成员方法的初始化
- 在类的内部，变量的定义顺序决定了初始化的顺序。创建一个对象时，会先初始化对象中的成员变量，然后才会初始化方法（包括构造器）。
- 成员变量 ==> 方法
x
class Window {
Window (int marker) {
System.out.println("Window: " + marker);
}
}
class House {
Window w1 = new Window(1);
House () {
System.out.println("init house");
w3 = new Window(4);
}
Window w2 = new Window(2);
Window w3 = new Window(3);
}
public class OrderOfInitialization {
public static void main (String[] args) {
House house = new House();
}
}
$ javac OrderOfInitialization.java
$ java OrderOfInitialization
Window: 1
Window: 2
Window: 3
init house
Window: 4
静态成员的初始化
- 静态成员变量的初始化要优先于成员变量，但是静态成员变量只有在必要的时候才会初始化。
- 如果不创建对象，或者不通过类名直接调用，静态成员永远也不会初始化。
- 静态成员变量只会初始化一次，就是在第一次创建对象时，后面无论创建多少次该对象，都不会再次初始化。
- 静态成员变量 ==> 成员变量 ==> 方法
class Bowl {
Bowl(int marker) {
System.out.println("Bowl class marker: " + marker);
}
void method(int marker) {
System.out.println("Bowl method: " + marker);
}
}
class Table {
static Bowl bowl1 = new Bowl(1);
Table() {
System.out.println("Table class marker");
bowl2.method(1);
}
void method(int marker) {
System.out.println("Table method: " + marker);
}
static Bowl bowl2 = new Bowl(2);
}
class Cupboard {
Bowl bowl3 = new Bowl(3);
static Bowl bowl4 = new Bowl(4);
Cupboard() {
System.out.println("Cupboard class marker");
bowl4.method(1);
}
void method(int marker) {
System.out.println("Cupboard method: " + marker);
}
static Bowl bowl5 = new Bowl(5);
}
public class StaticInitialization {
public static void main(String[] args) {
System.out.println("Creating new Cupboard in main");
new Cupboard();
System.out.println("Creating new Cupboard in main");
new Cupboard();
table.method(1);
cupboard.method(1);
}
static Table table = new Table();
static Cupboard cupboard = new Cupboard();
}
$ javac StaticInitialization.java
$ java StaticInitialization
Bowl class marker: 1
Bowl class marker: 2
Table class marker
Bowl method: 1
Bowl class marker: 4
Bowl class marker: 5
Bowl class marker: 3
Cupboard class marker
Bowl method: 1
Creating new Cupboard in main
Bowl class marker: 3
Cupboard class marker
Bowl method: 1
Creating new Cupboard in main
Bowl class marker: 3
Cupboard class marker
Bowl method: 1
Table method: 1
Cupboard method: 1
显式的静态初始化
- 静态代码块初始化的优先级要高于静态变量，只要对该类发送了消息。
- 无论是通过
new关键字对该类进行了初始化，还是通过Cups.cup1直接调用，静态代码块都会进行初始化。
class Cup {
Cup(int marker) {
System.out.println("Cup class marker: " + marker);
}
void method(int marker){
System.out.println("Cup method: " + marker);
}
}
class Cups {
static Cup cup1;
static Cup cup2;
static {
cup1 = new Cup(1);
cup2 = new Cup(2);
}
Cups (){
System.out.println("Cup class");
}
}
public class ExplicitStatic {
public static void main(String[] args) {
System.out.println("Inside main");
Cups.cup1.method(1);
}
//static Cups cups1 = new Cups();
//static Cups cups2 = new Cups();
}
$ javac ExplicitStatic.java
$ java ExplicitStatic
Inside main
Cup class marker: 1
Cup class marker: 2
Cup method: 1
