What are macro variables and macro substitution?

　In Java, macro variables and macro substitution are generally concepts related to the C/C++ preprocessor rather than features of Java itself. In Java, we usually don't use preprocessor directives such as macro definitions for code replacement. The compiler used by Java does not support preprocessor directives, so there is no concept of macro substitution.

　　However, to understand these concepts, we can briefly explain what macro variables and macro substitution mean, and demonstrate how they work using C/C++ code.

　　1. Macro variables

　　A macro variable is an identifier with a specific value created by a macro definition. They are usually used to represent constant values ​​or complex expressions. In the preprocessing stage, the compiler will replace the place where the macro variable appears with its value.

　　2. Macro replacement

　　Macro replacement refers to the replacement of macro variables appearing in the source code during the preprocessing stage. The preprocessor replaces macro variables in the code with their corresponding values ​​or expressions according to predefined macro rules.

　　The following is a simple C++ code example that demonstrates the process of macro definition and macro substitution:

#include <iostream>

#define PI 3.14159
#define SQUARE(x) (x) * (x)

int main() {
    int radius = 5;
    double area = PI * SQUARE(radius);

    std::cout << "The area of the circle with radius " << radius << " is: " << area << std::endl;

    return 0;
}

　　In the above code, we defined two macros:

　　1.PI

　　Represents pi, which is a constant macro.

　　2.SQUARE(x)

　　It means to calculate the square of a number, it is a macro with parameters.

　　In the preprocessing stage, the compiler replaces the source code, replacing macro variables with their values. The preprocessed code is as follows:

#include <iostream>

int main() {
    int radius = 5;
    double area = 3.14159 * (radius) * (radius);

    std::cout << "The area of the circle with radius " << radius << " is: " << area << std::endl;

    return 0;
}

　　It can be seen that the macro replacement replaces PI in the code with 3.14159, and replaces SQUARE(radius) with (radius)*(radius).

　　Again, this is how the C/C++ preprocessor works, and there is no equivalent concept of preprocessor directives or macro substitution in Java. In Java, constants are usually defined using the final keyword instead of macro variables defined using the preprocessor.