Java Basics 1.07

Passing By Copy; Primitives

When calling a function, a COPY of the argument is passed to the method

     void incrementValue(int inFunction) {

            inFunction ++;

            System.out.println("In function: " + inFunction);

      }

      

      int original = 10;

      System.out.println("Original before : " + original);

      incrementValue(original);

      System.out.println("Original after : " + original);

 

      // Original before: 10

      // In function: 11

      // Original after: 10

   }

 

Passing by Reference: Complex Objects

• Variable of Complex Object is a Reference to Memory
• A Copy of the Complex Object is passed, but references the same memory addresses

 

inf[] original = {10,20,30};

original[0] > {10,20,30} < inFunction[0]

 

Passing Strings: Is Complex Object but Acts Like Primitive

void changeString(String inFunction) {

     inFunction = “New!”;

     System.out.println(“In function: “ + inFunction);

}

 

• Strings are immutable, can’t change after declaration
• Copy of entire string passed to function

      // Original before: Original!

      // In function: New!

      // Original after: Original!

 

 

Extract Method with Error Handling

 

 

More Complex Programs and An Error Handling Declaration

 

Code:

import java.io.*;

 

public class Calculator2 {

 

    public static void main(String[] args) {

      String s1 = getInput("Enter a numeric value: ");

      String s2 = getInput("Enter a numeric value: ");

      String op = getInput("Enter 1=Add, 2=Subtract, 3=Multiply, 4=Divide: ");

      

      int opInt = Integer.parseInt(op);

      // declare variables BEFORE switch

      double result = 0;

      

      switch (opInt) {

      case 1:

            result = addValues(s1, s2);

            break;

      case 2:

            result = subtractValues(s1, s2);

            break;

      case 3:

            result = multiplyValues(s1, s2);

            break;

      case 4:

            result = divideValues(s1, s2);

            break;

      default:

            System.out.println("Unknown Operation: " + op);

            break;

      }

            

      System.out.println("The answer is " + result);

    }

 

    private static double divideValues(String s1, String s2) {

      double d1 = Double.parseDouble(s1);

      double d2 = Double.parseDouble(s2);

      double result = d1 / d2;

      return result;

    }

 

    private static double multiplyValues(String s1, String s2) {

      double d1 = Double.parseDouble(s1);

      double d2 = Double.parseDouble(s2);

      double result = d1 * d2;

      return result;

    }

 

    private static double subtractValues(String s1, String s2) {

      double d1 = Double.parseDouble(s1);

      double d2 = Double.parseDouble(s2);

      double result = d1 - d2;

      return result;

    }

 

    private static double addValues(String s1, String s2)

            throws NumberFormatException {

      double d1 = Double.parseDouble(s1);

      double d2 = Double.parseDouble(s2);

      double result = d1 + d2;

      return result;

    }

   

    private static String getInput(String prompt) {

      BufferedReader stdin = new BufferedReader(

                 new InputStreamReader(System.in));

 

      System.out.print(prompt);

      System.out.flush();

      

      try {

            return stdin.readLine();

      } catch (Exception e) {

            return "Error: " + e.getMessage();

      }

    }

 

}