If-Else Statement

The if-else statement in Java is very similar to the if statement. In Java, the syntax for an if-else statement is shown below:

if (<Boolean expression>) {
  <true block>
} else {
  <false block>
}

As expected, Java will first evaluate the <Boolean expression> to a single Boolean value. If that value is true, it will execute the instructions in the <true block>, which can be one or more lines of code, or even additional constructs as we’ll see later. If that value is false, then the program will execute the code in the <false block> instead. In essence, the if-else statement simply adds a second code block and the else keyword after an if- statement.

Let’s take a look at a few code examples, just to see how this construct works in practice. First, let’s consider the program represented by this flowchart from earlier in the chapter:

This flowchart corresponds to the following code in Java. In this case, we’ll assume x is hard-coded for now:

int x = 1;
if (x >= 0) {
  System.out.println(x);
} else {
  System.out.println(-1 * x);
}
System.out.println("Goodbye");

As we can see, this program uses x >= 0 as the Boolean logic expression inside of the if-else statement. If it is true, then it will output the value of x. If it is false, the program will output the value (-1 * x), which represents the inverse of x.

Here’s one more example. In this program, we’d like to calculate the difference between two numbers, but we’d only like to output a positive number. So, our code may look something like this:

int x = 3;
int y = 8;
if (x > y) {
  int difference = x - y;
  System.out.println(difference);
} else {
  int difference = y - x;
  System.out.println(difference);
}

In this program, we are simply checking to see if x > y. If so, we know that x - y will be a positive number. Otherwise, we can assume that y - x will be either a positive number or $ 0 $. In either case, we see that this program will produce the correct output.

int x = 4;
if (x == 5)
System.out.println(true);
else
System.out.println(false);

int x = 4;
if (x == 5)
x = 0;
System.out.println(true);
else
x++;
System.out.println(false);