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What does the assert keyword do in Java?

Educative Answers Team

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,

assert cond;

is equivalent to:

if (!cond)
   throw new AssertionError();
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For example, assert i == 0; is equivalent to:

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:

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

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

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assert
assertion
java
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