Assertions can be disabled. Since checks on parameters to non-private methods implement the requirements demanded of the caller, turning off such checks at runtime would mean that part of the contract is no longer being enforced.

Conversely, checks on arguments to private methods can indeed use assert. These checks are made to verify assumptions about internal implementation details, and not to check that the caller has followed the requirements of a non-private method's contract.

In addition, if an AssertionError can be thrown by a method, then this fact should not appear in the method's javadoc comment. The specification of a method's behaviour should never mention assertions, since assertions can be disabled.

Example

public final class SpaceShuttle {

  /**
  * The non-nullity of aName is part of the contract of the
  * constructor.
  *
  * @param aName is non-null.
  */
  public SpaceShuttle( final String aName ) {
    //WRONG way:
    assert (aName != null): "Name must not be null";
    //RIGHT way:
    if ( aName == null ) {
      throw new IllegalArgumentException("Name must not be null.");
    }
    fName = aName;
  }

  public String getName() {
    return fName;
  }

  // PRIVATE /////
  private final String fName;
}