Writing a Library

Exception

1. Always throw exception as early as possible.

// DO
class Animal{
  static final String name;
  static {
       name = LoadNameFromDisk()
       if (name == null) {
          throw IllegalStateException("Can not read name from disk"); // Throw exception here
       } 
  }
  public Animal(){}
}

// DON'T
class Animal{
  static final String name = LoadNameFromDisk()
  public Animal(){
    if (name == null) {
      throw IllegalStateException("Can not read name from disk"); // Too late to throw exception. Should throw it at installation time.
    }
}

2. Hide detailed implementation exception from the upper caller. Always throw meaningful exceptions to caller (client)

// DO
class Animal{
  static final String name;
  static {
       name = LoadNameFromDisk()
       if (name == null) {
          throw IllegalStateException("Can not read name from disk"); // Throw exception here
       } 
  }
  public Animal(){}
}

// DON'T
class Animal{
  static final String name;
  static {
       name = LoadNameFromDisk()
       if (name == null) {
          throw NullPointerException("name is null"); // This is implementation detail. Should be hid from caller.
       } 
  }
  public Animal(){}
}

IllegalArgumentException: the argument the caller passed to the library is incorrect.

IllegalStateException: something wrong with the internal library implementation.