What does the Java assert keyword do, and when should it be used?

Question

What are some real-life examples to understand the key role of assertions?

Answer 1

Assertions were added in Java 1.4. They are used to verify the accuracy of an invariant in the code. They shouldn't be triggered in production code, and are indicative of a bug or misuse of a code path. They can be initiated at run-time by way of the -ea option on the java command, but are not set on by default.

An example:

public Foo acquireFoo(int id) {

  Foo result = null;

  if (id > 50) {

    result = fooService.read(id);

  } else {

    result = new Foo(id);

  }

  assert result != null;

  return result;

}