Lab 8:  Boolean Values and Conditional Computation Solved

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  1. To explore logical (Boolean) expressions.
  2. To use the Java conditional statements if and if-else.
  3. To write a method that returns a boolean value.


  1. Go over the Lecture Notes: Topic 8 pages, Topic 10.
  2. Textbook Reading (optional): Chapter 6, Sections 6.1 and 6.2.

Exercise 1:  Evaluating logical expressions

In DrJava’s Interactions Pane, type in the following variable declarations and initializations:


int a = 12; int b = 7; boolean t = true; char c = ‘e’;


Now replace the question mark in each the following logical (also called “Boolean”)  expressions with one of the logical operators &&  | | ^   or one of the relational operators == != <  > as you type them in to the Interactions pane, so that each of the expressions evaluates to true. (Note that there may be more than one way to complete each expression correctly.)


(a > 0) &&  (b ? 0)


(‘a’ > c)  ? (‘E’ != c)


(a > b) ^ (t ? (c > ‘s’))


(a % b != 0) ? (b * 2 > a)


(t ? !(a > b)) && t

Exercise 2: Using an if statement in a while loop

In Lab 7 Exercise 3, you completed a program EnterCorrectInput that wanted a user to enter a number between 1 and 100, and kept prompting the user until the input was in the correct range. There is another common way of checking whether or not an input is in the correct range, using a boolean variable and an if statement to indicate whether or not the loop should continue. Study the code provided below; it is a variation of

EnterCorrectInput. The boolean variable cont is set to false when it is determined that the loop should not continue. (Note: we might have wanted to use the identifier continue for the boolean variable here, but that is not allowed because continue is a keyword in Java.) Fill in the blank space in the program below so that it functions the same as the program of Lab 7. (Hint: the condition in the if statement should now check if the input is in the correct range.) Test your program for boundary values, as you did in Lab 7 Exercise 3 b).


public class EnterCorrectInput2


public static void main(String[] args)

{    int num;

boolean cont = true;    // continue if cont is true

System.out.println(“Enter a number between 1 and 100.”);


while (cont)


num = SimpleInput.getIntNumber(“Enter number: “);


if (_________________________ )


cont = false;


System.out.println(“The number you entered is ” + num);






Exercise 3: Using an if statement in a Picture method

  1. Starting with the copyPictureTo method you used in Lab 5 (and Assignment 2), change it to a new method called copyExceptWhite that will copy all but the white pixels from the source picture to the target picture.
  2. Use this method to put the robot in jpg and/or the turtle in turtle.jpg on the moon in moon-surface.jpg. (These files are all in the mediasources folder).

Exercise 4:  Using an if-else statement in a Picture method

  1. Create a new method called blueExtremes()in the Picture class that removes blue from every pixel that has a blue value less than 128, and otherwise gives the pixel a maximum blue value.
  2. Test this method from the Interactions pane, using the image in jpg.