Next, we present the conditional operators, && and ||, in Table 2.10.
| TABLE 2 . 10 Conditional operators | ||||
| Operator | Example | Description | ||
| Conditional | a && b | Value is true only if both values are true. If the left side is false, then | ||
| AND | the right side will not be evaluated. | |||
| Conditional | c || d | Value is true if at least one of the values is true. If the left side is | ||
| OR | true, then the right side will not be evaluated. | |||
The conditional operators, often called short-circuit operators, are nearly identical to the logical operators, & and |, except that the right side of the expression may never be evalu-ated if the final result can be determined by the left side of the expression. For example, con-sider the following statement:
int hour = 10;
boolean zooOpen = true || (hour < 4);
System.out.println(zooOpen); // true
Referring to the truth tables, the value zooOpen can be false only if both sides of the expression are false. Since we know the left side is true, there’s no need to evaluate the right side, since no value of hour will ever make this code print false. In other words, hour could have been -10 or 892; the output would have been the same. Try it yourself with different values for hour!
A more common example of where conditional operators are used is checking for null objects before performing an operation. In the following example, if duck is null, the program will throw a NullPointerException at runtime:
if(duck!=null & duck.getAge()<5) { // Could throw a NullPointerException // Do something
}
The issue is that the logical AND (&) operator evaluates both sides of the expression. We could add a second if statement, but this could get unwieldy if we have a lot of variables to check. An easy-to-read solution is to use the conditional AND operator (&&):
if(duck!=null && duck.getAge()<5) {
// Do something
}
In this example, if duck is null, the conditional prevents a NullPointerException from ever being thrown, since the evaluation of duck.getAge() < 5 is never reached.
Be wary of short-circuit behavior on the exam, as questions are known to alter a variable on the right side of the expression that may never be reached. This is referred to as an unper-formed side effect. For example, what is the output of the following code?
int rabbit = 6;
boolean bunny = (rabbit >= 6) || (++rabbit <= 7); System.out.println(rabbit);
Because rabbit >= 6 is true, the increment operator on the right side of the expression is never evaluated, so the output is 6.
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