9 Zen

9.1 Boolean Zen

Boolean Zen is all about using boolean values efficiently and concisely.

The following code shows a tempting way to write an if statement based on boolean value.

Bad

if (test == true) {
    // do some work
}

Note that test itself is a boolean value. When it is true, we are asking whether true == true. true == true will evaluate to true, but remember that the if branch will execute as long as what is in its parentheses evaluates to true. So we can actually use test directly:

Good

if (test) {
    // do some work
}

Note

To check for the opposite of test, don’t check for test == false. Instead, use !test to check that the opposite of test evaluates to true.

Here’s an example that uses what we learned in the previous section about simplifying boolean expressions.

Bad

if (test) {
    return true;
} else {
    return false;
}

There is actually a much more concise way to do this. If we want to return true when test is true and false when test is false, then we actually just want to return the value of test.

Good

return test;

In general, make your use of booleans to simplify conditional and boolean expressions. This is something to look out for whenever you have conditions or boolean values.

Bad

Because this code always wants to do one or the other, (and doesn’t involve a return or exception) we want to express this code more simply as an if/else.

if (someTest) {
    System.out.println("hello!");
}
if (!someTest) {
    System.out.println("I'm redundant");
}

Bad

Note that the behavior inside this if block is exactly the same behavior as in the other if block. Instead of rewriting the same code twice, we can combine the two if conditions with || and just write the behavior once.

if (max < result) {
    return max;
}
if (max == 0) {
    return max;
}

Bad

It doesn’t matter if you think of conditions/cases or their negated versions, but after revising your code don’t include empty condition blocks with no line of code inside. Instead, just flip the condition to have if (max >= 0), and no else.

if (max < 0) {
    // do nothing
} else {
    ...
}

Don't use if / else statements or ternary operators to  to assign boolean values the result of a boolean expression.

Bad

boolean meetsReq;
if (score > = MIN_SCORE) {
    meetsReq = true;
} else {
    meetsReq = false;
}

Still Bad

boolean meetsReq = false;
if (score > = MIN_SCORE) {
    meetsReq = true;
}

Good

boolean meetsReq = score >= MIN_SCORE;

9.2 Loop Zen

9.2.1 Loop Bounds

When writing loops, choose loop bounds or loop conditions that help generalize code the best. For example, the code before this for loop is unnecessary.

Bad

System.out.print("\*");
for (int i = 0; i < 4; i++) {
    System.out.print("\*");
}

Instead, why not just run the loop one extra time? This way we avoid writing duplicate behavior.

Good

for (int i = 0; i < 5; i++) {
    System.out.print("\*");
}

9.2.2 Only Repeated Tasks

If you have something that only happens once (maybe at the end of a bunch of repeated tasks), then don’t put code for it inside of your loop.

Bad

for (int i = 0; i < 4; i++) {
    if (i != 3) {
        System.out.println("working hard!");
    } else {
        System.out.println("hardly working!");
    }
}

Good

for (int i = 0; i < 3; i++) {
    System.out.println("working hard!");
}
System.out.println("hardly working!");

Similarly, a loop that always runs one time is also a bad use of a loop. When reviewing the logic of your code, it might be helpful to check if a loop can be reduced to an if or just deleting the loop entirely.

9.3 Recursion Zen

Avoid adding extra if/else cases to recursive methods when there are more generalized conditions to use.

Bad

This private method is an example of arm’s length recursion: it has an extra base case for index == list.length - 1. If we omit this and just use the first base case, the behavior will actually still generalize and be correct, but our code will be more concise and easier to reason about.

private static int sum(int[] list, int index) {
    if (index == list.length) {
        return 0;
    } else if (index == list.length - 1) {
        return list[index];
    } else {
        return list[index] + sum(list, index + 1);
    }
}

Good

private static int sum(int[] list, int index) {
    if (index == list.length) {
        return 0;
    } else {
        return list[index] + sum(list, index + 1);
    }
}

Bad

This example also exhibits arm’s length recursion since the code in the public method would be more appropriate to handle in the private helper method.

public static int sum(int[] list) {
    if (list.length < 1) {
        return 0;
    } else {
        return list[0] + sum(list, 1);
    }
}

private static int sum(int[] list, int index) {
    if (index == list.length) {
        return 0;
    } else {
        return list[index] + sum(list, index + 1);
    }
}

Good

public static int sum(int[] list) {
    return sum(list, 0);
}

private static int sum(int[] list, int index) {
    if (index == list.length) {
        return 0;
    } else {
        return list[index] + sum(list, index + 1);
    }
}