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Creating your own exceptions

You’re not stuck using the Java exceptions. This is important because you’ll often need to create your own exceptions to denote a special error that your library is capable of creating, but which was not foreseen when the Java hierarchy was created.

To create your own exception class, you’re forced to inherit from an existing type of exception, preferably one that is close in meaning to your new exception. Inheriting an exception is quite simple:

//: Inheriting.java
// Inheriting your own exceptions

class MyException extends Exception {
  public MyException() {}
  public MyException(String msg) {
    super(msg);
  }
}

public class Inheriting {
  public static void f() throws MyException {
    System.out.println(
      "Throwing MyException from f()");
    throw new MyException();
  }
  public static void g() throws MyException {
    System.out.println(
      "Throwing MyException from g()");
    throw new MyException("Originated in g()");
  }
  public static void main(String[] args) {
    try {
      f();
    } catch(MyException e) {
      e.printStackTrace();
    }
    try {
      g();
    } catch(MyException e) {
      e.printStackTrace();
    }
  }
} ///:~

The inheritance occurs in the creation of the new class:

class MyException extends Exception {
  public MyException() {}
  public MyException(String msg) {
    super(msg);
  }
}

The key phrase here is extends Exception , which says “it’s everything an Exception is and more.” The added code is small – the addition of two constructors that define the way MyException is created. Remember that the compiler automatically calls the base-class default constructor if you don’t explicitly call a base-class constructor, as in the MyException( ) default constructor. In the second constructor, the base-class constructor with a String argument is explicitly invoked by using the super keyword.

The output of the program is:

Throwing MyException from f()
MyException
        at Inheriting.f(Inheriting.java:16)
        at Inheriting.main(Inheriting.java:24)
Throwing MyException from g()
MyException: Originated in g()
        at Inheriting.g(Inheriting.java:20)
        at Inheriting.main(Inheriting.java:29)

You can see the absence of the detail message in the MyException thrown from f( ).

The process of creating your own exceptions can be taken further. You can add extra constructors and members:

//: Inheriting2.java
// Inheriting your own exceptions

class MyException2 extends Exception {
  public MyException2() {}
  public MyException2(String msg) {
    super(msg);
  }
  public MyException2(String msg, int x) {
    super(msg);
    i = x;
  }
  public int val() { return i; }
  private int i;
}

public class Inheriting2 {
  public static void f() throws MyException2 {
    System.out.println(
      "Throwing MyException2 from f()");
    throw new MyException2();
  }
  public static void g() throws MyException2 {
    System.out.println(
      "Throwing MyException2 from g()");
    throw new MyException2("Originated in g()");
  }
  public static void h() throws MyException2 {
    System.out.println(
      "Throwing MyException2 from h()");
    throw new MyException2(
      "Originated in h()", 47);
  }
  public static void main(String[] args) {
    try {
      f();
    } catch(MyException2 e) {
      e.printStackTrace();
    }
    try {
      g();
    } catch(MyException2 e) {
      e.printStackTrace();
    }
    try {
      h();
    } catch(MyException2 e) {
      e.printStackTrace();
      System.out.println("e.val() = " + e.val());
    }
  }
} ///:~

A data member i has been added, along with a method that reads that value and an additional constructor that sets it. The output is:

Throwing MyException2 from f()
MyException2
        at Inheriting2.f(Inheriting2.java:22)
        at Inheriting2.main(Inheriting2.java:34)
Throwing MyException2 from g()
MyException2: Originated in g()
        at Inheriting2.g(Inheriting2.java:26)
        at Inheriting2.main(Inheriting2.java:39)
Throwing MyException2 from h()
MyException2: Originated in h()
        at Inheriting2.h(Inheriting2.java:30)
        at Inheriting2.main(Inheriting2.java:44)
e.val() = 47

Since an exception is just another kind of object, you can continue this process of embellishing the power of your exception classes. Keep in mind, however, that all this dressing up might be lost on the client programmers using your packages, since they might simply look for the exception to be thrown and nothing more. (That’s the way most of the Java library exceptions are used.) If this is the case, it’s possible to create a new exception type with almost no code at all:

//: SimpleException.java
class SimpleException extends Exception {
} ///:~

This relies on the compiler to create the default constructor (which automatically calls the base-class default constructor). Of course, in this case you don’t get a SimpleException(String) constructor, but in practice that isn’t used much.

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