java初始化块执行顺序

Z时代
2024-01-10
分类：综合

java

java中初始化块的执行顺序在构造器之前，多个初始化块之间定义在前的先执行。如下：

public class InitialBlockTest {
	// The first one
	{
		System.out.println("The first initial block");
	}
	// The second one
	{
		System.out.println("The second initial block");
	}
	// The constructor
	public InitialBlockTest() {
		System.out.println("Constructor of this class without any parameters");
	}
	public static void main(String args[]) {
		new InitialBlockTest();
	}
	// The third one
	{
		System.out.println("The third initial block");
	}
}

　结果如下：

The first initial block
The second initial block
The third initial block
Constructor of this class without any parameters

类中声明变量指定初始值也是初始化代码，与初始化块相比，其执行顺序由定义先后决定。

public class InitTest {
	{
		a = 6;
	}
	int a = 9;
	public static void main(String args[]) {
		System.out.println(new InitTest().a);
	}
}

　结果为：9

程序先为变量a分配内存，接着对其初始化。由于初始化块在前故先执行代码{ a = 6 }，再执行

a = 9；故a的值为9。

与构造器一样，创建一个java对象时，不仅会执行该类的初始化块和构造器，而且会一直上溯到Object类。如下：

public class Test {
	public static void main(String args[]) {
		new Xiaomi();
	}
}
class Phone {
	{
		System.out.println("Initial block in class Phone");
	}
	public Phone() {
		System.out.println("Constructor of Phone");
	}
}
class Xiaomi extends Phone {
	{
		System.out.println("Initial block in class Xiaomi");
	}
	public Xiaomi() {
		System.out.println("Constructor of Xiaomi");
	}
}

　结果为：

Initial block in class Phone
Constructor of Phone
Initial block in class Xiaomi
Constructor of Xiaomi

静态初始化块是由static修饰的初始化块，属于类的静态成员。通常用于对类变量执行初始化处理，系统在类初始化

阶段执行静态初始块。如下：

public class Test {
	public static void main(String args[]) {
		new Xiaomi();
		new Xiaomi();
	}
}
class Phone {
	static {
		System.out.println("Static block of Phone");
	}
	{
		System.out.println("Block of Phone");
	}
	public Phone() {
		System.out.println("Constructor of Phone");
	}
}
class Xiaomi extends Phone {
	static {
		System.out.println("Static block of Xiaomi");
	}
	{
		System.out.println("Block of Xiaomi");
	}
	public Xiaomi() {
		System.out.println("Constructor of Xiaomi");
	}
}