An **assertion** is defined as a claim or assumption. In programming, there are cases when we, as programmers, know that a certain variable takes a specific value(s) at a particular line of code. We usually represent such a claim using comments in our code. 

An assertion in Java is one single line of code which checks a certain boolean expression and throws a ***AssertionError*** if that expression is *false*. We say that an assertion has *failed* in this case.

 >**Note:** Assertions are *disabled by default* in Java. Include `-ea` command in your Java interpreter (or search specifically for your IDE on the web).


# Syntax

There are two ways to write an assertion using the *assert keyword*.

1. `assert expression;`

We can pass our own message as any type of variable (other than *void*) which will be shown in the stack trace in case the expression was false.

2. `assert expression : "Invalid value";`

# When to use assertions?

1. Using an assertion instead of a comment we put for our own convenience:

int x = 3;

if(x <= 2) {
	// ...
}
else {  // x > 2
	// ...
}

Commenting

int x = 3;

if(x <= 2) {
	// ...
}
else {
	assert x > 2 : "x is not greater 2";
}

Assertion

2. Checking control-flow. If we know that our code is not meant to reach a certain line, we can place the assertion in that prohibited area;but, it should be written in such a way that it always returns false:

int i = 5;
while(i > 0){
  if(i == 2){ // Can be any condition which you think will be surely met.
    return;
  }

  i--;
}
  // My program should not reach this place.

Normally

int i = 5;
while(i > 0){
  if(i == 2){ // Can be any condition which you think will be surely met.
    return;
  }

  i--;
}
assert false : "Condition not met.";

What are assertions in Java?

Assertions check boolean expressions in Java, throwing AssertionError on false. Disabled by default; use -ea to enable. Syntax: assert expression[: "message"].