**Member Variables**
A member variable is a variable that belongs to a class and represents a specific dimension member. These variables are used internally by Analysis Services to identify members of a dimension. The `MemberKeyColumn` property defines the member variable of a dimension. For example, a number between 1 and 12 could represent a month of the year as a member variable.
In Java, member variables are also known as fields. They are declared within the class body and can be accessed by any method or constructor within the class. Member variables include instance variables, class (static) variables, and constants. Here's an example from the `Clothes` class:
```java
package ch05.sample;
public class Clothes {
String id; // instance variable
private String colorType; // instance variable
private int size; // instance variable
private static String depart; // class variable
final String design = "yangzi"; // constant
}
```
The distinction between these types of variables is based on their modifiers. Instance variables are associated with an object, while class variables are shared among all instances of the class. Constants, defined with the `final` keyword, cannot be changed once initialized.
**Global Variables**
A global variable is a variable that is accessible throughout the entire program. In traditional programming languages, global variables can be accessed from any part of the code. However, in modern object-oriented languages like Java, C++, and C#, the concept of true global variables has been largely replaced by static class variables.
While local variables are limited to the scope of a method or block, global variables (or class-level variables) can be accessed by any method within the class. By declaring a variable as `public static`, it becomes effectively a global variable, accessible from anywhere in the program.
**Difference Between Member Variables and Global Variables**
- **Member Variables**: These are variables defined within a class. They can be either instance variables (non-static) or class variables (static). Member variables are part of the class and exist for the lifetime of the object.
- **Global Variables**: In some contexts, global variables are considered the same as member variables. However, in a broader sense, they refer to variables that are accessible across different parts of the program. In Java, this is typically achieved using `public static` variables.
**Key Differences:**
1. **Scope**: Member variables are scoped to the class, while global variables (if allowed) are accessible across the entire program.
2. **Initialization**: Member variables have default values (e.g., `0` for `int`, `null` for objects), whereas local variables must be explicitly initialized before use.
3. **Lifetime**: Member variables exist as long as the object exists, while local variables are destroyed once the method or block ends.
4. **Access Modifiers**: Member variables can have access modifiers like `private`, `protected`, and `public`, while local variables cannot.
**Example Code:**
```java
public class Variable {
int i; // member variable
void test() {
int j = 8; // local variable
if (j == i)
System.out.println("equal");
else
System.out.println("not equal");
}
public static void main(String[] args) {
Variable v = new Variable();
v.test();
}
}
```
In this example, `i` is a member variable and is automatically initialized to `0`. However, if we declare `i` inside the method, like in the next example, it will cause a compilation error because local variables must be explicitly initialized.
```java
public class Variable {
void test() {
int i; // local variable
int j = 8;
if (j == i)
System.out.println("equal");
else
System.out.println("not equal");
}
public static void main(String[] args) {
Variable v = new Variable();
v.test();
}
}
```
This code will result in an error: `variable i might not have been initialized`.
**Summary:**
- **Member Variables**: Defined in the class, can be accessed by all methods, have default values, and are stored in the heap.
- **Local Variables**: Declared inside a method or block, must be initialized, and are stored in the stack.
- **Global Variables (in Java)**: Achieved through `public static` variables, which act like global variables but are still encapsulated within the class.
Understanding the difference between these types of variables is essential for writing clean, efficient, and maintainable code.
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