https://developers.google.com/protocol-buffers/protobuf-java/: Core Protocol Buffers library. Protocol Buffers are a way of encoding structured data in an efficient yet extensible format. 
// Protocol Buffers - Google's data interchange format
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package com.google.protobuf;


Base interface for methods common to MessageLite and Builder to provide type equivalency. 
Author:jonp@google.com (Jon Perlow)
/**
 * Base interface for methods common to {@link MessageLite} and {@link MessageLite.Builder} to
 * provide type equivalency.
 *
 * @author jonp@google.com (Jon Perlow)
 */
public interface MessageLiteOrBuilder {
  
Get an instance of the type with no fields set. Because no fields are set, all getters for singular fields will return default values and repeated fields will appear empty. This may or may not be a singleton. This differs from the getDefaultInstance() method of generated message classes in that this method is an abstract method of the MessageLite interface whereas getDefaultInstance() is a static method of a specific class. They return the same thing. 
/**
   * Get an instance of the type with no fields set. Because no fields are set, all getters for
   * singular fields will return default values and repeated fields will appear empty. This may or
   * may not be a singleton. This differs from the {@code getDefaultInstance()} method of generated
   * message classes in that this method is an abstract method of the {@code MessageLite} interface
   * whereas {@code getDefaultInstance()} is a static method of a specific class. They return the
   * same thing.
   */
  MessageLite getDefaultInstanceForType();

  
Returns true if all required fields in the message and all embedded messages are set, false
otherwise.

See also: MessageOrBuilder.getInitializationErrorString() 
/**
   * Returns true if all required fields in the message and all embedded messages are set, false
   * otherwise.
   *
   * <p>See also: {@link MessageOrBuilder#getInitializationErrorString()}
   */
  boolean isInitialized();
}