Chapter 5  Conditionals and Logic

The programs in the previous chapters do the same thing every time they are run, regardless of the input. For more-complex computations, programs usually react to inputs, check for certain conditions, and generate applicable results. This chapter introduces Java language features for expressing logic and making decisions.

5.1  Relational Operators

Java has six relational operators that test the relationship between two values (e.g., whether they are equal, or whether one is greater than the other). The following expressions show how they are used:

The result of a relational operator is one of two special values: true or false. These values belong to the data type boolean, named after the mathematician George Boole. He developed an algebraic way of representing logic.

You are probably familiar with these operators, but notice how Java is different from mathematical symbols like =, ≠, and ≥. A common error is to use a single = instead of a double == when comparing values. Remember that = is the assignment operator, and == is a relational operator. Also, the operators =< and => do not exist.

The two sides of a relational operator have to be compatible. For example, the expression 5 < "6" is invalid because 5 is an int and "6" is a String. When comparing values of different numeric types, Java applies the same conversion rules you saw previously with the assignment operator. For example, when evaluating the expression 5 < 6.0, Java automatically converts the 5 to 5.0.

5.2  The if-else Statement

To write useful programs, we almost always need to check conditions and react accordingly. Conditional statements give us this ability. The simplest conditional statement in Java is the if statement:

The expression in parentheses is called the condition. If it is true, the statements in braces get executed. If the condition is false, execution skips over that block of code. The condition in parentheses can be any boolean expression.

A second form of conditional statement has two possibilities, indicated by if and else. The possibilities are called branches, and the condition determines which branch gets executed:

If the remainder when x is divided by 2 is 0, we know that x is even, and the program displays a message to that effect. If the condition is false, the second print statement is executed instead. Since the condition must be true or false, exactly one of the branches will run.

The braces are optional for branches that have only one statement. So we could have written the previous example this way:

However, it’s better to use braces—even when they are optional—to avoid making the mistake of adding statements to a one-line if or else block. This code is misleading because it’s not indented correctly:

Since there are no braces, only the first println is part of the if statement. Here is what the compiler actually sees:

As a result, the second println runs no matter what. Even experienced programmers make this mistake; search the web for Apple’s “goto fail” bug.

In all previous examples, notice that there is no semicolon at the end of the if or else lines. Instead, a new block should be defined using braces. Another common mistake is to put a semicolon after the condition, like this:

This code will compile, but the program will output "x is even" regardless of the value of x. Here is the same incorrect code with better formatting:

Because of the semicolon, the if statement compiles as if there are no braces, and the subsequent block runs independently. As a general rule, each line of Java code should end with a semicolon or brace—but not both.

The compiler won’t complain if you omit optional braces or write empty statements. Doing so is allowed by the Java language, but it often results in bugs that are difficult to find. Development tools like Checkstyle (see Appendix A.5) can warn you about these and other kinds of programming mistakes.

5.3  Chaining and Nesting

Sometimes you want to check related conditions and choose one of several actions. One way to do this is by chaining a series of if and else blocks:

These chains can be as long as you want, although they can be difficult to read if they get out of hand. One way to make them easier to read is to use standard indentation, as demonstrated in these examples. If you keep all the statements and braces lined up, you are less likely to make syntax errors.

Notice that the last branch is simply else, not else if (x == 0). At this point in the chain, we know that x is not positive and x is not negative. There is no need to test whether x is 0, because there is no other possibility.

In addition to chaining, you can also make complex decisions by nesting one conditional statement inside another. We could have written the previous example as follows:

The outer conditional has two branches. The first branch contains a print statement, and the second branch contains another conditional statement, which has two branches of its own. These two branches are also print statements, but they could have been conditional statements as well.

These kinds of nested structures are common, but they can become difficult to read very quickly. Good indentation is essential to make the structure (or intended structure) apparent to the reader.

5.4  The switch Statement

If you need to make a series of decisions, chaining else if blocks can get long and redundant. For example, consider a program that converts integers like 1, 2, and 3 into words like "one", "two", and "three":

This chain could go on and on, especially for banking programs that write numbers in long form (e.g., “one hundred twenty-three and 45/100 dollars”). An alternative way to evaluate many possible values of an expression is to use a switch statement:

The body of a switch statement is organized into one or more case blocks. Each case ends with a break statement, which exits the switch body. The default block is optional and executed only if none of the cases apply.

Although switch statements appear longer than chained else if blocks, they are particularly useful when multiple cases can be grouped:

5.5  Logical Operators

In addition to the relational operators, Java also has three logical operators: &&, ||, and !, which respectively stand for and, or, and not. The results of these operators are similar to their meanings in English. For example:

• x > 0 && x < 10 is true when x is greater than 0 and less than 10.
• x < 0 || x > 10 is true if either condition is true; that is, if x is less than 0 or greater than 10.
• !(x > 0) is true if x is not greater than 0. The parentheses are necessary in this example because, in the order of operations, ! comes before >.

In order for an expression with && to be true, both sides of the && operator must be true. And in order for an expression with || to be false, both sides of the || operator must be false.

The && operator can be used to simplify nested if statements. For example, the following code can be rewritten with a single condition:

Likewise, the || operator can simplify chained if statements. Since the branches are the same, there is no need to duplicate that code:

Then again, if the statements in the branches were different, we could not combine them into one block. But it’s useful to explore different ways of representing the same logic, especially when it’s complex.

Logical operators evaluate the second expression only when necessary. For example, true || anything is always true, so Java does not need to evaluate the expression anything. Likewise, false && anything is always false.

Ignoring the second operand, when possible, is called short-circuit evaluation, by analogy with an electrical circuit. Short-circuit evaluation can save time, especially if anything takes a long time to evaluate. It can also avoid unnecessary errors, if anything might fail.

5.6  De Morgan’s Laws

Sometimes you need to negate an expression containing a mix of relational and logical operators. For example, to test if x and y are both nonzero, you could write the following:

This condition is difficult to read because of the ! and parentheses. A better way to negate logic expressions is to apply De Morgan’s laws:

• !(A && B)  is the same as  !A || !B
• !(A || B)  is the same as  !A && !B

In words, negating a logical expression is the same as negating each term and changing the operator. The ! operator takes precedence over && and ||, so you don’t have to put parentheses around the individual terms !A and !B.

De Morgan’s laws also apply to the relational operators. In this case, negating each term means using the “opposite” relational operator:

• !(x < 5 && y == 3)  is the same as  x >= 5 || y != 3
• !(x >= 1 || y != 7)  is the same as  x < 1 && y == 7

It may help to read these examples out loud in English. For instance, “If I don’t want the case where x is less than 5 and y is 3, then I need x to be greater than or equal to 5, or I need y to be anything but 3.”

Returning to the previous example, here is the revised condition. In English, it reads, “If x is not zero and y is not zero.” The logic is the same, and the source code is easier to read:

5.7  Boolean Variables

To store a true or false value, you need a boolean variable. You can declare and assign them like other variables. In this example, the first line is a variable declaration, the second is an assignment, and the third is both:

Since relational and logical operators evaluate to a boolean value, you can store the result of a comparison in a variable:

The parentheses are unnecessary, but they make the code easier to understand. A variable defined in this way is called a flag, because it signals, or “flags”, the presence or absence of a condition.

You can use flag variables as part of a conditional statement:

Flags may not seem that useful at this point, but they will help simplify complex conditions later. Each part of a condition can be stored in a separate flag, and these flags can be combined with logical operators.

Notice that we didn’t have to write  if (evenFlag == true). Since evenFlag is a boolean, it’s already a condition. To check if a flag is false, we simply negate the flag:

In general, you should never compare anything to true or false. Doing so makes the code more verbose and awkward to read out loud.

5.8  Boolean Methods

Methods can return boolean values, just like any other type, which is often convenient for hiding tests inside methods. For example:

The name of this method is isSingleDigit. It is common to give boolean methods names that sound like yes/no questions. Since the return type is boolean, the return statement has to provide a boolean expression.

The code itself is straightforward, although it is longer than it needs to be. Remember that the expression x > -10 && x < 10 has type boolean, so there is nothing wrong with returning it directly (without the if statement):

In main, you can invoke the method in the usual ways:

The first line displays true because 2 is a single-digit number. The second line sets bigFlag to true, because 17 is not a single-digit number.

Conditional statements often invoke boolean methods and use the result as the condition:

Examples like this one almost read like English: “If is single digit z, print z is small else print z is big.”

5.9  Validating Input

One of the most important tasks in any computer program is to validate input from the user. People often make mistakes while typing, especially on smartphones, and incorrect inputs may cause your program to fail.

Even worse, someone (i.e., a hacker) may intentionally try to break into your system by entering unexpected inputs. You should never assume that users will input the right kind of data.

Consider this simple program that prompts the user for a number and computes its logarithm:

In mathematics, the natural logarithm (base e) is undefined when x ≤ 0. In Java, if you ask for Math.log(-1), it returns NaN, which stands for “not a number”. We can check for this condition and print an appropriate message:

The output is better now, but there is another problem. What if the user doesn’t enter a number at all? What would happen if they typed the word “hello”, either by accident or on purpose?

If the user inputs a String when we expect a double, Java reports an “input mismatch” exception. We can prevent this run-time error from happening by testing the input first.

The Scanner class provides hasNextDouble, which checks whether the next input can be interpreted as a double. If not, we can display an error message:

In contrast to in.nextLine, which returns an entire line of input, the in.next method returns only the next token of input. We can use in.next to show the user exactly which word they typed was not a number.

This example also uses System.err, which is an OutputStream for error messages and warnings. Some development environments display output to System.err with a different color or in a separate window.

5.10  Example Program

In this chapter, you have seen relational and logical operators, if statements, boolean methods, and validating input. The following program shows how the individual code examples in the previous section fit together:

Notice that the return statement allows you to exit a method before you reach the end of it. Returning from main terminates the program.

What started as five lines of code at the beginning of Section 5.9 is now a 30-line program. Making programs robust (and secure) often requires a lot of additional checking, as shown in this example.

It’s important to write comments every few lines to make your code easier to understand. Comments not only help other people read your code, but also help you document what you’re trying to do. If there’s a mistake in the code, finding it will be a lot easier when there are good comments.

5.11  Vocabulary

boolean:

A data type with only two possible values, true and false.

relational operator:

An operator that compares two values and produces a boolean indicating the relationship between them.

conditional statement:

A statement that uses a condition to determine which statements to execute.

block:

A sequence of statements, surrounded by braces, that generally runs as the result of a condition.

branch:

One of the alternative blocks after a conditional statement. For example, an if-else statement has two branches.

chaining:

A way of joining several conditional statements in sequence.

nesting:

Putting a conditional statement inside one or both branches of another conditional statement.

logical operator:

An operator that combines boolean values and produces a boolean value.

short circuit:

A way of evaluating logical operators that evaluates the second operand only if necessary.

De Morgan’s laws:

Mathematical rules that show how to negate a logical expression.

flag:

A variable (usually boolean) that represents a condition or status.

validate:

To confirm that an input value is of the correct type and within the expected range.

hacker:

A programmer who breaks into computer systems. The term hacker may also apply to someone who enjoys writing code.

NaN:

A special floating-point value that stands for “not a number”.

5.12  Exercises

The code for this chapter is in the ch05 directory of ThinkJavaCode2. See page ?? for instructions on how to download the repository. Before you start the exercises, we recommend that you compile and run the examples.

If you have not already read Appendix A.5, now might be a good time. It describes Checkstyle, a tool that analyzes many aspects of your source code.