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.
Every if statement needs a condition that results in true or false .
This means if statements work hand-in-hand with boolean values:
Example
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
- Use if to specify a block of code to be executed, if a specified condition is true
- Use else to specify a block of code to be executed, if the same condition is false
- Use else if to specify a new condition to test, if the first condition is false
- Use switch to specify many alternative blocks of code to be executed
The if Statement
The if statement specifies a block of code to be executed if a condition is true :
Syntax
if (condition) {
// block of code to be executed if the condition is true
}The condition inside the if statement must result in a boolean value - it can be either a boolean expression (like x > y ) or a boolean variable (like isLightOn ).
Also note that if is in lowercase letters. Uppercase letters (If or IF) will generate an error.
In the example below, we test two values to find out if 20 is greater than 18. If the condition is true , print some text:
Example
if (20 > 18) {
System.out.println("20 is greater than 18");
}Example
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 and y , 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 are equal , using the == operator:
Example
int x = 20;
int y = 20;
if (x == y) {
System.out.println("x is equal to y");
}Here the condition x == y is true, because both x and y are 20, so the message "x is equal to y" is printed.
Using Boolean Variables
You can also test boolean variables directly in an if statement:
Example
boolean isLightOn = true;
if (isLightOn) {
System.out.println("The light is on.");
}Note
Writing if (isLightOn) is the same as writing if (isLightOn == true) , but shorter and easier to read.
Here is the same example with the value false to see that the program continues even when the code block does not run:
Example
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 an if statement has only one line of code, you can write it without curly braces { } :
Example
if (20 > 18)
System.out.println("20 is greater than 18");Potential Problem
Without braces, only the first line after the if belongs to it. Any other lines will run no matter what, which can lead to unexpected results:
Example
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 the if statement:
Example
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 if System.out.println("I am outside if, not part of if!");Tip
Always using braces { } makes your code clearer, easier to read, and prevents subtle bugs.
In the next chapters, you will also learn how to handle else (when the condition is false), else if (to test multiple conditions), and switch (to handle many possible values).