3.5. Compound Boolean Expressions¶

What if you want two things to be true before the body of the conditional is executed? Use && as a logical and to join two Boolean expressions and the body of the condition will only be executed if both are true.

Coding Exercise

What if you want to go out and your parents say you can go out if you clean your room and do your homework? Run the code below and try different values for cleanedRoom and didHomework and see what they have to be for it to print You can go out .

What if it is okay if only one of two things is true? Use || as a logical or to join two Boolean expressions and the body of the condition will be executed if one or both are true.

Coding Exercise

For example, your parents might say you can go out if you can walk or they don’t need the car. Try different values for walking and carIsAvailable and see what the values have to be to print You can go out .

In English, we often use an exclusive-or like in the sentence “do you want to be player 1 or player 2?” where you can’t be both player 1 and player 2. In programming, the or-operator is an inclusive-or which means that the whole expression is true if either one or the other or both conditions are true.

With numerical values, the or-operator is often used to check for error conditions on different ends of the number line, while the and-operator is often used to see if a number is in an range.

Coding Exercise

Explore how && and || are used with numbers below. Try different values for score like -10 and 110 in the code below.

The not (!) operator can be used to negate a boolean value. We’ve seen ! before in != (not equal). In Java, ! has precedence (is executed before) &&, and && has precedence over ||. Parentheses can be used to force the order of execution in a different way. If you mix ! with && and ||, be careful because the results are often the opposite of what you think. We’ll see examples of this in the next lesson.

Coding Exercise

The code below says if homework is not done, you can’t go out. Try different values for homeworkDone .

In Java, ! will be executed before &&, and && will be executed before ||, unless there are parentheses. Anything inside parentheses is executed first.

3.5.2. Truth Tables¶

The following table (also called a truth table) shows the result for P && Q when P and Q are both expressions that can be true or false. An expression involving logical operators like (P && Q) evaluates to a Boolean value, true or false. As you can see below the result of P && Q is only true if both P and Q are true.

Check your understanding

The following table shows the result for P || Q when P and Q are both expressions that can be true or false. As you can see below the result of P || Q is true if either P or Q is true. It is also true when both of them are true.

Check your understanding

Check your understanding

3-5-7: What is printed when the following code executes and x has been set to 3 and y has been set to 9?

1if (x > 0 && (y / x) == 3) 2 3 System.out.println("first case"); 4> 5else 6 7 System.out.println("second case"); 8> 

3-5-8: What is printed when the following code executes and x has been set to 3 and y has been set to 6?

1if (x > 0 && (y / x) == 3) 2 3 System.out.println("first case"); 4> 5else 6 7 System.out.println("second case"); 8> 

3-5-9: What is printed when the following code executes and x has been set to 3 and y has been set to 6? Notice that it is now an or (||) instead of and.

1if (x > 0 || (y / x) == 3) 2 3 System.out.println("first case"); 4> 5else 6 7 System.out.println("second case"); 8> 

3.5.3. Short Circuit Evaluation¶

Both && and || use short circuit evaluation. That means that the second condition isn’t necessarily checked if the result from the first condition is enough to tell if the result is true or false. In a complex conditional with a logical and ( && ) both conditions must be true, so if the first is false, then the second doesn’t have to be evaluated. If the complex conditional uses a logical or ( || ) and the first condition is true, then the second condition won’t be executed, since only one of the conditions needs to be true.

In a complex conditional using a logical and ( && ) the evaluation will short circuit (not execute the second condition) if the first condition is false. In a complex conditional using a logical or ( || ) the evaluation will short circuit if the first condition is true.

Check your understanding

3-5-10: What is printed when the following code executes and x has been set to 0 and y to 3?

1if (x > 0 && (y / x) == 3) 2 3 System.out.println("first case"); 4> 5else 6 7 System.out.println("second case"); 8> 

3-5-11: What is printed when the following code executes and x has been set to zero and y is set to 3?

1if (x == 0 || (y / x) == 3) 2 3 System.out.println("first case"); 4> 5else 6 7 System.out.println("second case"); 8> 

3-5-12: What is printed when the following code executes and x has been set to zero and y is set to 3?

1if ((y / x) == 3 || x == 0 ) 2 3 System.out.println("first case"); 4> 5else 6 7 System.out.println("second case"); 8> 

3.5.4. Programming Challenge : Truth Tables¶

Explore the following problems:

1. Draw or print a Venn diagram of 4 intersecting circles. Put the names of 4 people in each circle. Write down the age of each person in the circles. If two or more people are the same age, put the age in the intersecting parts of their circles. Write a Boolean expression that compares the age of each person in the group using ==, , and &&, for example Ada’s age > Alan’s age && Alan’s age == Grace’s age. Then, ask each person in your group their favorite movie. If two or more people have the same favorite movie, put the movie in the intersecting parts of their circles. Write a Boolean expression that compares the favorite movies in the group using ==, !=, and &&, for example Ada’s movie == Alan’s movie && Alan’s movie != Grace’s movie. Think of 1 more comparison and write it in the circles and as a Boolean expression. Share the Boolean expressions with the class. (Thank you to Jill Westerlund of Hoover High School and Art Lopez of Sweetwater High School for this activity suggestion).
2. Write the sentence “If it’s sunny, OR if the temperature is greater than 80 and it’s not raining, I will go to the beach.” as a Java if statement using an int variable temperature and boolean variables sunny and raining . If the conditional is true, print out “Go to the beach!”. So, you will go to beach on days that it is sunny in any temperature, or you will go to the beach on days when the temperature is over 80 degrees and it’s not raining.
3. Complete a truth table for the if statement that you wrote in #2 with columns for sunny, temperature > 80, raining, and go to the beach.
4. Write Java code below to test your if statement and try all the values in your truth table to see if you filled it out correctly. You will need test case for each of the 8 rows in your truth table, for example when sunny is true and false, when raining is true or false, and for a value of temperature greater than 80, for example 90, and less than 80, for example 60.

3.5.5. Summary¶

• Logical operators !(not), &&(and), and ||(or) are used with Boolean values.
• (A && B) is true if both A and B are true.
• (A || B) is true if either A or B (or both) are true.
• !(A) is true if A is false.
• In Java, ! has precedence (is executed before) && which has precedence over ||. Parentheses can be used to force the order of execution in a different way.
• When the result of a logical expression using && or || can be determined by evaluating only the first Boolean operand, the second is not evaluated. This is known as short-circuit evaluation.

© Copyright 2015 Barb Ericson, 2019 revised by Beryl Hoffman Mobile CSP, 2020 revised by Linda Seiter and Dan Palmer. Created using Runestone 7.3.7.