Java中的if-else语句

1. Overview

In this tutorial, we’ll learn how to use the if-else statement in Java.

The if-else statement is the most basic of all control structures, and it’s likely also the most common decision-making statement in programming.

It allows us to execute a certain code section only if a specific condition is met.

2. Syntax of If-Else

The if statement always needs a boolean expression as its parameter.

if (condition) {
    // Executes when condition is true.
} else {
    // Executes when condition is false.
}

It can be followed by an optional else statement, whose contents will be executed if the boolean expression is false.

3. Example of If

So, let’s start with something very basic.

Let’s say that we only want something to happen if our count variable is larger than one:

if (count > 1) {
    System.out.println("Count is higher than 1");
}

The message Count is higher than 1 will only be printed out if the condition passes.

Also, note that we technically can remove the braces in this case since there is only one line in the block. But, we should always use braces to improve readability; even when it’s only a one-liner.

We can, of course, add more instructions to the block if we like:

if (count > 1) {
    System.out.println("Count is higher than 1");
    System.out.println("Count is equal to: " + count);
}

4. Example of If-Else

Next, we can choose between two courses of action using if and else together:

if (count > 2) {
    System.out.println("Count is higher than 2");
} else {
    System.out.println("Count is lower or equal than 2");
}

Please note that else can’t be by itself. It has to be joined with an if.

5. Example of If-Else If-Else

And finally, let’s end with a combined if/else/else if syntax example.

We can use this to choose between three or more options:

if (count > 2) {
    System.out.println("Count is higher than 2");
} else if (count <= 0) {
    System.out.println("Count is less or equal than zero");
} else {
    System.out.println("Count is either equal to one, or two");
}

6. Conclusion

In this quick article, we learned what if-else statement is and how to use it to manage flow control in our Java programs.

All code presented in this article is available over on GitHub.