发现代码之美
困扰
写代码时，很讨厌写对null对象的判断。突然有一天发现了系统自带的代码之类
java.util.Objects
public static boolean equals(Object a, Object b) {
return (a == b) || (a != null && a.equals(b));//对象比较，简单封装
}
public static String toString(Object o, String nullDefault) {
return (o != null) ? o.toString() : nullDefault;//如果为空，就用默认的nullDefault
}
public static <T> int compare(T a, T b, Comparator<? super T> c) {
return (a == b) ? 0 : c.compare(a, b);//a b比较，直接采用封装完美的方法
}
public static <T> T requireNonNull(T obj, String message) {
if (obj == null)
throw new NullPointerException(message);//如果为null，直接报异常
return obj;
}
Objects详细
/*
* Copyright (c) 2009, 2013, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.util;
import java.util.function.Supplier;
/**
* This class consists of {@code static} utility methods for operating
* on objects. These utilities include {@code null}-safe or {@code
* null}-tolerant methods for computing the hash code of an object,
* returning a string for an object, and comparing two objects.
*
* @since 1.7
*/
public final class Objects {
private Objects() {
throw new AssertionError("No java.util.Objects instances for you!");
}
/**
* Returns {@code true} if the arguments are equal to each other
* and {@code false} otherwise.
* Consequently, if both arguments are {@code null}, {@code true}
* is returned and if exactly one argument is {@code null}, {@code
* false} is returned. Otherwise, equality is determined by using
* the {@link Object#equals equals} method of the first
* argument.
*
* @param a an object
* @param b an object to be compared with {@code a} for equality
* @return {@code true} if the arguments are equal to each other
* and {@code false} otherwise
* @see Object#equals(Object)
*/
public static boolean equals(Object a, Object b) {
return (a == b) || (a != null && a.equals(b));
}
/**
* Returns {@code true} if the arguments are deeply equal to each other
* and {@code false} otherwise.
*
* Two {@code null} values are deeply equal. If both arguments are
* arrays, the algorithm in {@link Arrays#deepEquals(Object[],
* Object[]) Arrays.deepEquals} is used to determine equality.
* Otherwise, equality is determined by using the {@link
* Object#equals equals} method of the first argument.
*
* @param a an object
* @param b an object to be compared with {@code a} for deep equality
* @return {@code true} if the arguments are deeply equal to each other
* and {@code false} otherwise
* @see Arrays#deepEquals(Object[], Object[])
* @see Objects#equals(Object, Object)
*/
public static boolean deepEquals(Object a, Object b) {
if (a == b)
return true;
else if (a == null || b == null)
return false;
else
return Arrays.deepEquals0(a, b);
}
/**
* Returns the hash code of a non-{@code null} argument and 0 for
* a {@code null} argument.
*
* @param o an object
* @return the hash code of a non-{@code null} argument and 0 for
* a {@code null} argument
* @see Object#hashCode
*/
public static int hashCode(Object o) {
return o != null ? o.hashCode() : 0;
}
/**
* Generates a hash code for a sequence of input values. The hash
* code is generated as if all the input values were placed into an
* array, and that array were hashed by calling {@link
* Arrays#hashCode(Object[])}.
*
* <p>This method is useful for implementing {@link
* Object#hashCode()} on objects containing multiple fields. For
* example, if an object that has three fields, {@code x}, {@code
* y}, and {@code z}, one could write:
*
* <blockquote><pre>
* @Override public int hashCode() {
* return Objects.hash(x, y, z);
* }
* </pre></blockquote>
*
* <b>Warning: When a single object reference is supplied, the returned
* value does not equal the hash code of that object reference.</b> This
* value can be computed by calling {@link #hashCode(Object)}.
*
* @param values the values to be hashed
* @return a hash value of the sequence of input values
* @see Arrays#hashCode(Object[])
* @see List#hashCode
*/
public static int hash(Object... values) {
return Arrays.hashCode(values);
}
/**
* Returns the result of calling {@code toString} for a non-{@code
* null} argument and {@code "null"} for a {@code null} argument.
*
* @param o an object
* @return the result of calling {@code toString} for a non-{@code
* null} argument and {@code "null"} for a {@code null} argument
* @see Object#toString
* @see String#valueOf(Object)
*/
public static String toString(Object o) {
return String.valueOf(o);
}
/**
* Returns the result of calling {@code toString} on the first
* argument if the first argument is not {@code null} and returns
* the second argument otherwise.
*
* @param o an object
* @param nullDefault string to return if the first argument is
* {@code null}
* @return the result of calling {@code toString} on the first
* argument if it is not {@code null} and the second argument
* otherwise.
* @see Objects#toString(Object)
*/
public static String toString(Object o, String nullDefault) {
return (o != null) ? o.toString() : nullDefault;
}
/**
* Returns 0 if the arguments are identical and {@code
* c.compare(a, b)} otherwise.
* Consequently, if both arguments are {@code null} 0
* is returned.
*
* <p>Note that if one of the arguments is {@code null}, a {@code
* NullPointerException} may or may not be thrown depending on
* what ordering policy, if any, the {@link Comparator Comparator}
* chooses to have for {@code null} values.
*
* @param <T> the type of the objects being compared
* @param a an object
* @param b an object to be compared with {@code a}
* @param c the {@code Comparator} to compare the first two arguments
* @return 0 if the arguments are identical and {@code
* c.compare(a, b)} otherwise.
* @see Comparable
* @see Comparator
*/
public static <T> int compare(T a, T b, Comparator<? super T> c) {
return (a == b) ? 0 : c.compare(a, b);
}
/**
* Checks that the specified object reference is not {@code null}. This
* method is designed primarily for doing parameter validation in methods
* and constructors, as demonstrated below:
* <blockquote><pre>
* public Foo(Bar bar) {
* this.bar = Objects.requireNonNull(bar);
* }
* </pre></blockquote>
*
* @param obj the object reference to check for nullity
* @param <T> the type of the reference
* @return {@code obj} if not {@code null}
* @throws NullPointerException if {@code obj} is {@code null}
*/
public static <T> T requireNonNull(T obj) {
if (obj == null)
throw new NullPointerException();
return obj;
}
/**
* Checks that the specified object reference is not {@code null} and
* throws a customized {@link NullPointerException} if it is. This method
* is designed primarily for doing parameter validation in methods and
* constructors with multiple parameters, as demonstrated below:
* <blockquote><pre>
* public Foo(Bar bar, Baz baz) {
* this.bar = Objects.requireNonNull(bar, "bar must not be null");
* this.baz = Objects.requireNonNull(baz, "baz must not be null");
* }
* </pre></blockquote>
*
* @param obj the object reference to check for nullity
* @param message detail message to be used in the event that a {@code
* NullPointerException} is thrown
* @param <T> the type of the reference
* @return {@code obj} if not {@code null}
* @throws NullPointerException if {@code obj} is {@code null}
*/
public static <T> T requireNonNull(T obj, String message) {
if (obj == null)
throw new NullPointerException(message);
return obj;
}
/**
* Returns {@code true} if the provided reference is {@code null} otherwise
* returns {@code false}.
*
* @apiNote This method exists to be used as a
* {@link java.util.function.Predicate}, {@code filter(Objects::isNull)}
*
* @param obj a reference to be checked against {@code null}
* @return {@code true} if the provided reference is {@code null} otherwise
* {@code false}
*
* @see java.util.function.Predicate
* @since 1.8
*/
public static boolean isNull(Object obj) {
return obj == null;
}
/**
* Returns {@code true} if the provided reference is non-{@code null}
* otherwise returns {@code false}.
*
* @apiNote This method exists to be used as a
* {@link java.util.function.Predicate}, {@code filter(Objects::nonNull)}
*
* @param obj a reference to be checked against {@code null}
* @return {@code true} if the provided reference is non-{@code null}
* otherwise {@code false}
*
* @see java.util.function.Predicate
* @since 1.8
*/
public static boolean nonNull(Object obj) {
return obj != null;
}
/**
* Checks that the specified object reference is not {@code null} and
* throws a customized {@link NullPointerException} if it is.
*
* <p>Unlike the method {@link #requireNonNull(Object, String)},
* this method allows creation of the message to be deferred until
* after the null check is made. While this may confer a
* performance advantage in the non-null case, when deciding to
* call this method care should be taken that the costs of
* creating the message supplier are less than the cost of just
* creating the string message directly.
*
* @param obj the object reference to check for nullity
* @param messageSupplier supplier of the detail message to be
* used in the event that a {@code NullPointerException} is thrown
* @param <T> the type of the reference
* @return {@code obj} if not {@code null}
* @throws NullPointerException if {@code obj} is {@code null}
* @since 1.8
*/
public static <T> T requireNonNull(T obj, Supplier<String> messageSupplier) {
if (obj == null)
throw new NullPointerException(messageSupplier.get());
return obj;
}
}
网友评论