Question: Explain the concept of scope and lifetime of variables in computer programming. Give examples to illustrate your explanation.

Answer: Scope and Lifetime of Variables:

Scope refers to the region in a program where a variable is valid and can be accessed. Lifetime, on the other hand, refers to the period during which a variable is allocated and exists in the memory. Understanding the scope and lifetime of variables is crucial for writing clean and efficient code.

1. Local Variables: Local variables are declared inside a block of code, typically within a function or method. They have a limited scope and are only accessible within that specific block. Once the block of code is executed, the local variables cease to exist.

public class Example {
   public static void main(String[] args) {
      int x = 10; // local variable
      
      if (x > 5) {
         int y = 20; // local variable
         System.out.println(x + y); // valid
      }
      
      System.out.println(y); // Error: y is out of scope
   }
}

In the example above, variable x is declared inside the main method and can be accessed within the entire method. Variable y, however, is declared inside the if block and can only be accessed within that block. Trying to access y outside the block would result in a compilation error.

2. Instance Variables: Instance variables are declared within a class, but outside any method or block. They belong to the instance of the class and can be accessed throughout the entire class. Instance variables have a longer lifetime compared to local variables.

public class Example {
   int x; // instance variable

   public void setX(int value) {
      x = value;
   }

   public int getX() {
      return x;
   }

   public static void main(String[] args) {
      Example obj = new Example();
      obj.setX(5);
      System.out.println(obj.getX()); // 5
   }
}

In the example above, variable x is an instance variable. It is accessible within the setX() and getX() methods because they belong to the class. The instance of the class named obj is created in the main method and used to set the value of x and retrieve it.

3. Class Variables (Static Variables): Class variables, also known as static variables, are declared with the static keyword within a class. They are accessible to all instances of the class and have the longest lifetime. Class variables are shared among instances of the class.

public class Example {
   static int count = 0; // class variable
  
   public Example() {
      count++;
   }

   public static void main(String[] args) {
      Example obj1 = new Example();
      Example obj2 = new Example();
     
      System.out.println(obj1.count); // 2
      System.out.println(obj2.count); // 2
   }
}

In the example above, variable count is a class variable declared with the static keyword. Every time an instance of the Example class is created using the Example() constructor, the value of count is incremented. Since the class variable is shared among instances, both obj1.count and obj2.count will have the same value.

Understanding the scope and lifetime of variables helps ensure proper memory management and prevents conflicts or unexpected behavior when accessing variables in different parts of the program.