If, else

In Java we use an if-statement (with optional "elses") to make decisions. We use expressions and operators—these things affect performance.

Ordering ifs

An important consideration with if-statements is the ordering of tests. If we place the most common first, we can speed up our program.

First example

This program uses the if-statement in a loop. It also includes else-if and else blocks. The for-loop iterates through the values 0, 1 and finally 2.

public class Program {
    public static void main(String[] args) {

        // Loop through three numbers.
        for (int i = 0; i <= 2; i++) {
            if (i == 0) {
                System.out.println("Zero");
            } else if (i == 1) {
                System.out.println("One");
            } else {
                System.out.println("Else reached");
            }
        }
    }
}
Zero
One
Else reached

Negation

To compare two numbers for inequality, we use the "!=" operator. We can also negate an entire expression by using a leading exclamation mark and parentheses.

public class Program {
    public static void main(String[] args) {

        int value = 5;

        // This expression...
        if (value != 6) {
            System.out.println("Not 6!");
        }

        // Is the same as this one.
        if (!(value == 6)) {
            System.out.println("Not 6!");
        }
    }
}
Not 6!
Not 6!

And, or

Often we chain expressions within an if-statement. We can use binary (two-part) operators for this. With AND both expressions must evaluate to true.

public class Program {
    public static void main(String[] args) {

        int width = 10;
        int height = 5;

        // Both expressions must evaluate to true.
        if (width == 10 && height == 5) {
            System.out.println("width 10 and height 5");
        }

        // Only one expression must be true.
        if (width == 100 || height > 0) {
            System.out.println("width 100 or height greater than 0");
        }
    }
}
width 10 and height 5
width 100 or height greater than 0

Boolean, store expressions

If-statements can become complex. Often an expression is repeated. We can store the result of an expression in a boolean, and then just test that.

public class Program {
    public static void main(String[] args) {

        int width = 10;
        int height = 15;
        int weight = 200;

        // Use a boolean to store computed result.
        boolean fits = width <= 10 && height <= 20;

        // We can use the boolean, not a complex expression.
        if (fits && weight <= 150) {
            System.out.println("It fits");
        } else {
            System.out.println("Does not fit");
        }
    }
}
Does not fit

True and false

A boolean variable can equal true or false. We can test expressions and variables with the true or false keywords.

public class Program {
    public static void main(String[] args) {

        boolean vacant = true;
        boolean occupied = false;

        // Test boolean variables.
        if (vacant && !occupied) {
            System.out.println(true);
        }
    }
}
true

Method call

Often we call methods in if-statements. In this example we test for an odd number. We can read the if-statement as: "if is odd number."

public class Program {

    static boolean isOdd(int value) {
        // See if number is not evenly divisible by 2.
        return (value % 2) != 0;
    }

    public static void main(String[] args) {
        int number = 5;
        // Call method in if-statement.
        if (isOdd(number)) {
            System.out.println(true);
        }
    }
}
true

Type mismatch

An expression in an if-statement must be evaluated to a boolean. In this program, we try to test an int, but this does not evaluate to true or false. It causes an error.

public class Program {
    public static void main(String[] args) {

        int value = 1;
        // This does not compile: we must have a boolean expression.
        if (value) {
            System.out.println(1);
        }
    }
}
Exception in thread "main" java.lang.Error:
    Unresolved compilation problem:
    Type mismatch: cannot convert from int to boolean...

Benchmark, reorder

Ifs are sequentially evaluated. The first expression is tested first. We can exploit this to optimize if-statement execution.

public class Program {
    public static void main(String[] args) {

        String value = "java";
        int count = 0;

        long t1 = System.currentTimeMillis();

        // Version 1: if-statements ordered most common last.
        for (int i = 0; i < 100000000; i++) {
            if (value == "tea") {
                count++;
            } else if (value == "java") {
                count += 2;
            }
            if (count == 0) {
                System.out.println(false);
            }
        }

        long t2 = System.currentTimeMillis();

        // Version 2: if-statements ordered most common first.
        for (int i = 0; i < 100000000; i++) {
            if (value == "java") {
                count += 2;
            } else if (value == "tea") {
                count++;
            }
            if (count == 0) {
                System.out.println(false);
            }
        }

        long t3 = System.currentTimeMillis();

        // ... Benchmark times.
        System.out.println(t2 - t1);
        System.out.println(t3 - t2);
    }
}
35 ms:  unoptimized order
32 ms:  optimized order

Ternary

In this kind of expression, we can assign a variable (or return a value) based on a condition. A ternary requires the question mark and ":" operators.

Summary

The if-statement, and its friends else-if and else, are important constructs. In Java, they are part of nearly every program. We use them for simple and complex selections.