Java Variables and Scopes

What is variable in java?

• Variable: is the basic unit of storage in a java program.
• A variable is defined by the combination of an identifier, a type, and initialized value(optional).
• All variables have a scope in the program, scope defines the variables visibility, and a lifetime in the java code.
• Declaring a variable in java:
  • All variables must be declared before using in the java code.
  • Syntax: type identifier [=value][, identifier [= value]...];
  • type is Java simple data types or name of a class or interface.
  • identifier is the name of the variable.
  • we can initialize the variable by specifying an equal sign and a value.
  • More than one variable can be declared using the comma-separated list.
    Here are some examples of variable declarations of different data types.

    // declares two integer variables a and b.
    int a, b;
    
    // declares three integer variables num1, num2, and num3, also num1 and num3 are initialized with 15 and 25.
    int num1 = 15, num2, num3 = 25;
    
    // declares double variable pi with initialized value 3.14.
    double pi = 3.14;
    
    // declares character variable with initialized char value 'J'.
    char ch = 'J';
    

What is dynamic initialization in java?

• Java allows any variables to be initialized dynamically using expression.
• Privious examples are based on constants to initialize variables(which is static initialization).
• Dynamic initialization example:

  // Dynamic initialization in java                                               
  class DynamicInitializationTest {                                               
   public static void main(String args[]) {                                       
    double a = 6.0, b = 5.0;                                                      
                                                                                  
    // double variable c is here dynamically initialized                          
    // based on below expression                                                  
    double c = ((a * a) + (b * b));                                               
                                                                                  
    System.out.println("c = ((a*a) +(b*b)): " + c);                               
   }                                                                              
  }  
  

  Output:

  c = ((a*a) +(b*b)): 61.0
  

What is Scope and Lifetime of variables?

• Most programming languages define two general categories of scopes: glopal and local.
• Two major scopes in java are those defined by the class and those defined by the method.
• Scope defined by a method begins with its opening curly brace.
• Variables declared inside the a scope are not visible(accessible) to code that is defined outside the scope. When declaring a variable within a scope, localizing that variable and protecting it from unauthorized access or modification. this scope rules provide the foundation for encapsulation in java.

  // Scope and lifetime of variables in java                                      
  class ScopeTest {                                                               
   public static void main(String args[]) {                                       
    //Visible or accessible this double variables within main method              
    double a = 6.0, b = 5.0;                                                      
    double d = 0.0d;                                                              
    if (a > 5.0) {                                                                
     // double variable c is visible to only if block                             
     double c = 7.0;                                                              
     // variables a and b are visible in if block since inner block for method main.
     System.out.println("a and b: " + a + " " + b);                               
     d = a + b + c;                                                               
    }                                                                             
    // c is not visible here, gets the java compiler error                        
    //c = 4.0;                                                                    
                                                                                  
    System.out.println("d = (a + b + c): " + d);                                  
   }                                                                              
  }   
  

  Output:

  a and b: 6.0 5.0
  d = (a + b + c): 18.0