Settings by enum

Client.java

if(Settings._.hasError()){
  System.err.println("something wrong.");
}

System.out.println("main.foo: " + Settings.main$foo.value());
System.out.println("main.bar: " + Settings.main$bar.value());
System.out.println("sub.buz: " + Settings.sub$buz.value());
System.out.println("sub.quux: " + Settings.sub$quux.value());

Settings.java

public enum Settings {
  _, // dummy
  main$foo,
  main$bar,
  sub$buz("buzval"),
  sub$quux("quuxval")
  ;

  private static enum singleton {
    instance
    ;

    public boolean hasError = false;

    Properties props = new Properties(); {
      try {
        props.load(new FileInputStream("yourapp.properties"));
      } catch (Exception e) {
        hasError = true;
      }
    }
  }

  private final String defaultValue;
  private final String key;

  public String value() {
    if(hasError()) throw new IllegalStateException("properties haven't been loaded.");

    return singleton.instance.props.getProperty(key, defaultValue);
  }

  public String key() {
    return key;
  }

  public boolean hasError() {
    return singleton.instance.hasError;
  }

  private Settings() {
    this(null);
  }

  private Settings(String defaultValue) {
    this.defaultValue = defaultValue;
    this.key = this.name().replace('$', '.');
  }
}

yourapp.properties

main.foo=aaa
main.bar=bbb
sub.buz=ccc
#sub.quux=DEFAULT_VALUE_IS_CHOSEN

singleton prevents multiple and unnecessarily loading with below restrictions within enum.

1. enum cannot contain static fields
2. instance fields are initialized for each entries in the enum itself