说明Java继承中的类加载和静态块

类加载是指读取.class文件并将相应的二进制数据存储在方法区中。对于每个.class文件，JVM 都会在 Method Area 中存储相应的信息。现在在类加载中加入继承。在Java继承中，JVM首先加载并初始化父类，然后加载并初始化子类。

例子

Java

// Java program to Illustrate Class Loading
 
// Importing input output files
import java.io.*;
 
// Helper class 1
class A {
 
    // First static block of this class
    static
    {
 
        // Print message
        System.out.println(
            "Loading class A 1st static block ");
    }
 
    // Second static block of this class
    static
    {
 
        // Print message
        System.out.println(
            "Loading class A 2nd static block B.c="
            + B.c);
    }
 
    // Third static block of this class
    static
    {
 
        // Print message
        System.out.println(
            "Loading class A 3rd static block ");
    }
 
    static int a = 50;
    A() {}
}
 
// Helper class 2 extending from
// helper class 1
class B extends A {
 
    static
    {
        // Print message
        System.out.println(
            "Loading class B static block A.a=" + A.a);
    }
 
    static int c = 100;
}
 
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Execution statement in main class
        new B();
    }
}

输出

Loading class A 1st static block 
Loading class A 2nd static block B.c=0
Loading class A 3rd static block 
Loading class B static block A.a=50

From the above output. it can be said that the parent class loads before the child class and also child class initialize after the parent class. One can check this by executing it with java -verbose Test.

静态块将在类加载时执行，因此在类加载时，如果我们想要执行任何活动，我们必须在静态块中定义它。在静态块继承中，必须记住以下两条规则：

1. 不能覆盖静态块，因为将生成非静态 else 编译时错误。
2. 非静态块不能被覆盖为静态块。

对于上述场景，静态块在继承中的使用如下所示，因为将生成的编译器错误分别显示如下：

情况 1：不能覆盖静态块，因为将生成非静态 else 编译时错误

例子

Java

// Java Program to illustrate static blocks in inheritance
 
// Importing all input output classes
import java.io.*;
 
// Class 1- Parent Class
class GFG {
 
    // Declaring a static method m1
    public static void m1() {}
}
 
// Class 2 -  child class of GFG
class c extends GFG {
 
    // Declaring a non- static method m1
    public void m1() {}
}

输出：

情况 2：非静态块不能被覆盖为静态块

例子

Java

// Java Program to illustrate static blocks in inheritance
 
// Importing all input output classes
import java.io.*;
 
// Class 1 - Parent class
class GFG {
 
    // Declaring a non-static method m1
    public void m1() {}
}
 
// Class 2 - child class of parent
// child class of GFG
class C extends GFG {
 
    // Declaring a static method m1
    public static void m1() {}
}

输出：

Note: No compile-time error will be generated only and only if both parent and child class methods are static.