/*
 * Copyright (C) 2007 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.base;

import java.util.NoSuchElementException;

/**
 * Contains simple static methods to be called at the start of your own methods
 * to verify correct arguments and state. This allows constructs such as
 * <pre>
 *     if (count <= 0) {
 *       throw new IllegalArgumentException("must be positive: " + count);
 *     }</pre>
 *
 * to be replaced with the more compact
 * <pre>
 *     checkArgument(count > 0, "must be positive: %s", count);</pre>
 *
 * Note that the sense of the expression is inverted; with {@code Preconditions}
 * you declare what you expect to be <i>true</i>, just as you do with an
 * <a href="http://java.sun.com/j2se/1.4.2/docs/guide/lang/assert.html">
 * {@code assert}</a> or a JUnit {@code assertTrue()} call.
 *
 * <p>Take care not to confuse precondition checking with other similar types
 * of checks! Precondition exceptions -- including those provided here, but also
 * {@link IndexOutOfBoundsException}, {@link NoSuchElementException}, {@link
 * UnsupportedOperationException} and others -- are used to signal that the
 * <i>calling method</i> has made an error. This tells the caller that it should
 * not have invoked the method when it did, with the arguments it did, or
 * perhaps <i>ever</i>. Postcondition or other invariant failures should not
 * throw these types of exceptions, perhaps using {@link AssertionError}
 * instead.
 *
 * <p><b>Note:</b> The methods of the {@code Preconditions} class are highly
 * unusual in one way: they are <i>supposed to</i> throw exceptions, and promise
 * in their specifications to do so even when given perfectly valid input. That
 * is, {@code null} is a valid parameter to the method {@link
 * #checkNotNull(Object)} -- and technically this parameter could be even marked
 * as {@link Nullable}! -- yet the method will still throw an exception anyway,
 * because that's what its contract says to do.
 *
 * @author Kevin Bourrillion
 */
public final class Preconditions {
  private Preconditions() {}

  /**
   * Ensures the truth of an expression involving one or more parameters to the
   * calling method.
   *
   * @param expression a boolean expression
   * @throws IllegalArgumentException if {@code expression} is false
   */
  public static void checkArgument(boolean expression) {
    if (!expression) {
      throw new IllegalArgumentException();
    }
  }

  /**
   * Ensures the truth of an expression involving one or more parameters to the
   * calling method.
   *
   * @param expression a boolean expression
   * @param errorMessage the exception message to use if the check fails; will
   *     be converted to a string using {@link String#valueOf(Object)} only if
   *     needed
   * @throws IllegalArgumentException if {@code expression} is false
   */
  public static void checkArgument(boolean expression, Object errorMessage) {
    if (!expression) {
      throw new IllegalArgumentException(String.valueOf(errorMessage));
    }
  }

  /**
   * Ensures the truth of an expression involving one or more parameters to the
   * calling method.
   *
   * @param expression a boolean expression
   * @param errorMessageFormat the {@link java.util.Formatter format string} for
   *     the desired error message
   * @param errorMessageArgs the arguments referenced by the format specifiers
   *     in {@code errorMessageFormat}.
   * @throws IllegalArgumentException if {@code expression} is false
   * @throws NullPointerException if the check fails and either {@code
   *     errorMessageFormat} or {@code errorMessageArgs} is null (don't let this
   *     happen)
   */
  public static void checkArgument(boolean expression,
      String errorMessageFormat, Object... errorMessageArgs) {
    if (!expression) {
      throw new IllegalArgumentException(
          String.format(errorMessageFormat, errorMessageArgs));
    }
  }

  /**
   * Ensures the truth of an expression involving the state of the calling
   * instance, but not involving any parameters to the calling method.
   *
   * @param expression a boolean expression
   * @throws IllegalStateException if {@code expression} is false
   */
  public static void checkState(boolean expression) {
    if (!expression) {
      throw new IllegalStateException();
    }
  }

  /**
   * Ensures the truth of an expression involving the state of the calling
   * instance, but not involving any parameters to the calling method.
   *
   * @param expression a boolean expression
   * @param errorMessage the exception message to use if the check fails; will
   *     be converted to a string using {@link String#valueOf(Object)} only if
   *     needed
   * @throws IllegalStateException if {@code expression} is false
   */
  public static void checkState(boolean expression, Object errorMessage) {
    if (!expression) {
      throw new IllegalStateException(String.valueOf(errorMessage));
    }
  }

  /**
   * Ensures the truth of an expression involving the state of the calling
   * instance, but not involving any parameters to the calling method.
   *
   * @param expression a boolean expression
   * @param errorMessageFormat the {@link java.util.Formatter format string} for
   *     the desired error message
   * @param errorMessageArgs the arguments referenced by the format specifiers
   *     in {@code errorMessageFormat}.
   * @throws IllegalStateException if {@code expression} is false
   * @throws NullPointerException if the check fails and either {@code
   *     errorMessageFormat} or {@code errorMessageArgs} is null (don't let this
   *     happen)
   */
  public static void checkState(boolean expression,
      String errorMessageFormat, Object... errorMessageArgs) {
    if (!expression) {
      throw new IllegalStateException(
          String.format(errorMessageFormat, errorMessageArgs));
    }
  }

  /**
   * Ensures that an object reference passed as a parameter to the calling
   * method is not null, throwing a {@code NullPointerException} if it is.
   *
   * @param reference an object reference
   * @return the non-null reference that was validated
   * @throws NullPointerException if {@code reference} is null
   */
  public static <T> T checkNotNull(T reference) {
    if (reference == null) {
      throw new NullPointerException();
    }
    return reference;
  }

  /**
   * Ensures that an object reference passed as a parameter to the calling
   * method is not null, throwing a {@code NullPointerException} if it is.
   *
   * @param reference an object reference
   * @param errorMessage the exception message to use if the check fails; will
   *     be converted to a string using {@link String#valueOf(Object)} only if
   *     needed
   * @return the non-null reference that was validated
   * @throws NullPointerException if {@code reference} is null
   */
  public static <T> T checkNotNull(T reference, Object errorMessage) {
    if (reference == null) {
      throw new NullPointerException(String.valueOf(errorMessage));
    }
    return reference;
  }

  /**
   * Ensures that an object reference passed as a parameter to the calling
   * method is not null, throwing a {@code NullPointerException} if it is.
   *
   * @param reference an object reference
   * @param errorMessageFormat the {@link java.util.Formatter format string} for
   *     the desired error message
   * @param errorMessageArgs the arguments referenced by the format specifiers
   *     in {@code errorMessageFormat}.
   * @return the non-null reference that was validated
   * @throws NullPointerException if {@code reference} is null
   */
  public static <T> T checkNotNull(T reference, String errorMessageFormat,
      Object... errorMessageArgs) {
    if (reference == null) {
      // If either of these parameters is null, the right thing happens anyway
      throw new NullPointerException(
          String.format(errorMessageFormat, errorMessageArgs));
    }
    return reference;
  }

  /**
   * Ensures that an {@code Iterable} object passed as a parameter to the
   * calling method is not null and contains no null elements.
   *
   * @param iterable any {@code Iterable} object
   * @return the non-null {@code iterable} reference just validated
   * @throws NullPointerException if {@code iterable} is null or contains at
   *     least one null element
   */
  public static <T extends Iterable<?>> T checkContentsNotNull(T iterable) {
    for (Object element : iterable) {
      checkNotNull(element);
    }
    return iterable;
  }

  /**
   * Ensures that an {@code Iterable} object passed as a parameter to the
   * calling method is not null and contains no null elements.
   *
   * @param iterable any {@code Iterable} object
   * @param errorMessage the exception message to use if the check fails; will
   *     be converted to a string using {@link String#valueOf(Object)} only if
   *     needed
   * @return the non-null {@code iterable} reference just validated
   * @throws NullPointerException if {@code iterable} is null or contains at
   *     least one null element
   */
  public static <T extends Iterable<?>> T checkContentsNotNull(
      T iterable, Object errorMessage) {
    checkNotNull(iterable, errorMessage);

    for (Object element : iterable) {
      checkNotNull(element, errorMessage);
    }
    return iterable;
  }

  /**
   * Ensures that an {@code Iterable} object passed as a parameter to the
   * calling method is not null and contains no null elements.
   *
   * @param iterable any {@code Iterable} object
   * @param errorMessageFormat the {@link java.util.Formatter format string} for
   *     the desired error message
   * @param errorMessageArgs the arguments referenced by the format specifiers
   *     in {@code errorMessageFormat}.
   * @return the non-null {@code iterable} reference just validated
   * @throws NullPointerException if {@code iterable} is null or contains at
   *     least one null element
   */
  public static <T extends Iterable<?>> T checkContentsNotNull(T iterable,
      String errorMessageFormat, Object... errorMessageArgs) {
    checkNotNull(iterable, errorMessageFormat, errorMessageArgs);

    for (Object element : iterable) {
      checkNotNull(element, errorMessageFormat, errorMessageArgs);
    }
    return iterable;
  }
}