Java if...else (if-then-else) Statement

In Java, the syntax of the if-then statement is:

$\begin{lstlisting} if (expression) { // statements } \end{lstlisting}$

Here expression is a boolean expression. A boolean expression returns either true or false.

if the expression is evaluated to true, statement(s) inside the body of if(statements inside parenthesis) are executed
if the expression is evaluated to false, statement(s) inside the body of if are skipped from execution

Working of Java if statement

$\begin{lstlisting} class IfStatement { public static void main(String[] args) {... ...ystem.out.println(''This statement is always executed.''); } } \end{lstlisting}$

The output for the above code is as follows.

$\begin{lstlisting} The number is positive. This statement is always executed. \end{lstlisting}$

In the above example, we have a variable named number. Here, the test expression checks if the number is greater than 0 (number > 0). Since the number is greater than 0. So the test expression evaluates to true. Hence code inside the body of if is executed. Now, change the value of the number to a negative integer. Let's say -5. If we run the above program with the new value of the number, the output will be This statement is always executed.

$\begin{lstlisting} class Main { public static void main(String[] args) { // cr... ...m.out.println(''This is best programming language.''); } } } \end{lstlisting}$

Above program display This is best programming language as an output. Here, we are comparing two strings in the if block.

The if statement executes a certain section of code if the test expression is evaluated to true. However, if the test expression is evaluated to false, it does nothing. In this case, we can use an optional else block. Statements inside the body of else block are executed if the test expression is evaluated to false. This is known as the if-then-else statement in Java. The syntax of the if-then-else statement is:

$\begin{lstlisting} if (expression) { // codes } else { // some other code } \end{lstlisting}$

Here, our program will do one task (task inside if block) if the test expression is true and another task (task inside else block) if the test expression is false.

$\begin{lstlisting} class IfElse { public static void main(String[] args) { in... ...ystem.out.println(''This statement is always executed.''); } } \end{lstlisting}$

The output for the above code is

$\begin{lstlisting} The number is positive. This statement is always executed. \end{lstlisting}$

In the above example, we have a variable named number. Here, the test expression checks if the number is greater than 0 (number > 0). Since the value of the number is 10, the test expression evaluates to true. Hence code inside the body of if is executed. Now, change the value of the number to a negative integer. Let's say -5, (int number = -5;). If we run the program with the new value of the number, the output will be:

$\begin{lstlisting} The number is not positive. This statement is always executed. \end{lstlisting}$

Here, the value of number is -5. So the test expression evaluates to false. Hence code inside the body of else is executed.

In Java, we have an if...else...if ladder, that can be used to execute one block of code among multiple other blocks.

$\begin{lstlisting} if (expression1) { // codes } else if(expression2) { // codes } else if (expression3) { // codes } . . else { // codes } \end{lstlisting}$

Here, if statements are executed from the top towards the bottom. As soon as the test expression is true, codes inside the body of that the if statement is executed. Then, the control of the program jumps outside the if-else-if ladder. If all test expressions are false, codes inside the body of else is executed.

$\begin{lstlisting} class Ladder { public static void main(String[] args) { \pa... ... } else { System.out.println(''The number is 0.''); } } } \end{lstlisting}$

Output from above code looks like The number is 0. In the above example, we are checking whether the number is positive, negative or zero. Here, we have two test expressions:

number > 0 - checks if the number is greater than 0
number < 0 - checks if the number is less than 0

Here, the value of number is 0. So both the test expression evaluates to false. Hence the statement inside the body of else is executed.