {
_Pj5u&nR)}2L1e(p+n0 static Bowl b1 = new Bowl(1);
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css,v+U0 Table() {51Testing软件测试网Bp)\-Y"Z;q{@
System.out.println("Table()");
$Pb8DEw y0 b2.f(1);
(Qi9bOfM l0 }
GA
`%F?.mc@0 void f2(int marker) {51Testing软件测试网O6jeY&ZPn3E
System.out.println("f2(" + marker + ")");
gYiv:T$Q;k0 }51Testing软件测试网0H.Xh*^*v0rK|
static Bowl b2 = new Bowl(2);
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Z mWq0}
!L+b7m\b
@i0class Cupboard {
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ztNJ:cva\0 Bowl b3 = new Bowl(3);
jKIK;^U%e0 static Bowl b4 = new Bowl(4);51Testing软件测试网o Z4fH+YYw
Cupboard() {
h-gtR9re;g}%Y$Xz~#x0 System.out.println("Cupboard()");51Testing软件测试网1t)O%ug3f
b4.f(2);51Testing软件测试网zC
k(Ek(fbE0z
}
H7X7D } s0 void f3(int marker) {
System.out.println("f3(" + marker + ")");
l^;b"k3WDP0k-D0 }
ZC%b0t9Zx9o ^0 static Bowl b5 = new Bowl(5);51Testing软件测试网RU p]%m/T T K9D
}
8ek"rs;nS:if0public class E1 {51Testing软件测试网-T*^z:_9EN'v
public static void main(String[] args) {51Testing软件测试网+|5^:Y|0j eFN
System.out.println("Creating new Cupboard() in main");
K?6\H/h5A(^E n0 new Cupboard();
#I-KC4l"RF0 System.out.println("Creating new Cupboard() in main");
9P:\A s;Q;s!Rm0 new Cupboard();
Table t2 = new Table();
pN$|L
W[O0 Cupboard t3 = new Cupboard();
@O/@!w8v1wy:BoxH0 t2.f2(1);51Testing软件测试网lut.Z3|i
t3.f3(1);51Testing软件测试网;ZH_\+i(@5k
}51Testing软件测试网
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}
程序的输出如下:
Creating new Cupboard() in main
Bowl(4)
Bowl(5)
Bowl(3)
Cupboard()
f(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f(2)
Bowl(1)
Bowl(2)
Table()
f(1)
Bowl(3)
Cupboard()
f(2)
f2(1)
f3(1)
从程序的输出可以看出,static成员只会在需要的时候进行初始化。如果没有创建Table对象,就永远不可能用到Table.b1或者Table.b2,因此也不会去创建static的Bowl b1和b2。只有创建了第一个Table对象之后(或者第一次访问static成员的时候),它们才会被初始化。此后,static对象就不会再作初始化了。
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初始化
编程基础