Null Pointer Exception

In a computer, a variable is like a note or an address book that says where something is. For example, when you tap a friend's name in your phone's contact list, the computer reads the phone number note linked to that name and makes the call.

But what if the friend's name is in the list, yet the actual phone number field is "empty (Null)"? If you tell the computer to press the call button anyway, it tries to dial a number that doesn't exist, gets badly confused, and stops working.

Here, a "pointer" is a finger pointing at where a piece of data sits in memory, and "null" means nothing is there. It's like telling someone to pick something up by pointing a finger at thin air. You're holding the paper (the variable), but the actual contents (the object) the paper points to are completely empty.

You've probably had a phone app go dark without any warning, or kick you back to the home screen. One common cause of those forced shutdowns is exactly this error.

For example, say your internet connection is shaky and the app can't get profile pictures or comment data from the server in time. If the app is told to draw the screen when there's no data ready, the computer tries to force open an empty space and throws an error.

If no safeguard has been prepared, the program can't run its next line of code and just collapses on the spot. That's why developers need to write checks done in advance (null checks) that first ask, "Could this value be empty?" before they handle any data.

In the history of computer programming, the idea of null has a very famous story. In 1965, Tony Hoare, a renowned British computer scientist, added the null reference to a programming language for the first time, as an easy way to express "no value exists."

Decades later, though, he admitted in public that his invention was a billion-dollar mistake. Because null values were allowed, countless crashes and security flaws showed up in software around the world, and fixing them cost an astronomical amount.

Many newer languages, such as Kotlin and Swift, come with clever safeguards built in to escape this long-running pain. A technique called null safety is spreading, which strictly checks whether a variable can hold an empty value from the moment a programmer writes the code.