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The OpenTripPlanner multimodal journey planning system

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package org.opentripplanner.util.lang;

import java.util.function.Function;
import javax.annotation.Nullable;

/**
 * Simple generic utility functions.
 */
public class ObjectUtils {

  /**
   * Similar to the {@link java.util.Objects#requireNonNullElse(Object, Object)}, but allow
   * the defaultValue to be {@code null}.
   */
  @Nullable
  public static  T ifNotNull(@Nullable T value, @Nullable T defaultValue) {
    return value != null ? value : defaultValue;
  }

  /**
   * Similar to {@link #ifNotNull(Object, Object)}, but take a function to access the
   * entity field. The given {@code defaultValue} can be {@code null}.
   * */
  @Nullable
  public static  T ifNotNull(
    @Nullable E entity,
    Function getter,
    @Nullable T defaultValue
  ) {
    if (entity == null) {
      return defaultValue;
    }
    return ifNotNull(getter.apply(entity), defaultValue);
  }

  public static  T requireNotInitialized(T oldValue, T newValue) {
    if (oldValue != null) {
      throw new IllegalStateException(
        "Field is already set! Old value: " + oldValue + ", new value: " + newValue
      );
    }
    return newValue;
  }

  /**
   * Map an object to a string. This is null safe and a empty string is returned if the given input
   * is {@code null}.
   */
  public static String toString(Object object) {
    return object == null ? "" : object.toString();
  }
}