Fields should be declared at the top of the class, before any method declarations, constructors, initializers or inner classes. Example:
public class HelloWorldBean {

  // Field declared before methods / inner classes - OK
  private String _thing;

  public String getMessage() {
    return "Hello World!";
  }

  // Field declared after methods / inner classes - avoid this
  private String _fieldInWrongLocation;
}

This rule is deprecated, use {rule:squid:S1213} instead.