Java Tutorial/Development/Assertions

A Program with Assertions

public class Main {
  public static void main(String[] args) {
    assert args.length > 0;
  }
}

Assertions

public class MainClass {
  public static void main(String[] arg) {
    int a = 0;
    assert a == 0;
  }
}

java -enableassertions MyProg
You can also use its abbreviated form -ea:
   java -ea MyProg

Assert with an informative message

public class MainClass {
  public static void main(String[] args) {
    assert false: "Here"s a message saying what happened";
  }
}

Catch assert exception with message

public class Main {
  public static void main(String[] argv) throws Exception {
    try {
      assert argv.length > 0 : "my message";
    } catch (AssertionError e) {
      String message = e.getMessage();
      System.out.println(message);
    }
  }
}

Compile "assert"

// Non-informative style of assert
// Compile with: javac -source 1.4 Assert1.java
// {JVMArgs: -ea} // Must run with -ea
public class MainClass {
  public static void main(String[] args) {
    assert false;
  }
}

Enabling Assertions from the Command Line: -ea and -da enable and disable assertion in a package subtree or in a class.

java -ea MyApp
java -ea:... MyApp
java -ea:com.mycompany... MyApp
java -ea:com.mycompany... -da:com.mycompany.MyClass MyApp

Handling an Assertion Error

public class Main {
  public static void main(String[] argv) throws Exception {
    try {
      assert argv.length > 0;
    } catch (AssertionError e) {
      String message = e.getMessage();
      System.out.println(message);
    }
  }
}

More Complex Assertions

assert logical_expression : string_expression;

public class MainClass {
  public static void main(String[] arg) {
    int value = 80;
    assert false : "value: " + value;
  }
}

Using the class loader to enable assertions

// Compile with: javac -source 1.4 LoaderAssertions.java
public class MainClass {
  public static void main(String[] args) {
    ClassLoader.getSystemClassLoader()
      .setDefaultAssertionStatus(true);
    new Loaded().go();
  }
}
class Loaded {
  public void go() {
    assert false: "Loaded.go()";
  }
}