Java programming language

Control structures

Boolean type and operators

  

In this section we are going to talk about the basics of structured programming using logical information. So, we are going to introduce the boolean data types that lets us check some conditions and decide which piece of code we should run next. This way, we can choose among different results depending on a given input condition, or even repeat the execution of a given piece of code many times without having to re-type it again.

1. Boolean data type

Boolean data type is another basic data type (just like integers, characters or floating point numbers) that lets us represent two opposite values: true and false. In Java, this data type is represented by the word boolean. We can declare variables of this type, and also assign an initial value to them (from the range true and false):

boolean b = true;

We can even read boolean values from user input, using nextBoolean method from the Scanner variable. In this case, user must type true or false in the keyboard, which is not very intuitive.

Scanner sc = new Scanner(System.in);
boolean b = sc.nextBoolean();

2. Some additional operators

Related with this boolean data type, there are some Java operators that we can use. In this section we are going to talk about relational and logical operators.

2.1. Relational operators

Relational operators let us compare two different values, and check if one of them is greater, or lower, or equal than the other. This is the complete list of relational operators:

Operator Meaning
> Greater than
>= Greater or equal than
< Lower than
<= Lower or equal than
== Equal to
!= Not equal to

Note that, if we want to check if two values are equal, we use == comparator instead of just =, which is used for assignment purposes. We can join these operators with boolean values to determine if some comparisons are true or false:

int n = 10;
boolean check1 = n > 5;     // true
boolean check2 = n != 10;   // false

Regarding strings, we can’t use these comparators, because they don’t work as expected (our program compiles, but results may be unpredictable). If we want to check if two strings are the same, we use equals instruction instead of == comparator:

String s1 = "Hello";

boolean check1 = s1 == "Hello";      // Does not work as expected
boolean check2 = s1.equals("Hello"); // OK

We’ll learn more about how to deal with string values in later sections.

2.2. Logical operators

Logical operators join two or more simple comparisons to build a complex one. This way we can check if every comparison in the list is true, or at least one of them. The final result of this complex expression is also a boolean value. This is the list of logical operators:

Operator Meaning
&& AND operator
|| OR operator
! NOT operator

Regarding AND operator &&, it joins two comparisons, so that the final result will be true if both comparisons are true. Otherwise it will be false:

int n = 10, m = 5;

boolean c1 = n > 5 && m < 10;   // true && true = true
boolean c2 = n > 5 && m > 10;   // true && false = false

OR operator || also joins two comparisons, but in this case the final result will be true if any of the comparisons joined (or both) are true:

int n = 10, m = 5;

boolean c1 = n > 5 || m > 10;   // true || false = true
boolean c2 = n < 5 || m > 10;   // false || false = false

Finally, the NOT operator ! is a unary operator, this is, it affects only one expression (not two), and changes the value of this expression (this is, if the expression was true, the final result is false, and vice versa).

int n = 10;

bool c1 = n > 5;    // true
bool c2 = !c1;      // false

The precedence of these operators is important:

  1. First of all, we evaluate every expression between parentheses
  2. Then, we evaluate NOT operators
  3. Next, we check AND operators
  4. Finally, we look for OR operators

Also, you must take into account that both AND and OR operators work in short circuit mode. This means that:

Exercise 1:

Try to guess the final result of these expressions e1, e2 and e3. You can then write a short program to check your answers:

int a = 3, b = 5, c = 8;

boolean e1 = a < 2 && b >= 5 || c == 8;
boolean e2 = a < 2 && (b >= 5 || c == 8);
boolean e3 = !(a < 2) && (b >= 5 || c == 8);