JavaIf ... Else

Java Conditions and If Statements

Conditions and if statements let you control the flow of your program - deciding which code runs, and which code is skipped.

Think of it like real life:If it rains, take an umbrella. Otherwise, do nothing.

Everyif statement needs a condition that results intrue orfalse.

This meansif statements work hand-in-hand withboolean values:

boolean isRaining = true;if (isRaining) {  System.out.println("Bring an umbrella!");}

Most often, conditions are created using comparison operators, like the ones below:

• Less than:a < b
• Less than or equal to:a <= b
• Greater than:a > b
• Greater than or equal to:a >= b
• Equal to:a == b
• Not equal to:a != b

You can use these conditions to perform different actions for different decisions.

Java has the following conditional statements:

• Useif to specify a block of code to be executed, if a specified condition is true
• Useelse to specify a block of code to be executed, if the same condition is false
• Useelse if to specify a new condition to test, if the first condition is false
• Useswitch to specify many alternative blocks of code to be executed

The if Statement

Theif statement specifies a block of code to be executed if a condition istrue:

if (condition) {  // block of code to be executed if the condition is true}

In the example below, we test two values to find out if 20 is greater than 18. If the condition istrue, print some text:

if (20 > 18) {  System.out.println("20 is greater than 18");}

You can also compare variables:

int x = 20;int y = 18;if (x > y) {  System.out.println("x is greater than y");}

Example explained

In the example above we use two variables,x andy, to test whether x is greater than y (using the> operator). As x is 20, and y is 18, and we know that 20 is greater than 18, we print to the screen that "x is greater than y".

Comparison is also often used to check if two values areequal, using the== operator:

int x = 20;int y = 20;if (x == y) {  System.out.println("x is equal to y");}

Here the conditionx == y is true, because bothx andy are 20, so the message "x is equal to y" is printed.

Using Boolean Variables

You can also test boolean variables directly in anif statement:

boolean isLightOn = true;if (isLightOn) {  System.out.println("The light is on.");}

Note: Writingif (isLightOn) is the same as writingif (isLightOn == true), but shorter and easier to read.

Here is the same example with the valuefalse to see that the program continues even when the code block does not run:

boolean isLightOn = false;if (isLightOn) {  System.out.println("The light is on.");  // This will not be printed}System.out.println("This line runs no matter what, because it is outside the if statement.");

If Without Braces

If anif statement has only one line of code, you can write it without curly braces{ }:

if (20 > 18)  System.out.println("20 is greater than 18");

Potential Problem

Without braces, only thefirst line after theif belongs to it. Any other lines will run no matter what, which can lead to unexpected results:

int x = 20;int y = 18;if (x > y)  System.out.println("x is greater than y");  // Belongs to if  System.out.println("This line runs no matter what (not part of the if statement)");// Output:// x is greater than y// This line runs no matter what (not part of the if statement)

The Safe Way

To avoid mistakes, always use curly braces{ }. This makes it clear which lines belong to theif statement:

int x = 20;int y = 18;if (x > y) {  System.out.println("x is greater than y");  System.out.println("Both lines are part of the if");}// Some code outside ifSystem.out.println("I am outside if, not part of if!");

In the next chapters, you will also learn how to handleelse (when the condition is false),else if (to test multiple conditions), andswitch (to handle many possible values).