参考http://www.cnblogs.com/lwbqqyumidi/p/3693015.html

1,在idea中进入object类

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2,阅读源码

2,1 object类的结构

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没有定义属性,一共有13个方法

2.2 public Object()

构造方法,实际是存在的,但没有写

2.3

 private static native void registerNatives();
    static {
        registerNatives();
    }

registerNatives函数前面有native关键字修饰，Java中，用native关键字修饰的函数表明该方法的实现并不是在Java中去完成，而是由C/C++去完成，并被编译成了.dll，由Java去调用。方法的具体实现体在dll文件中，对于不同平台，其具体实现应该有所不同。用native修饰，即表示操作系统，需要提供此方法，Java本身需要使用。具体到registerNatives()方法本身，其主要作用是将C/C++中的方法映射到Java中的native方法，实现方法命名的解耦。
后面的静态代码块,执行了此方法

2.4

public final native Class<?> getClass();

以下为getClass的源码解释(根据自己的理解给出翻译):

 /**
     * Returns the runtime class of this {@code Object}.
     返回的是运行对象的类
      The returned
     * {@code Class} object is the object that is locked by {@code
     * static synchronized} methods of the represented class.
  
     * <p><b>The actual result type is {@code Class<? extends |X|>}
     * where {@code |X|} is the erasure of the static type of the
     * expression on which {@code getClass} is called.</b>
       
        For
     * example, no cast is required in this code fragment:</p>
     *   For example不需要强制类型转化
     * <p>
     * {@code Number n = 0;                             }<br>
     * {@code Class<? extends Number> c = n.getClass(); }
     * </p>
     *
     * @return The {@code Class} object that represents the runtime
     *         class of this object.
      返回的class对象表示当前运行的类的对象
     * @jls 15.8.2 Class Literals
     */

 public static void main(String[] args) {
        Number n = 0;
        Class<? extends Number> c = n.getClass();
        System.out.println(c);
    }

运行结果:java.lang.Integer
getClass()也是一个native方法，返回的是此Object对象的类对象/运行时类对象Class<?>。效果与Object.class相同。

首先解释下"类对象"的概念：在Java中，类是是对具有一组相同特征或行为的实例的抽象并进行描述，对象则是此类所描述的特征或行为的具体实例。作为概念层次的类，其本身也具有某些共同的特性，如都具有类名称、由类加载器去加载，都具有包，具有父类，属性和方法等。于是，Java中有专门定义了一个类，Class，去描述其他类所具有的这些特性，因此，从此角度去看，类本身也都是属于Class类的对象。为与经常意义上的对象相区分，在此称之为"类对象"。

2.5 public native int hashCode();

 /**
     * Returns a hash code value for the object. 
       返回当前对象的hashcode的值
 This method is
     * supported for the benefit of hash tables such as those provided by
     * {@link java.util.HashMap}.此方法被java.uitl.HashMap支持
     * <p>
     * The general contract of {@code hashCode} is:具体内容如下
     * <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.
            在 Java 应用程序执行期间，
            在对同一对象多次调用 hashCode 方法时，
            必须一致地返回相同的整数，
            前提是将对象进行 equals 比较时所用的信息没有被修改。
            从某一应用程序的一次执行到
            同一应用程序的另一次执行，该整数无需保持一致。
     * <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.
           如果根据 equals(Object) 方法，两个对象是相等的，
          那么对这两个对象中的每 个对象调用 hashCode 方法 
          都必须生成相同的整数结果
     * <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.
            如果根据 equals(java.lang.Object) 方法，两个对象不相等，
            那么对这两个对象中的任一对象上调用 hashCode 方法不 
           要求一定生成不同的整数结果。
            但是，程序员应该意识到，为不相等的对象生成不同整数结果可以提高哈希表的性能。

     * </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
     */

2.6

public boolean equals(Object obj) {
        return (this == obj);
    }

/**
     * Indicates whether some other object is "equal to" this one.
其他对象是否等于地对象
     * <p>
     * The {@code equals} method implements an equivalence relation
     * on non-null object references:
      equals方法实现等价关系在非空对象引用上：
     * <ul>
     * <li>It is <i>reflexive</i>: for any non-null reference value
     *     {@code x}, {@code x.equals(x)} should return
     *     {@code true}.
对应任意的非空值应该返回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}.
a.equals(b)等价于b.equals(a)
     * <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}.
可传递的
     * <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.
对于任何的非空的对象,它都是连续的
     * <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}).
对应两个类的对象,返回true的可能当且仅当两个值为同一个对象
     * <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.
     *当重写此方法时,一定要重写hashcode方法,因为规定相同的对象一定要有相同的
      哈希值
     * @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
     */

Object原生的equals()方法内部调用的正是==，与==具有相同的含义。既然如此，为什么还要定义此equals()方法？

equlas()方法的正确理解应该是：判断两个对象是否相等。那么判断对象相等的标尺又是什么？
如上，在object类中，此标尺即为==。当然，这个标尺不是固定的，其他类中可以按照实际的需要对此标尺含义进行重定义。如String类中则是依据字符串内容是否相等来重定义了此标尺含义。如此可以增加类的功能型和实际编码的灵活性。当然了，如果自定义的类没有重写equals()方法来重新定义此标尺，那么默认的将是其父类的equals()，直到object基类。
即严格的数学逻辑表示为： 两个对象相等 <=> equals()相等 => hashCode()相等。因此，重写equlas()方法必须重写hashCode()方法，以保证此逻辑严格成立，同时可以推理出：hasCode()不相等 => equals（）不相等 <=> 两个对象不相等。

2.7

可能有人在此产生疑问：既然比较两个对象是否相等的唯一条件（也是冲要条件）是equals，那么为什么还要弄出一个hashCode()，并且进行如此约定，弄得这么麻烦？

其实，这主要体现在hashCode()方法的作用上，其主要用于增强哈希表的性能。

以集合类中，以Set为例，当新加一个对象时，需要判断现有集合中是否已经存在与此对象相等的对象，如果没有hashCode()方法，需要将Set进行一次遍历，并逐一用equals()方法判断两个对象是否相等，此种算法时间复杂度为o(n)。通过借助于hasCode方法，先计算出即将新加入对象的哈希码，然后根据哈希算法计算出此对象的位置，直接判断此位置上是否已有对象即可。（注：Set的底层用的是Map的原理实现）

在此需要纠正一个理解上的误区：对象的hashCode()返回的不是对象所在的物理内存地址。甚至也不一定是对象的逻辑地址，hashCode()相同的两个对象，不一定相等，换言之，不相等的两个对象，hashCode()返回的哈希码可能相同。

2.8

/**
     * Creates and returns a copy of this object.  The precise meaning
     * of "copy" may depend on the class of the object. The general
     * intent is that, for any object {@code x},
       返回当前对象的拷贝
 the expression:
     * <blockquote>
     * <pre>
     * x.clone() != x</pre></blockquote>
     * will be true, and that the expression:
     * <blockquote>
     * <pre>
     * x.clone().getClass() == x.getClass()</pre></blockquote>
     * will be {@code true}, but these are not absolute requirements.
两个对象不相同,但是两个对象的class对象相同,这些不是绝对的要求
     * While it is typically the case that:
     * <blockquote>
     * <pre>
     * x.clone().equals(x)</pre></blockquote>
     * will be {@code true}, this is not an absolute requirement.
     * <p>
     * By convention, the returned object should be obtained by calling
     * {@code super.clone}.  If a class and all of its superclasses (except
     * {@code Object}) obey this convention, it will be the case that
     * {@code x.clone().getClass() == x.getClass()}.
     * <p>
     * By convention, the object returned by this method should be independent
     * of this object (which is being cloned).  To achieve this independence,
     * it may be necessary to modify one or more fields of the object returned
     * by {@code super.clone} before returning it.  Typically, this means
     * copying any mutable objects that comprise the internal "deep structure"
     * of the object being cloned and replacing the references to these
     * objects with references to the copies.  If a class contains only
     * primitive fields or references to immutable objects, then it is usually
     * the case that no fields in the object returned by {@code super.clone}
     * need to be modified.
     * <p>
     * The method {@code clone} for class {@code Object} performs a
     * specific cloning operation. First, if the class of this object does
     * not implement the interface {@code Cloneable}, then a
     * {@code CloneNotSupportedException} is thrown. Note that all arrays
     * are considered to implement the interface {@code Cloneable} and that
     * the return type of the {@code clone} method of an array type {@code T[]}
     * is {@code T[]} where T is any reference or primitive type.
     * Otherwise, this method creates a new instance of the class of this
     * object and initializes all its fields with exactly the contents of
     * the corresponding fields of this object, as if by assignment; the
     * contents of the fields are not themselves cloned. Thus, this method
     * performs a "shallow copy" of this object, not a "deep copy" operation.
     * <p>
     * The class {@code Object} does not itself implement the interface
     * {@code Cloneable}, so calling the {@code clone} method on an object
     * whose class is {@code Object} will result in throwing an
     * exception at run time.
     *
     * @return     a clone of this instance.
     * @throws  CloneNotSupportedException  if the object's class does not
     *               support the {@code Cloneable} interface. Subclasses
     *               that override the {@code clone} method can also
     *               throw this exception to indicate that an instance cannot
     *               be cloned.
     * @see java.lang.Cloneable
     */

2.9

/**
     * 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:
返回对象的字符串表示形式。一般来说，
@code to字符串方法返回一个字符串
“文本代表”这个对象。结果应该是
是一个简洁但信息丰富的表达，对于
要阅读的人。
建议所有子类都重写此方法。
类@code对象的@code to字符串方法
返回由类的名称组成的字符串，其中
对象是一个实例，at符号字符“@code@”，
的哈希代码的无符号十六进制表示形式
对象。换句话说，此方法返回的字符串等于
值
     * <blockquote>
     * <pre>
     * getClass().getName() + '@' + Integer.toHexString(hashCode())
     * </pre></blockquote>
     *
     * @return  a string representation of the object.
     */