Step 1 of 6

if

So far every line of a program ran every time. An if statement runs a block of code only when a condition is true:

int temperature = 31;
if (temperature > 30) {
    System.out.println("Hot day: drink water.");
}
System.out.println("Temperature: " + temperature);

Output

Hot day: drink water.
Temperature: 31

The condition goes in parentheses after if. Here it's temperature > 30, which is either true or false.
The block in braces { } runs only when the condition is true. With temperature = 25, the program would print only the second line.
The line after the block runs either way.

No semicolon after the condition

A semicolon right after if (...) ends the if there, with nothing in it. The block below it then runs every time, and Java doesn't warn about it:

int speed = 30;
if (speed > 50); {
    System.out.println("Too fast!");
}

Output

Too fast!

Only 30, and yet "Too fast!". When an if seems to be ignored, check for this semicolon first.