guava/src/com/google/common/base/Verify.java

/*
 * Copyright (C) 2013 The Guava Authors
 *
 * 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 static com.google.common.base.Preconditions.format;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;

import javax.annotation.Nullable;

/**
 * Static convenience methods that serve the same purpose as Java language
 * <a href="http://docs.oracle.com/javase/7/docs/technotes/guides/language/assert.html">
 * assertions</a>, except that they are always enabled. These methods should be used instead of Java
 * assertions whenever there is a chance the check may fail "in real life". Example: <pre>   {@code
 *
 *   Bill bill = remoteService.getLastUnpaidBill();
 *
 *   // In case bug 12345 happens again we'd rather just die
 *   Verify.verify(bill.status() == Status.UNPAID,
 *       "Unexpected bill status: %s", bill.status());}</pre>
 *
 * <h3>Comparison to alternatives</h3>
 *
 * <p><b>Note:</b> In some cases the differences explained below can be subtle. When it's unclear
 * which approach to use, <b>don't worry</b> too much about it; just pick something that seems
 * reasonable and it will be fine.
 *
 * <ul>
 * <li>If checking whether the <i>caller</i> has violated your method or constructor's contract
 *     (such as by passing an invalid argument), use the utilities of the {@link Preconditions}
 *     class instead.
 *
 * <li>If checking an <i>impossible</i> condition (which <i>cannot</i> happen unless your own class
 *     or its <i>trusted</i> dependencies is badly broken), this is what ordinary Java assertions
 *     are for. Note that assertions are not enabled by default; they are essentially considered
 *     "compiled comments."
 *
 * <li>An explicit {@code if/throw} (as illustrated above) is always acceptable; we still recommend
 *     using our {@link VerifyException} exception type. Throwing a plain {@link RuntimeException}
 *     is frowned upon.
 *
 * <li>Use of {@link java.util.Objects#requireNonNull(Object)} is generally discouraged, since
 *     {@link #verifyNotNull(Object)} and {@link Preconditions#checkNotNull(Object)} perform the
 *     same function with more clarity.
 * </ul>
 *
 * <h3>Warning about performance</h3>
 *
 * <p>Remember that parameter values for message construction must all be computed eagerly, and
 * autoboxing and varargs array creation may happen as well, even when the verification succeeds and
 * the message ends up unneeded. Performance-sensitive verification checks should continue to use
 * usual form: <pre>   {@code
 *
 *   Bill bill = remoteService.getLastUnpaidBill();
 *   if (bill.status() != Status.UNPAID) {
 *     throw new VerifyException("Unexpected bill status: " + bill.status());
 *   }}</pre>
 *
 * <h3>Only {@code %s} is supported</h3>
 *
 * <p>As with {@link Preconditions} error message template strings, only the {@code "%s"} specifier
 * is supported, not the full range of {@link java.util.Formatter} specifiers. However, note that
 * if the number of arguments does not match the number of occurrences of {@code "%s"} in the
 * format string, {@code Verify} will still behave as expected, and will still include all argument
 * values in the error message; the message will simply not be formatted exactly as intended.
 *
 * <h3>More information</h3>
 *
 * See
 * <a href="http://code.google.com/p/guava-libraries/wiki/ConditionalFailuresExplained">Conditional
 * failures explained</a> in the Guava User Guide for advice on when this class should be used.
 *
 * @since 17.0
 */
@Beta
@GwtCompatible
public final class Verify {
  /**
   * Ensures that {@code expression} is {@code true}, throwing a {@code VerifyException} with no
   * message otherwise.
   */
  public static void verify(boolean expression) {
    if (!expression) {
      throw new VerifyException();
    }
  }

  /**
   * Ensures that {@code expression} is {@code true}, throwing a {@code VerifyException} with a
   * custom message otherwise.
   *
   * @param expression a boolean expression
   * @param errorMessageTemplate a template for the exception message should the
   *     check fail. The message is formed by replacing each {@code %s}
   *     placeholder in the template with an argument. These are matched by
   *     position - the first {@code %s} gets {@code errorMessageArgs[0]}, etc.
   *     Unmatched arguments will be appended to the formatted message in square
   *     braces. Unmatched placeholders will be left as-is.
   * @param errorMessageArgs the arguments to be substituted into the message
   *     template. Arguments are converted to strings using
   *     {@link String#valueOf(Object)}.
   * @throws VerifyException if {@code expression} is {@code false}
   */
  public static void verify(
      boolean expression,
      @Nullable String errorMessageTemplate,
      @Nullable Object... errorMessageArgs) {
    if (!expression) {
      throw new VerifyException(format(errorMessageTemplate, errorMessageArgs));
    }
  }

  /**
   * Ensures that {@code reference} is non-null, throwing a {@code VerifyException} with a default
   * message otherwise.
   *
   * @return {@code reference}, guaranteed to be non-null, for convenience
   */
  public static <T> T verifyNotNull(@Nullable T reference) {
    return verifyNotNull(reference, "expected a non-null reference");
  }

  /**
   * Ensures that {@code reference} is non-null, throwing a {@code VerifyException} with a custom
   * message otherwise.
   *
   * @param errorMessageTemplate a template for the exception message should the
   *     check fail. The message is formed by replacing each {@code %s}
   *     placeholder in the template with an argument. These are matched by
   *     position - the first {@code %s} gets {@code errorMessageArgs[0]}, etc.
   *     Unmatched arguments will be appended to the formatted message in square
   *     braces. Unmatched placeholders will be left as-is.
   * @param errorMessageArgs the arguments to be substituted into the message
   *     template. Arguments are converted to strings using
   *     {@link String#valueOf(Object)}.
   * @return {@code reference}, guaranteed to be non-null, for convenience
   */
  public static <T> T verifyNotNull(
      @Nullable T reference,
      @Nullable String errorMessageTemplate,
      @Nullable Object... errorMessageArgs) {
    verify(reference != null, errorMessageTemplate, errorMessageArgs);
    return reference;
  }

  // TODO(kevinb): consider <T> T verifySingleton(Iterable<T>) to take over for
  // Iterables.getOnlyElement()

  private Verify() {}
}