Java tutorial
/* * Copyright (C) 2007 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 com.google.common.annotations.GwtCompatible; import javax.annotation.Nullable; /** * Static convenience methods that help a method or constructor check whether it was invoked * correctly (whether its <i>preconditions</i> have been met). These methods generally accept a * {@code boolean} expression which is expected to be {@code true} (or in the case of {@code * checkNotNull}, an object reference which is expected to be non-null). When {@code false} (or * {@code null}) is passed instead, the {@code Preconditions} method throws an unchecked exception, * which helps the calling method communicate to <i>its</i> caller that <i>that</i> caller has made * a mistake. Example: <pre> {@code * * /** * * Returns the positive square root of the given value. * * * * @throws IllegalArgumentException if the value is negative * *}{@code / * public static double sqrt(double value) { * Preconditions.checkArgument(value >= 0.0, "negative value: %s", value); * // calculate the square root * } * * void exampleBadCaller() { * double d = sqrt(-1.0); * }}</pre> * * In this example, {@code checkArgument} throws an {@code IllegalArgumentException} to indicate * that {@code exampleBadCaller} made an error in <i>its</i> call to {@code sqrt}. * * <h3>Warning about performance</h3> * * <p>The goal of this class is to improve readability of code, but in some circumstances this may * come at a significant performance cost. 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 precondition check then succeeds (as it should almost always do in production). In some * circumstances these wasted CPU cycles and allocations can add up to a real problem. * Performance-sensitive precondition checks can always be converted to the customary form: * <pre> {@code * * if (value < 0.0) { * throw new IllegalArgumentException("negative value: " + value); * }}</pre> * * <h3>Other types of preconditions</h3> * * <p>Not every type of precondition failure is supported by these methods. Continue to throw * standard JDK exceptions such as {@link java.util.NoSuchElementException} or {@link * UnsupportedOperationException} in the situations they are intended for. * * <h3>Non-preconditions</h3> * * <p>It is of course possible to use the methods of this class to check for invalid conditions * which are <i>not the caller's fault</i>. Doing so is <b>not recommended</b> because it is * misleading to future readers of the code and of stack traces. See * <a href="https://github.com/google/guava/wiki/ConditionalFailuresExplained">Conditional * failures explained</a> in the Guava User Guide for more advice. * * <h3>{@code java.util.Objects.requireNonNull()}</h3> * * <p>Projects which use {@code com.google.common} should generally avoid the use of {@link * java.util.Objects#requireNonNull(Object)}. Instead, use whichever of {@link * #checkNotNull(Object)} or {@link Verify#verifyNotNull(Object)} is appropriate to the situation. * (The same goes for the message-accepting overloads.) * * <h3>Only {@code %s} is supported</h3> * * <p>In {@code Preconditions} error message template strings, only the {@code "%s"} specifier is * supported, not the full range of {@link java.util.Formatter} specifiers. * * <h3>More information</h3> * * <p>See the Guava User Guide on * <a href="https://github.com/google/guava/wiki/PreconditionsExplained">using {@code * Preconditions}</a>. * * @author Kevin Bourrillion * @since 2.0 */ @GwtCompatible 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)} * @throws IllegalArgumentException if {@code expression} is false */ public static void checkArgument(boolean expression, @Nullable 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 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 IllegalArgumentException if {@code expression} is false * @throws NullPointerException if the check fails and either {@code errorMessageTemplate} or * {@code errorMessageArgs} is null (don't let this happen) */ public static void checkArgument(boolean expression, @Nullable String errorMessageTemplate, @Nullable Object... errorMessageArgs) { if (!expression) { throw new IllegalArgumentException(format(errorMessageTemplate, 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)} * @throws IllegalStateException if {@code expression} is false */ public static void checkState(boolean expression, @Nullable 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 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 IllegalStateException if {@code expression} is false * @throws NullPointerException if the check fails and either {@code errorMessageTemplate} or * {@code errorMessageArgs} is null (don't let this happen) */ public static void checkState(boolean expression, @Nullable String errorMessageTemplate, @Nullable Object... errorMessageArgs) { if (!expression) { throw new IllegalStateException(format(errorMessageTemplate, errorMessageArgs)); } } /** * Ensures that an object reference passed as a parameter to the calling method is not null. * * @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. * * @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)} * @return the non-null reference that was validated * @throws NullPointerException if {@code reference} is null */ public static <T> T checkNotNull(T reference, @Nullable 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. * * @param reference an object reference * @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 the non-null reference that was validated * @throws NullPointerException if {@code reference} is null */ public static <T> T checkNotNull(T reference, @Nullable String errorMessageTemplate, @Nullable Object... errorMessageArgs) { if (reference == null) { // If either of these parameters is null, the right thing happens anyway throw new NullPointerException(format(errorMessageTemplate, errorMessageArgs)); } return reference; } /* * All recent hotspots (as of 2009) *really* like to have the natural code * * if (guardExpression) { * throw new BadException(messageExpression); * } * * refactored so that messageExpression is moved to a separate String-returning method. * * if (guardExpression) { * throw new BadException(badMsg(...)); * } * * The alternative natural refactorings into void or Exception-returning methods are much slower. * This is a big deal - we're talking factors of 2-8 in microbenchmarks, not just 10-20%. (This * is a hotspot optimizer bug, which should be fixed, but that's a separate, big project). * * The coding pattern above is heavily used in java.util, e.g. in ArrayList. There is a * RangeCheckMicroBenchmark in the JDK that was used to test this. * * But the methods in this class want to throw different exceptions, depending on the args, so it * appears that this pattern is not directly applicable. But we can use the ridiculous, devious * trick of throwing an exception in the middle of the construction of another exception. Hotspot * is fine with that. */ /** * Ensures that {@code index} specifies a valid <i>element</i> in an array, list or string of size * {@code size}. An element index may range from zero, inclusive, to {@code size}, exclusive. * * @param index a user-supplied index identifying an element of an array, list or string * @param size the size of that array, list or string * @return the value of {@code index} * @throws IndexOutOfBoundsException if {@code index} is negative or is not less than {@code size} * @throws IllegalArgumentException if {@code size} is negative */ public static int checkElementIndex(int index, int size) { return checkElementIndex(index, size, "index"); } /** * Ensures that {@code index} specifies a valid <i>element</i> in an array, list or string of size * {@code size}. An element index may range from zero, inclusive, to {@code size}, exclusive. * * @param index a user-supplied index identifying an element of an array, list or string * @param size the size of that array, list or string * @param desc the text to use to describe this index in an error message * @return the value of {@code index} * @throws IndexOutOfBoundsException if {@code index} is negative or is not less than {@code size} * @throws IllegalArgumentException if {@code size} is negative */ public static int checkElementIndex(int index, int size, @Nullable String desc) { // Carefully optimized for execution by hotspot (explanatory comment above) if (index < 0 || index >= size) { throw new IndexOutOfBoundsException(badElementIndex(index, size, desc)); } return index; } private static String badElementIndex(int index, int size, String desc) { if (index < 0) { return format("%s (%s) must not be negative", desc, index); } else if (size < 0) { throw new IllegalArgumentException("negative size: " + size); } else { // index >= size return format("%s (%s) must be less than size (%s)", desc, index, size); } } /** * Ensures that {@code index} specifies a valid <i>position</i> in an array, list or string of * size {@code size}. A position index may range from zero to {@code size}, inclusive. * * @param index a user-supplied index identifying a position in an array, list or string * @param size the size of that array, list or string * @return the value of {@code index} * @throws IndexOutOfBoundsException if {@code index} is negative or is greater than {@code size} * @throws IllegalArgumentException if {@code size} is negative */ public static int checkPositionIndex(int index, int size) { return checkPositionIndex(index, size, "index"); } /** * Ensures that {@code index} specifies a valid <i>position</i> in an array, list or string of * size {@code size}. A position index may range from zero to {@code size}, inclusive. * * @param index a user-supplied index identifying a position in an array, list or string * @param size the size of that array, list or string * @param desc the text to use to describe this index in an error message * @return the value of {@code index} * @throws IndexOutOfBoundsException if {@code index} is negative or is greater than {@code size} * @throws IllegalArgumentException if {@code size} is negative */ public static int checkPositionIndex(int index, int size, @Nullable String desc) { // Carefully optimized for execution by hotspot (explanatory comment above) if (index < 0 || index > size) { throw new IndexOutOfBoundsException(badPositionIndex(index, size, desc)); } return index; } private static String badPositionIndex(int index, int size, String desc) { if (index < 0) { return format("%s (%s) must not be negative", desc, index); } else if (size < 0) { throw new IllegalArgumentException("negative size: " + size); } else { // index > size return format("%s (%s) must not be greater than size (%s)", desc, index, size); } } /** * Ensures that {@code start} and {@code end} specify a valid <i>positions</i> in an array, list * or string of size {@code size}, and are in order. A position index may range from zero to * {@code size}, inclusive. * * @param start a user-supplied index identifying a starting position in an array, list or string * @param end a user-supplied index identifying a ending position in an array, list or string * @param size the size of that array, list or string * @throws IndexOutOfBoundsException if either index is negative or is greater than {@code size}, * or if {@code end} is less than {@code start} * @throws IllegalArgumentException if {@code size} is negative */ public static void checkPositionIndexes(int start, int end, int size) { // Carefully optimized for execution by hotspot (explanatory comment above) if (start < 0 || end < start || end > size) { throw new IndexOutOfBoundsException(badPositionIndexes(start, end, size)); } } private static String badPositionIndexes(int start, int end, int size) { if (start < 0 || start > size) { return badPositionIndex(start, size, "start index"); } if (end < 0 || end > size) { return badPositionIndex(end, size, "end index"); } // end < start return format("end index (%s) must not be less than start index (%s)", end, start); } /** * Substitutes each {@code %s} in {@code template} with an argument. These are matched by * position: the first {@code %s} gets {@code args[0]}, etc. If there are more arguments than * placeholders, the unmatched arguments will be appended to the end of the formatted message in * square braces. * * @param template a non-null string containing 0 or more {@code %s} placeholders. * @param args the arguments to be substituted into the message template. Arguments are converted * to strings using {@link String#valueOf(Object)}. Arguments can be null. */ // Note that this is somewhat-improperly used from Verify.java as well. static String format(String template, @Nullable Object... args) { template = String.valueOf(template); // null -> "null" // start substituting the arguments into the '%s' placeholders StringBuilder builder = new StringBuilder(template.length() + 16 * args.length); int templateStart = 0; int i = 0; while (i < args.length) { int placeholderStart = template.indexOf("%s", templateStart); if (placeholderStart == -1) { break; } builder.append(template.substring(templateStart, placeholderStart)); builder.append(args[i++]); templateStart = placeholderStart + 2; } builder.append(template.substring(templateStart)); // if we run out of placeholders, append the extra args in square braces if (i < args.length) { builder.append(" ["); builder.append(args[i++]); while (i < args.length) { builder.append(", "); builder.append(args[i++]); } builder.append(']'); } return builder.toString(); } }