Original article: Java: How does the assert keyword work?

By default, an `assert` statement does nothing at all. However, if you launch your program with the VM option `-enableassertions` (`-ea` for short):

```
$ java -ea com.example.Main
```

Then this statement,
```java
assert cond;
```
is equivalent to:

```java
if (!cond)
   throw new AssertionError();
```



For example, `assert i == 0;` is equivalent to:

```java
if (i != 0)
   throw new AssertionError();
```

The **Java Language Specification** states the following:

> ### 14.10. The assert Statement
>An assertion is an `assert` statement that contains a boolean expression. An assertion is *either enabled or disabled*. If the assertion is enabled, then the execution of the assertion causes an evaluation of the boolean expression and, if the expression evaluates to `false`, *an error is reported*. If the assertion is disabled, execution of the assertion has no effect whatsoever.

Here, "*enabled or disabled*" is controlled with the `-ea` switch. "*An error is reported*" means that an `AssertionError` is thrown.

Assertions allow us to check the validity of a value. If an `AssertionError` occurs, the code does not obey your assumptions.

# Adding an error message
A lesser-known feature of `assert` is that you can append an error message. For example:

```java
assert o != null : "o is null";
```

This error message is then passed to the `AssertionError` constructor and ​printed along with the stack trace.

What does the assert keyword do in Java? 

Use the `assert` keyword in Java, enabled with `-ea`, to check conditions and throw `AssertionError` if false. Add messages for clarity.