Building Java Programs

Lab: Parameters

Except where otherwise noted, the contents of this document are Copyright 2019 Stuart Reges and Marty Stepp.

Lab goals

Goals for this problem set:

• practice declaring and passing parameters to methods

Parameters

A parameter allows you to pass in a value to a method as you call it.

public static void name(type name) {   // declare

methodName(expression);                // call

Example:

public static void squared(int num) {
    System.out.println(num + " times " + num + " is " + (num * num));
}
...
squared(3);          // 3 times 3 is 9
squared(8);          // 8 times 8 is 64

Exercise : Parameters output

• What two values are printed by each call below? Write them in the boxes provided.

  public class MysteryNums {
      public static void main(String[] args) {
          int x = 15;
          sentence(x, 42);      // 15 42
  
          int y = x - 5;
          sentence(y, x + y);   // 10 25
      }
      
      public static void sentence(int num1, int num2) {
          System.out.println(num1 + " " + num2);
      }
  }
  

Solving "Parameter Mystery" problems

• Note the values passed, the order in which they are passed, and the order the method prints them.

  public class MysterySoda {
      public static void main(String[] args) {
          String soda = "coke";
          String pop = "pepsi";
          String pepsi = "soda";
          
          // #1 = "coke", #2 = "pepsi", #3 = "soda"
          carbonated(soda, pop, pepsi);
      }
      //                                    #1           #2           #3
      public static void carbonated(String coke, String soda, String pop) {
          //                           #2               #3             #1
          System.out.println("say " + soda + " not " + pop + " or " + coke);
      }
  }
  

• output: // say#2pepsi not#3soda or#1coke

Exercise : Parameter Mystery

• Fill in the boxes with the output that each method call would produce:

  public class MysterySoda {
      public static void main(String[] args) {
          String soda = "coke";
          String pop = "pepsi";
          String coke = "pop";
          String pepsi = "soda";
          String say = pop;
  
          carbonated(soda, pop, pepsi);        // say pepsi not soda or coke
          carbonated(coke, soda, pop);         // say coke not pepsi or pop
          carbonated(pop, pepsi, pepsi);       // say soda not soda or pepsi
          carbonated("pop", pop, "koolaid");   // say pepsi not koolaid or pop
          carbonated(say, "say", pop);         // say say not pepsi or pepsi
      }
      public static void carbonated(String coke, String soda, String pop) {
          System.out.println("say " + soda + " not " + pop + " or " + coke);
      }
  }
  

Exercise : Parameter Mystery

• Fill in the boxes with the output that each method call would produce:

  public class MysteryNumbers {
      public static void main(String[] args) {
          String one = "two";
          String two = "three";
          String three = "1";
          int number = 20;
  
          sentence(one, two, 3);                  // three times two = 6
          sentence(two, three, 14);               // 1 times three = 28
          sentence(three, three, number + 1);     // 1 times 1 = 42
          sentence(three, two, 1);                // three times 1 = 2
          sentence("eight", three, number / 2);   // 1 times eight = 20
      }
      public static void sentence(String three, String one, int number) {
          System.out.println(one + " times " + three + " = " + (number * 2));
      }
  }
  

Exercise : Parameter Mystery

• Fill in the boxes with the output that each method call would produce:

  public class Mystery {
      public static void main(String[] args) {
          String hear = "bad";
          String song = "good";
          String good = "hear";
          String walk = "talk";
          String talk = "feel";
          String feel = "walk";
          claim(feel, song, feel);       // to walk the walk is good
          claim(good, hear, song);       // to hear the good is bad
          claim(talk, "song", feel);     // to feel the walk is song
          claim("claim", talk, walk);    // to claim the talk is feel
      }
  
      public static void claim(String hear, String good, String song) {
          System.out.println("to " + hear + " the " + song + " is " + good);
      }
  }
  

Exercise : Syntax errors

• The following Java program has 9 errors. Can you find all of them?

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

  public class Parameters { public static void main() { double bubble = 867.5309; double x = 10.01; printer(double x, double y); printer(x); printer("barack", "obama"); System.out.println("z = " + z); } public static void printer(x, y double) { int z = 5; System.out.println("x = " + double x + " and y = " + y); System.out.println("The value from main is: " + bubble); } }

Exercise - answer

1. line 5: cannot use variable y without declaring and initializing it
2. line 5: cannot declare the type of y in the method call
3. line 6: cannot call printer without the correct number of parameters (2, in this case)
4. line 7: cannot call printer by passing the correct type of parameters (double, in this case)
5. line 8: cannot refer to the variable z: it is in scope inside printer, not main
6. line 11: must provide a type for x
7. line 11: must provide a variable name for the second parameter
8. line 12: must refer to the parameters using the exact same spelling
9. line 13: cannot refer to variables in main that were not passed into printer as a parameter

Exercise - Corrected version

• Here is a corrected version of the program:

  public class Parameters {
      public static void main(String[] args) {
          double bubble = 867.5309;
          double x = 10.01;
          double y = 5.3;
          printer(double x, double y);
          printer(x, 0.0);
          printer("barack", "obama");
          int z = 5;
          System.out.println("z = " + z);
      }
  
      public static void printer(double x, double y) {
          System.out.println("x = " + x + " and y = " + y);
          System.out.println("The value from main is: " + bubble);
          int z = 5;
      }
  }
  

Exercise : printGrid

• Write a method named printGrid that accepts two integers representing a number of rows and columns and prints a grid of integers from 1 to (rows*columns) in column-major order.
• For example, the call:

  printGrid(4, 6);
  

  should produce the following output:

  1 5 9 13 17 21
  2 6 10 14 18 22
  3 7 11 15 19 23
  4 8 12 16 20 24
  

Exercise : printSquare

• Write a method printSquare that accepts min and max integers and prints a square of lines of increasing numbers. The first line should start with the minimum; each line that follows should start with the next-higher number. The numbers on a line wrap back to the minimum after it hits the maximum. For example, the call:

  printSquare(3, 6);
  

  should produce the following output:

  3456
  4563
  5634
  6345
  

• If the maximum passed is less than the minimum, the method produces no output.
• Hint: remember the mod operator or think of each lines as having two parts...