Boolean Operators

Java also supports most of the Boolean operators discussed earlier in the chapter. Let’s look at a few examples.

Not !

In Java, the not operator is the exclamation point !, placed before a Boolean value. It will invert the value of the variable, changing true to false and false to true.

Here is a quick example:

boolean b = true;
boolean c = !b;
System.out.println(c); // false

This program will output false, which is the inverted value of b.

And &&

To perform the and operation, Java uses two ampersands && placed between Boolean values. Let’s look at an example:

boolean a = true;
boolean b = true;
boolean c = false;
System.out.println(a && b); // true
System.out.println(b && c); // false

This program will output true on the first line, since we know that both a and b are true. On the second line, it will output false, since c is false, even though b is true.

Or ||

Similarly, Java uses two vertical pipes || to represent the or operator. On most keyboards, we can find that key above the Enter key. Here’s an example of how it can be used in code:

boolean a = true;
boolean b = false;
boolean c = false;
System.out.println(a || b); // true
System.out.println(b || c); // false

Once again, this program will output true on the first line, since a is true, even though b is false. On the second line, it will output false, since both b and c are false.

Short-Circuit Operators

According to the Java documentation, Java also supports using a single & for and, and a single | for or when dealing with Boolean values. Why shouldn’t we just use these operators instead?

It turns out that there is a fundamental difference in how the Java compiler handles these operators. The double-character operators && and || are called logical operators and short-circuit operators. In some scenarios, the system only needs to look at the first value in the statement to determine the result. For example, the statement x || (y || z) will always be true if x is true, without needing to consider the values in y or z. The same works for x && (y && z) when x is false, which will result in the entire statement being false. For larger Boolean expressions, the use of these short-circuit operators can make our programs run much faster and more efficiently.

The single-character operators & and | bit-wise comparison operators and are not short-circuited. So, the system will evaluate each part of the statement before determining the outcome. For boolean values, the bit-wise operators will evaluate to the same answer as the logical operators.

However you should not use them for this purpose. First, because they will slow your programs executions. Second, because it will obscure your intent to future readers of your program. At some distance point in the future, a programmer will see you used a bit-wise operator, assume there must be a reason you did not using the logical one, and lose valuable time trying to figure out why you did not use the logical operator.

You can read more about Bitwise Operations on Wikipedia. We will not use them in this course.

Exclusive Or

Java does not support a logical exclusive or, but we can build a similar statement using other operators.

boolean a = true;
boolean b = false;
boolean c = true;
System.out.println((a || b) && !(a && b)); // true
System.out.println((a || c) && !(a && c)); // false

In this example, the first line will be true, since a is true but b is false. However, the second line will output false, since both a and c are true .

Comparison Operators

We can also use the comparison operators ==, !=, <, <=, >, and >= to compare variables containing numbers, which will result in a Boolean value. Here’s an example showing those operators in action:

int x = 1;
int y = 2;
double z = 3.0;

System.out.println(x < y);  // true
System.out.println(x <= 1); // true
System.out.println(x > 2);  // false
System.out.println(x >= z); // false

System.out.println(x == 1); // true
System.out.println(x != y); // true

As we can see, each of the comparison operators works just as we’d expect, and outputs a Boolean value of either true or false, depending on the result of the comparison.

Chaining Operators is Not Allowed

Like most high level languages, Java does not allow the chaining of comparison operators. 10 <= x <=20, which is a pretty standard math notation for x is between 10 and 20, will not work. First the compiler will evaluate 10 <=x as a boolean, then it will throw a fit about trying to compare a boolean and and int for the <= 20 part.

You will need to write this as 10 <= x && x <= 20.

Order of Operations

Now that we’ve introduced some additional operators, we should also see where they fit in with the other operators in Java. Here is an updated list giving the appropriate operator precedence for Java, with new entries in bold:

  1. Parentheses
  2. Postfix: Increment ++ & Decrement -- after variable*
  3. Prefix: Inverse -, Not !, Increment ++ & Decrement -- before variable*
  4. Multiplicative: Multiplication *, Division /, and Modulo %
  5. Additive: Addition +, Subtraction -
  6. Relational: <, >, <=, >=
  7. Equality: ==, !=
  8. Logical And: &&
  9. Logical Or: ||
  10. Assignment: =, +=, -=, *=, /=, %=