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What are assertions in Java?

Educative Answers Team

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).

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Program flow with an assert statement


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.

  1. 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
	// ...
  1. 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.

  // My program should not reach this place.


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