Читать книгу OCP Oracle Certified Professional Java SE 17 Developer Study Guide - Jeanne Boyarsky - Страница 164

The final operator you should be familiar with for the exam is the conditional operator, ? :, otherwise known as the ternary operator. It is notable in that it is the only operator that takes three operands. The ternary operator has the following form:

booleanExpression ? expression1 : expression2

The first operand must be a boolean expression, and the second and third operands can be any expression that returns a value. The ternary operation is really a condensed form of a combined if and else statement that returns a value. We cover if/else statements in a lot more detail in Chapter 3, so for now we just use simple examples.

For example, consider the following code snippet that calculates the food amount for an owl:

int owl = 5; int food; if(owl < 2) { food = 3; } else { food = 4; } System.out.println(food); // 4

Compare the previous code snippet with the following ternary operator code snippet:

int owl = 5; int food = owl < 2 ? 3 : 4; System.out.println(food); // 4

These two code snippets are equivalent. Note that it is often helpful for readability to add parentheses around the expressions in ternary operations, although doing so is certainly not required. It is especially helpful when multiple ternary operators are used together, though. Consider the following two equivalent expressions:

int food1 = owl < 4 ? owl > 2 ? 3 : 4 : 5; int food2 = (owl < 4 ? ((owl > 2) ? 3 : 4) : 5);

While they are equivalent, we find the second statement far more readable. That said, it is possible the exam could use multiple ternary operators in a single line.

For the exam, you should know that there is no requirement that second and third expressions in ternary operations have the same data types, although it does come into play when combined with the assignment operator. Compare the two statements following the variable declaration:

int stripes = 7; System.out.print((stripes > 5) ? 21 : "Zebra"); int animal = (stripes < 9) ? 3 : "Horse"; // DOES NOT COMPILE

Both expressions evaluate similar boolean values and return an int and a String, although only the first one will compile. System.out.print() does not care that the expressions are completely different types, because it can convert both to Object values and call toString() on them. On the other hand, the compiler does know that "Horse" is of the wrong data type and cannot be assigned to an int; therefore, it does not allow the code to be compiled.