Object类是处于所有class的root等级的一个类。java中其他任何的类都是他的子类。所有的类包括数组都要实现这个类中的方法
Object类的源码如下所示：
public class Object { private static native void registerNatives(); static { registerNatives(); } public final native Class<?> getClass(); public native int hashCode(); public boolean equals(Object obj) { return (this == obj); } protected native Object clone() throws CloneNotSupportedException; public String toString() { return getClass().getName() + "@" + Integer.toHexString(hashCode()); } public final native void notify(); public final native void notifyAll(); public final native void wait(long timeout) throws InterruptedException; public final void wait(long timeout, int nanos) throws InterruptedException { if (timeout < 0) { throw new IllegalArgumentException("timeout value is negative"); } if (nanos < 0 || nanos > 999999) { throw new IllegalArgumentException( "nanosecond timeout value out of range"); } if (nanos > 0) { timeout++; } wait(timeout); } public final void wait() throws InterruptedException { wait(0); } protected void finalize() throws Throwable { }以上就是Object类的所有的方法成员了。下面一个一个的来进行学习。
在学习这个方法之前首先先了解一下java中的native关键字 在java当中，native关键字是用于与C语言或者C++语言进行协同工作的。被native修饰的方法都是操作系统实现的。java中共只是调用这个方法而已。总而言之java是跨平台的语言，既然是跨了平台，所付出的代价就是牺牲一些对底层的控制，而java要实现对底层的控制，就需要一些其他语言的帮助，这个就是native的作用了。
private static native void registerNatives(); static { registerNatives(); }对于这个函数而言，java将这个方法的执行放到了一个静态代码快当中，也就是说一开始这个方法是最先被执行的。 这个方法就是为了注册这个类中的除这个类之外的其他本地方法。为什么要注册呢？
首先java调用本地方法要分为两个步骤：(https://blog.csdn.net/Saintyyu/article/details/90452826) 第一，通过System.loadLibrary()将包含本地方法实现的动态文件加载进内存；第二，当Java程序需要调用本地方法时，虚拟机在加载的动态文件中定位并链接该本地方法，从而得以执行本地方法。registerNatives()方法的作用就是取代第二步，让程序主动将本地方法链接到调用方，当Java程序需要调用本地方法时就可以直接调用，而不需要虚拟机再去定位并链接。
/** * Returns the runtime class of this {@code Object}. 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> * * <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. * @jls 15.8.2 Class Literals */ public final native Class<?> getClass(); class Class{ Filed[] f;//Filed代表字段,可以存String,int存成员变量 Constructor c;//存构造方法 Method m;//存方法 }getClass方法的返回值是一个class对象，通过返回的class对象可以获取这个类所有的成员方法，类型为Method。同时也可以获取所有的成员变量，类型为Filed，也可以获取到构造方法，类型为constructor。
/** * 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™ 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(); public static void main(String[] args) { Object o = new Object(); int hashcode = o.hashCode(); System.out.println(hashcode); } 输出：460141958该方法返回一个对象的hashcode值。这是为了提高诸如java.util.HashMap中所提供的那些哈希表的性能。
在同一次的java程序应用过程中，对应同样的对象多次调用hashCode方法，hashCode方法必须一致性的返回同样的一个地址值，前提是这个对象不能改变两个对象相同是依据equals方法来的，那么其中的每一个对象调用hashCode方法都必须返回相同的一个integer值，也就是对象的地址。equals方法相等，那么hashCode方法也必须相等。如果两个对象依据equals方法返回的结果不相等，那么对于其中的每一个对象调用hashCode方法返回的结果也不是一定必须得相等（也就是说，equals方法的结果为false，那么hashCode方法返回的结果可以相同也可以不相同），但是，对于我们开发者来说，针对两个对象的不相等如果生成相同的hashCode则可以提高应用程序的性能。 /** * 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); } 自反性 ： x.equals(x) 结果应该返回true。对称性 ： x.equals(y) 结果返回true当且仅当y.equals(x)也应该返回true。传递性 ： x.equals(y) 返回true，并且y.equals(z) 返回true，那么x.equals(z) 也应该返回true。一致性 ： x.equals(y)的第一次调用为true，那么x.equals(y)的第二次，第三次等多次调用也应该为true，但是前提条件是在进行比较之前，x和y都没有被修改。 x.equals(null) 应该返回false。 这个方法返回true当且仅当x和y指向了同样的对象(x==y)，这句话也就是说明了在默认情况下，Object类中的equals方法默认比较的是对象的地址，因为只有是相同的地址才会相等(x == y)，如果没有重写equals方法，那么默认就是比较的是地址。 注意：无论何时这个equals方法被重写那么都是有必要去重写hashCode方法，这个是因为为了维持hashCode的一种约定，相同的对象必须要有相同的hashCode值。 /** * 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}, 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. * 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 */ protected native Object clone() throws CloneNotSupportedException;clone函数主要是用来进行对象的的克隆。在java当中，一个对象中的数据分为两种，一种是基本数据类型，另外一种是引用类型。那么为什么要克隆呢，是因为想得到一个对象的副本。通过重新创建出来一个赋值也是可以的。但是由于clone是一个native方法，那么执行的速度肯定是更快的。一个类如果能够使用clone方法应该要做到以下的几点：
对象的类实现Cloneable接口；覆盖Object类的clone()方法 （覆盖clone()方法，访问修饰符设为public，默认是protected）；在clone()方法中调用super.clone()； . 但是如果仅仅做到这些所复制出来的是钱拷贝，就是只能复制基本数据类型，引用类型和原来的那个还是指向的同一块地址。实现深拷贝和浅拷贝可以看这个：Object的clone方法测地剖析 /** * 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()); }toString方法是我们很熟悉的一个方法了，如果我们不在自己的类中重载这个方法的话，那么他默认返回的是这个类的类名和hashcode值。
/** * Wakes up a single thread that is waiting on this object's * monitor. If any threads are waiting on this object, one of them * is chosen to be awakened. The choice is arbitrary and occurs at * the discretion of the implementation. A thread waits on an object's * monitor by calling one of the {@code wait} methods. */ public final native void notify(); public final native void notifyAll();这个方法和notifyAll要区分开，notify方法用于唤醒等待在该对象上的任意一个线程。 notifyAll方法用来唤醒所有的线程
* Causes the current thread to wait until either another thread invokes the * {@link java.lang.Object#notify()} method or the * {@link java.lang.Object#notifyAll()} method for this object, or a * specified amount of time has elapsed. public final native void wait(long timeout) throws InterruptedException; public final void wait() throws InterruptedException { wait(0); } public final void wait(long timeout, int nanos) throws InterruptedException { if (timeout < 0) { throw new IllegalArgumentException("timeout value is negative"); } if (nanos < 0 || nanos > 999999) { throw new IllegalArgumentException( "nanosecond timeout value out of range"); } if (nanos > 0) { timeout++; } wait(timeout); }这些方法在java并发编程里面都是讲过的。wait方法用来是当前线程阻塞在这个对象上，只有另外的线程调用了该对象的notify方法或者notifyAll方法，wait才能被唤醒。 可以看到，在OBject类中一个有三个重载的wait方法。其中只有一个方法是native方法。其他两个方法都只是调用了那个native方法。如果wait没有指定超时时间，那么就会传入超时时间是0.表示一直等待。若否则就等待超时时间，不然就会自动唤醒。
/** * Called by the garbage collector on an object when garbage collection * determines that there are no more references to the object. * A subclass overrides the {@code finalize} method to dispose of * system resources or to perform other cleanup. */ protected void finalize() throws Throwable { }在java虚拟机当中已经学习过了。在java的GC中，若要宣布一个对象的死亡，需要经过两次标记过程。如果对象在第一次经过可达性分析之后，发现没有GC root相连接的引用链，那他将会被第一次标记，并进行一次筛选。筛选的条件就是此对象是否有必要执行finalize方法。当对象没有覆盖这个方法，或者说这个方法已经被执行过一次了，那么虚拟机都会认为这两种情况没有必要执行finalize。如果判定为有必要执行，那么这个方法会被放到一个对列中等待执行。这个方法是该对象逃脱被回收的最后一个方法。如果在该方法中该对象与其他任意一个对象建立联系，那么第二次标记就会将这个对象移除带回收对象的集合。不然他就会真正的被回收了。注意任何的finalize方法只会被执行一次，而且在java中这个方法并不被倡导使用。在JDK9之后就被淘汰了。
