java中根类Object的常用方法

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Object类，一般我们习惯称之为超类或根父类，这代表了它至高无上的低位。它是所有类的父类，它当中的一些属性将被所有类继承。

这里我主要总结了toString()、equals()以及hashCode()这3个方法。

toString()方法

这里首先引入一段jdk源码：

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/**
    * Returns a string representation of the object. In general, the
    * {@code toString} method returns a string that
    * "textually represents" this object. The result should
    * be a concise but informative representation that is easy for a
    * person to read.
    * It is recommended that all subclasses override this method.
    * <p>
    * The {@code toString} method for class {@code Object}
    * returns a string consisting of the name of the class of which the
    * object is an instance, the at-sign character `{@code @}', and
    * the unsigned hexadecimal representation of the hash code of the
    * object. In other words, this method returns a string equal to the
    * value of:
    * <blockquote>
    * <pre>
    * getClass().getName() + '@' + Integer.toHexString(hashCode())
    * </pre></blockquote>
    *
    * @return  a string representation of the object.
    */
   public String toString() {
       return getClass().getName() + "@" + Integer.toHexString(hashCode());
   }

这个方法主要完成Object对象的文本化输出。输出内容由getClass().getName()和Integer.toHexString(hashCode())两部分组成。第一部分是：对象所属的包名加类名，第二个部分是：对象地址的hash表示。

在实际的开发中，我们一般会通过IDE自动重写该方法。

equals()方法

二话不说，上jdk源码：

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/**
     * Indicates whether some other object is "equal to" this one.
     * <p>
     * The {@code equals} method implements an equivalence relation
     * on non-null object references:
     * <ul>
     * <li>It is <i>reflexive</i>: for any non-null reference value
     *     {@code x}, {@code x.equals(x)} should return
     *     {@code true}.
     * <li>It is <i>symmetric</i>: for any non-null reference values
     *     {@code x} and {@code y}, {@code x.equals(y)}
     *     should return {@code true} if and only if
     *     {@code y.equals(x)} returns {@code true}.
     * <li>It is <i>transitive</i>: for any non-null reference values
     *     {@code x}, {@code y}, and {@code z}, if
     *     {@code x.equals(y)} returns {@code true} and
     *     {@code y.equals(z)} returns {@code true}, then
     *     {@code x.equals(z)} should return {@code true}.
     * <li>It is <i>consistent</i>: for any non-null reference values
     *     {@code x} and {@code y}, multiple invocations of
     *     {@code x.equals(y)} consistently return {@code true}
     *     or consistently return {@code false}, provided no
     *     information used in {@code equals} comparisons on the
     *     objects is modified.
     * <li>For any non-null reference value {@code x},
     *     {@code x.equals(null)} should return {@code false}.
     * </ul>
     * <p>
     * The {@code equals} method for class {@code Object} implements
     * the most discriminating possible equivalence relation on objects;
     * that is, for any non-null reference values {@code x} and
     * {@code y}, this method returns {@code true} if and only
     * if {@code x} and {@code y} refer to the same object
     * ({@code x == y} has the value {@code true}).
     * <p>
     * Note that it is generally necessary to override the {@code hashCode}
     * method whenever this method is overridden, so as to maintain the
     * general contract for the {@code hashCode} method, which states
     * that equal objects must have equal hash codes.
     *
     * @param   obj   the reference object with which to compare.
     * @return  {@code true} if this object is the same as the obj
     *          argument; {@code false} otherwise.
     * @see     #hashCode()
     * @see     java.util.HashMap
     */
    public boolean equals(Object obj) {
        return (this == obj);
    }

如注释所述，equals方法主要用于判断非空对象与另一给定对象，是否为同一对象（在堆内存中位于同一地址空间）。在实际开发中，我们一般会选择重写该方法去完成更具体的功能。例如，在jdk中的String类中，就重写了equals方法，去判断两个字符串是否相等。

hashCode()方法

jdk源码如下：

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/**
    * Returns a hash code value for the object. This method is
    * supported for the benefit of hash tables such as those provided by
    * {@link java.util.HashMap}.
    * <p>
    * The general contract of {@code hashCode} is:
    * <ul>
    * <li>Whenever it is invoked on the same object more than once during
    *     an execution of a Java application, the {@code hashCode} method
    *     must consistently return the same integer, provided no information
    *     used in {@code equals} comparisons on the object is modified.
    *     This integer need not remain consistent from one execution of an
    *     application to another execution of the same application.
    * <li>If two objects are equal according to the {@code equals(Object)}
    *     method, then calling the {@code hashCode} method on each of
    *     the two objects must produce the same integer result.
    * <li>It is <em>not</em> required that if two objects are unequal
    *     according to the {@link java.lang.Object#equals(java.lang.Object)}
    *     method, then calling the {@code hashCode} method on each of the
    *     two objects must produce distinct integer results.  However, the
    *     programmer should be aware that producing distinct integer results
    *     for unequal objects may improve the performance of hash tables.
    * </ul>
    * <p>
    * As much as is reasonably practical, the hashCode method defined by
    * class {@code Object} does return distinct integers for distinct
    * objects. (This is typically implemented by converting the internal
    * address of the object into an integer, but this implementation
    * technique is not required by the
    * Java&trade; programming language.)
    *
    * @return  a hash code value for this object.
    * @see     java.lang.Object#equals(java.lang.Object)
    * @see     java.lang.System#identityHashCode
    */
   public native int hashCode();

hashCode方法，返回的是对象在内存中地址的hash编码。这在HashTable中会用到，主要是用作对象存储的index使用。