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When evaluating a logical expression, the final outcome can often be determined without the requirement to evaluate all of the operands.
Once the final outcome is determined, no attempt is made to evaluate the remainder of the expression. This is short-circuit evaluation.
Code from Question 1
Consider the following code fragment from Question 1 :
int x = 5, y = 6;
if((x>y) || (y<x/0)){
... |
The (||) operator is the logical or operator.
Boolean operands required
This operator requires that its left and right operands both be of type boolean . This operator performs an inclusive or on its left and right operands. The rules for an inclusive or are:
If either of its operands is true, the operator returns true. Otherwise, it returns false.
Left operand is false
In this particular expression, the value of x is not greater than the value of y . Therefore, the left operand of the logical or operator is not true.
Right operand must be evaluated
This means that the right operand must be evaluated in order to determine the final outcome.
Right operand attempts to divide by zero
However, when an attempt is made to evaluate the right operand, an attempt is made to divide x by zero. This throws an exception, which is not caught and handled by the program, so the program terminates as described in Question 1 .
Similar code from Question 2
Now consider the following code fragment from Question 2 .
int x = 5, y = 6;
if((x<y) || (y<x/0)){
System.out.println("A");... |
Note that the right operand of the logical or operator still contains an expression that attempts to divide the integer x by zero.
No runtime error in this case
This program does not terminate with a runtime error. Why not?
And the answer is ...
In this case, x is less than y . Therefore, the left operand of the logical or operator is true.
Remember the rule for inclusive or
It doesn't matter whether the right operand is true or false. The final outcome is determined as soon as it is determined that the left operand is true.
The bottom line
Because the final outcome has been determined as soon as it is determined that the left operand is true, no attempt is made toevaluate the right operand.
Therefore, no attempt is made to divide x by zero, and no runtime error occurs.
Short-circuit evaluation
This behavior is often referred to as short-circuit evaluation .
Only as much of a logical expression is evaluated as is required to determine the final outcome.
Once the final outcome is determined, no attempt is made to evaluate the remainder of the logical expression.
This is not only true for the logical or operator, it is also true for the logical and operator, which consists of two ampersand characters with no space between them.
B. Runtime Error
Divide by zero
Whenever a Java program attempts to evaluate an expression requiring that a value of one of the integer types be divided by zero, it will throw an ArithmeticException . If this exception is not caught and handled by the program, it will cause the program to terminate.
Attempts to divide x by 0
This program attempts to evaluate the following expression:
(y<x/0) |
This expression attempts to divide the variable named x by zero. This causes the program to terminate with the following error message when running under JDK 1.3:
java.lang.ArithmeticException: / by zero
at Worker.doLogical(Ap039.java:13)at Ap039.main(Ap039.java:6) |
-end-
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