Lifetime and Scope

Objectives

1. Define and discuss lifetime and scope of variables.

Notes

• For variables,
  • Scope is where a variable can be accessed
  • Lifetime is when the variable has a value
• For the purposes of this discussion variables can be declared
  • As a static class variable
  • As a non-static class variable, but we did not do this
  • As a local variable in a function
  • As a parameter to a function
  • As a variable as part of a structure
  • As a variable inside a structure code block.
• The Scope of a variable is from where it is declared until the code block it is declared in exits
  • BUT if an inner code block declares the variable it goes out of scope.
  • And if it the flow of control is transferred to another function/class the variable goes out of scope as well.
• This program illustrates the scope of a variable.

  public class Scope {
  
      public static double x = 3.14;
  
      public static void main(String[] args) {
          // the variable in scope will be the class member variable
          System.out.print("On line 7 x = ");
          System.out.println(x);
          int x = 7;
  
          // the variable in scope will be the local variable
          System.out.print("On line 12 x = ");
          System.out.println(x);
  
          SillyFunction1();
          SillyFunction2();
          SillyFunction3(99);
  
          System.out.print("On line 19 x = ");
          System.out.println(x);
  
          for(int i = 0; i < 10; ++i) {
             // other languages allow this but java does not.
             //boolean x = false;
             System.out.print("On line 19 x = ");
             System.out.println(x);
          }
  
          System.out.print("On line 19 x = ");
          System.out.println(x);
      }
  
      public static void SillyFunction1() {
          // the variable in scope will be the parameter
          String x = "Hello";
          System.out.print("In SillyFunction1 x = ");
          System.out.println(x);
      }
  
      public static void SillyFunction2() {
          // the variable in scope will be the class member variable
          System.out.print("In SillyFunction2 x = ");
          System.out.println(x);
      }
  
      public static void SillyFunction3(int x){
          // the vairable in scope will be the parameter
          System.out.print("In SillyFunction3 x = ");
          System.out.println(x);
      }
  } 

• The lifetime of a variable is from when the variable is declared until the code block that variable is declared in exits.
  • Class variables have a lifetime of the object. (more on this next semester)
  • The word static has an impact on this, but is beyond the focus of our class.
  • I believe you will learn more about this next semester as well.
  • In general, you probably want to avoid static class variables as soon as you learn about multiple class program construction.

• public class Lifetime {
       public static String a = "has liftime of the program";
  
       public static void main(String[] args){
           String b = "Has lifetime of main";
  
           int h = 0;
           // i has lifetime for the loop, scope as well.
           for(int i = 0; i < 5; ++i) {
              // j has lifetime for a single iteration of the loop.
              int j = 0;
              System.out.printf("h = %d, i = %d; j = %d\n",h,i,j);
              h += 2;
              j += 5;
           }
  
           Function1(3);
       }
  
       public static void Function1(int param) {
           int localVar;
       }
  } 

• So how do I deal with this:
  • Follow the tradition of declaring variables at the top of the function
    • This eliminates most of the need for these.
  • Don't use class level variables unless you must
    • Again, more on this next semester
    • But this also eliminates confusion.