5、Object：所有类的超类
Object类是Java中所有类的始祖，在Java中，每个类都扩展了Object。如果没有明确的指出这个类的超类，则Object就被认为是这个类的超类。由于Java中每个类都是由Object类扩展而来的，所以熟悉这个类提供的所有服务十分重要。
5.1、Object类型的变量
可以使用Object类型的变量引用任何类型的对象，但只能作为各种值的一种泛型容器，如果需要具体操作，还需要知道对象的原始类型，并进行相应的强制类型转换。
在java中，只有基本类型不是对象，例如：数值、字符和布尔类型。
5.2、Object中的方法
5.2.1、equals 方法
equals方法用于检测一个对象是否等于另外一个对象。Object类中实现equals的方法将确定两个对象引用是否相等。这对于很多类来说，已经足够了。看一下Object中的equals源码,留下了重要的注释,如下：
/**
* Indicates whether some other object is "equal to" this one.
*用于检测一个对象是否等于另外一个对象
*
* Note that it is generally necessary to override the {@code hashCode}
* 覆盖equals时需要覆盖hashCode方法
*
* 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.
* 相等的对象，必须有相等的hashCode
*
* @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.
*/
public boolean equals(Object obj) {
return (this == obj);
}
equals方法必须满足如下特性：
- 自反性：对于任何非空引用x，x.equals(x)应该返回true。
- 对称性：对于任何引用x和y，当且仅当y.equals(x)返回true时，x.equals(y)返回true。
- 传递性：对于任何引用x,y和z，如果x.equals(y)返回true，y.equals(z)返回true，x.equals(z)也应该返回true。
- 一致性：如果x和y引用的对象没有发生变化，反复调用x.equals(y)应该返回同样的结果。
- 对于任意非空引用x,x.equals(null)应该返回false。
编写一个完美的equals方法的建议：
- 显式参数命名为otherObject,稍后需要将它强制转换成另一个名为other的变量。
- 检测this与ohterObject是否相等。
if(this==otherObject)return true;
这条语句只是一个优化。实际上，这是一种经常采用的形式。因为检查身份要比逐个比较字段开销小。
3. 检测otherObject是否为null,如果为null，返回false,这项检查是很必要的。
if(otherObject==null)return false;
- 比较this与otherObject的类。如果equals的语义可以在子类中改变，就使用getClass检测：
if(getClass()!=otherObject.getClass())return false;
- 将otherObject强制转换为响应类类型的变量
ClassName other = (ClassName) otherObject;
- 现在根据相等性概念的要求来比较字段。使用==比较基本类型字段，使用Objects.equals比较对象字段。如果所有的字段都匹配，就返回true;否则返回false。
return field1 == other.field1&& Objects.equals(field2,other.field2)&&...;
- 如果子类中重新定义equals,就要在其中包含一个super.equals(other)调用。
5.2.2、hashCode()散列码
如果重新定义了equals方法，就必须为用户可能插入散列表(散列非常高效，使用散列将耗费O(1)时间来查找、插入以及删除一个元素。重新定义hashCode方法。散列码（hashcode）是由对象导出的一个整型值。散列码是没有规律的。如果x和y是两个不同的对象。x.hashCode()与y.hashCode()基本上不会相同。String 类使用以下算法计算散列码:
//代码5.2.2.1
int hash=0;
for(int i=0; i<length(); i++){
hash=31*hash+charAt(i);
}
- 补充：在哈希表中，当我们向其添加对象object时，首先调用hashCode()方法计算object的哈希码，通过哈希码可以直接定位object在哈希表中的位置(一般是哈希码对哈希表大小取余)。如果该位置没有对象，可以直接将object插入该位置；如果该位置有对象(可能有多个，通过链表实现)，则调用equals()方法比较这些对象与object是否相等，如果相等，则不需要保存object；如果不相等，则将该对象加入到链表中。“两个不同的键值对，哈希值相等”，这就是哈希冲突。如果覆盖了equals()方法，而不覆盖hashCode()方法，那么在put()的时候，会发现hashCode()不相等，就误认为是不一样的对象，和预期不符。
由于hashCode方法定义在Object类中，因此每个对象都有一个默认的散列码，其值由对象的存储地址得出。下面通过例子说明默认对象的散列码是由存储地址得出的。
//代码5.2.2.2
String s="OK";
StringBuilder sb=new StringBuilder(s);
String t="OK";
StringBuilder tb=new StringBuilder(t);
| 对象 | 散列码 |
|---|---|
| s | 2556 |
| t | 2556 |
| sb | 20526976 |
| tb | 20527144 |
由上表可以看到 内容相同的字符串s、t的散列码相同。而内容相同的sb、tb的散列码不相同。由代码5.2.2.1可知，String 的hashCode方法是由内容导出的，因此，内容相同的字符串s、t的hashCode也相同。而StringBuilder类中并没有定义hashCode方法，采用Object类中的默认方法是由存储地址导出的hash code。
如何覆盖hashCode()
合理组合实例字段的散列码，以便能够让不同对象产生的散列码分布更加均匀。散列码可以是任意的整数。两个相等的对象要求返回相等的散列码。
- java.lang.Object [int hashCode()] 返回对象的散列码
- java.util.Objects [static int hash(Object… objects)],返回一个由提供的所有对象的散列码组合而成的散列码。
- java.lang.(Integer|Long|short|Byte|Double|Float|Character|Boolean) [static int hashCode(xxxx value)],返回给定值的散列码。
- java.util.Arrays [static int hashCode(xxx[] a)],返回数组a的散列码。
5.2.3、toString()方法
在Object中还有一个重要的方法，就是toString方法，它会返回表示对象值的一个字符串。绝大多数toString方法都遵循这样的格式：类的名字，随后是一对方括号括起来的字段值。
@override
public String toString(){
return getClass().getName()
+"[id="+id
+...
+"]";
}
重写toString 方法的主要原因是：只要对象与一个字符串通过操作符+连接起来，java编译器就会自动调用toString方法来获得这个对象的字符串描述。这样做不仅自己受益，所有使用这个类的程序员也会从中受益匪浅。
注意：数组的打印需要调用Arrays.toString(xxx[] a);
附录
jdk11 中的Object类源码
/*
* Copyright (c) 1994, 2017, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*/
package java.lang;
import jdk.internal.HotSpotIntrinsicCandidate;
/**
* Class {@code Object} is the root of the class hierarchy.
* Every class has {@code Object} as a superclass. All objects,
* including arrays, implement the methods of this class.
*
* @author unascribed
* @see java.lang.Class
* @since 1.0
*/
public class Object {
private static native void registerNatives();
static {
registerNatives();
}
/**
* Constructs a new object.
*/
@HotSpotIntrinsicCandidate
public Object() {}
/**
* 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
*/
@HotSpotIntrinsicCandidate
public final native Class<?> getClass();
/**
* 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. (The hashCode may or may not be implemented
* as some function of an object's memory address at some point
* in time.)
*
* @return a hash code value for this object.
* @see java.lang.Object#equals(java.lang.Object)
* @see java.lang.System#identityHashCode
*/
@HotSpotIntrinsicCandidate
public native int hashCode();
/**
* 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);
}
/**
* 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
*/
@HotSpotIntrinsicCandidate
protected native Object clone() throws CloneNotSupportedException;
/**
* 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());
}
/**
* 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.
* <p>
* The awakened thread will not be able to proceed until the current
* thread relinquishes the lock on this object. The awakened thread will
* compete in the usual manner with any other threads that might be
* actively competing to synchronize on this object; for example, the
* awakened thread enjoys no reliable privilege or disadvantage in being
* the next thread to lock this object.
* <p>
* This method should only be called by a thread that is the owner
* of this object's monitor. A thread becomes the owner of the
* object's monitor in one of three ways:
* <ul>
* <li>By executing a synchronized instance method of that object.
* <li>By executing the body of a {@code synchronized} statement
* that synchronizes on the object.
* <li>For objects of type {@code Class,} by executing a
* synchronized static method of that class.
* </ul>
* <p>
* Only one thread at a time can own an object's monitor.
*
* @throws IllegalMonitorStateException if the current thread is not
* the owner of this object's monitor.
* @see java.lang.Object#notifyAll()
* @see java.lang.Object#wait()
*/
@HotSpotIntrinsicCandidate
public final native void notify();
/**
* Wakes up all threads that are waiting on this object's monitor. A
* thread waits on an object's monitor by calling one of the
* {@code wait} methods.
* <p>
* The awakened threads will not be able to proceed until the current
* thread relinquishes the lock on this object. The awakened threads
* will compete in the usual manner with any other threads that might
* be actively competing to synchronize on this object; for example,
* the awakened threads enjoy no reliable privilege or disadvantage in
* being the next thread to lock this object.
* <p>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the {@code notify} method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*
* @throws IllegalMonitorStateException if the current thread is not
* the owner of this object's monitor.
* @see java.lang.Object#notify()
* @see java.lang.Object#wait()
*/
@HotSpotIntrinsicCandidate
public final native void notifyAll();
/**
* Causes the current thread to wait until it is awakened, typically
* by being <em>notified</em> or <em>interrupted</em>.
* <p>
* In all respects, this method behaves as if {@code wait(0L, 0)}
* had been called. See the specification of the {@link #wait(long, int)} method
* for details.
*
* @throws IllegalMonitorStateException if the current thread is not
* the owner of the object's monitor
* @throws InterruptedException if any thread interrupted the current thread before or
* while the current thread was waiting. The <em>interrupted status</em> of the
* current thread is cleared when this exception is thrown.
* @see #notify()
* @see #notifyAll()
* @see #wait(long)
* @see #wait(long, int)
*/
public final void wait() throws InterruptedException {
wait(0L);
}
/**
* Causes the current thread to wait until it is awakened, typically
* by being <em>notified</em> or <em>interrupted</em>, or until a
* certain amount of real time has elapsed.
* <p>
* In all respects, this method behaves as if {@code wait(timeoutMillis, 0)}
* had been called. See the specification of the {@link #wait(long, int)} method
* for details.
*
* @param timeoutMillis the maximum time to wait, in milliseconds
* @throws IllegalArgumentException if {@code timeoutMillis} is negative
* @throws IllegalMonitorStateException if the current thread is not
* the owner of the object's monitor
* @throws InterruptedException if any thread interrupted the current thread before or
* while the current thread was waiting. The <em>interrupted status</em> of the
* current thread is cleared when this exception is thrown.
* @see #notify()
* @see #notifyAll()
* @see #wait()
* @see #wait(long, int)
*/
public final native void wait(long timeoutMillis) throws InterruptedException;
/**
* Causes the current thread to wait until it is awakened, typically
* by being <em>notified</em> or <em>interrupted</em>, or until a
* certain amount of real time has elapsed.
* <p>
* The current thread must own this object's monitor lock. See the
* {@link #notify notify} method for a description of the ways in which
* a thread can become the owner of a monitor lock.
* <p>
* This method causes the current thread (referred to here as <var>T</var>) to
* place itself in the wait set for this object and then to relinquish any
* and all synchronization claims on this object. Note that only the locks
* on this object are relinquished; any other objects on which the current
* thread may be synchronized remain locked while the thread waits.
* <p>
* Thread <var>T</var> then becomes disabled for thread scheduling purposes
* and lies dormant until one of the following occurs:
* <ul>
* <li>Some other thread invokes the {@code notify} method for this
* object and thread <var>T</var> happens to be arbitrarily chosen as
* the thread to be awakened.
* <li>Some other thread invokes the {@code notifyAll} method for this
* object.
* <li>Some other thread {@linkplain Thread#interrupt() interrupts}
* thread <var>T</var>.
* <li>The specified amount of real time has elapsed, more or less.
* The amount of real time, in nanoseconds, is given by the expression
* {@code 1000000 * timeoutMillis + nanos}. If {@code timeoutMillis} and {@code nanos}
* are both zero, then real time is not taken into consideration and the
* thread waits until awakened by one of the other causes.
* <li>Thread <var>T</var> is awakened spuriously. (See below.)
* </ul>
* <p>
* The thread <var>T</var> is then removed from the wait set for this
* object and re-enabled for thread scheduling. It competes in the
* usual manner with other threads for the right to synchronize on the
* object; once it has regained control of the object, all its
* synchronization claims on the object are restored to the status quo
* ante - that is, to the situation as of the time that the {@code wait}
* method was invoked. Thread <var>T</var> then returns from the
* invocation of the {@code wait} method. Thus, on return from the
* {@code wait} method, the synchronization state of the object and of
* thread {@code T} is exactly as it was when the {@code wait} method
* was invoked.
* <p>
* A thread can wake up without being notified, interrupted, or timing out, a
* so-called <em>spurious wakeup</em>. While this will rarely occur in practice,
* applications must guard against it by testing for the condition that should
* have caused the thread to be awakened, and continuing to wait if the condition
* is not satisfied. See the example below.
* <p>
* For more information on this topic, see section 14.2,
* "Condition Queues," in Brian Goetz and others' <em>Java Concurrency
* in Practice</em> (Addison-Wesley, 2006) or Item 69 in Joshua
* Bloch's <em>Effective Java, Second Edition</em> (Addison-Wesley,
* 2008).
* <p>
* If the current thread is {@linkplain java.lang.Thread#interrupt() interrupted}
* by any thread before or while it is waiting, then an {@code InterruptedException}
* is thrown. The <em>interrupted status</em> of the current thread is cleared when
* this exception is thrown. This exception is not thrown until the lock status of
* this object has been restored as described above.
*
* @apiNote
* The recommended approach to waiting is to check the condition being awaited in
* a {@code while} loop around the call to {@code wait}, as shown in the example
* below. Among other things, this approach avoids problems that can be caused
* by spurious wakeups.
*
* <pre>{@code
* synchronized (obj) {
* while (<condition does not hold> and <timeout not exceeded>) {
* long timeoutMillis = ... ; // recompute timeout values
* int nanos = ... ;
* obj.wait(timeoutMillis, nanos);
* }
* ... // Perform action appropriate to condition or timeout
* }
* }</pre>
*
* @param timeoutMillis the maximum time to wait, in milliseconds
* @param nanos additional time, in nanoseconds, in the range range 0-999999 inclusive
* @throws IllegalArgumentException if {@code timeoutMillis} is negative,
* or if the value of {@code nanos} is out of range
* @throws IllegalMonitorStateException if the current thread is not
* the owner of the object's monitor
* @throws InterruptedException if any thread interrupted the current thread before or
* while the current thread was waiting. The <em>interrupted status</em> of the
* current thread is cleared when this exception is thrown.
* @see #notify()
* @see #notifyAll()
* @see #wait()
* @see #wait(long)
*/
public final void wait(long timeoutMillis, int nanos) throws InterruptedException {
if (timeoutMillis < 0) {
throw new IllegalArgumentException("timeoutMillis value is negative");
}
if (nanos < 0 || nanos > 999999) {
throw new IllegalArgumentException(
"nanosecond timeout value out of range");
}
if (nanos > 0) {
timeoutMillis++;
}
wait(timeoutMillis);
}
/**
* 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.
* <p>
* The general contract of {@code finalize} is that it is invoked
* if and when the Java™ virtual
* machine has determined that there is no longer any
* means by which this object can be accessed by any thread that has
* not yet died, except as a result of an action taken by the
* finalization of some other object or class which is ready to be
* finalized. The {@code finalize} method may take any action, including
* making this object available again to other threads; the usual purpose
* of {@code finalize}, however, is to perform cleanup actions before
* the object is irrevocably discarded. For example, the finalize method
* for an object that represents an input/output connection might perform
* explicit I/O transactions to break the connection before the object is
* permanently discarded.
* <p>
* The {@code finalize} method of class {@code Object} performs no
* special action; it simply returns normally. Subclasses of
* {@code Object} may override this definition.
* <p>
* The Java programming language does not guarantee which thread will
* invoke the {@code finalize} method for any given object. It is
* guaranteed, however, that the thread that invokes finalize will not
* be holding any user-visible synchronization locks when finalize is
* invoked. If an uncaught exception is thrown by the finalize method,
* the exception is ignored and finalization of that object terminates.
* <p>
* After the {@code finalize} method has been invoked for an object, no
* further action is taken until the Java virtual machine has again
* determined that there is no longer any means by which this object can
* be accessed by any thread that has not yet died, including possible
* actions by other objects or classes which are ready to be finalized,
* at which point the object may be discarded.
* <p>
* The {@code finalize} method is never invoked more than once by a Java
* virtual machine for any given object.
* <p>
* Any exception thrown by the {@code finalize} method causes
* the finalization of this object to be halted, but is otherwise
* ignored.
*
* @apiNote
* Classes that embed non-heap resources have many options
* for cleanup of those resources. The class must ensure that the
* lifetime of each instance is longer than that of any resource it embeds.
* {@link java.lang.ref.Reference#reachabilityFence} can be used to ensure that
* objects remain reachable while resources embedded in the object are in use.
* <p>
* A subclass should avoid overriding the {@code finalize} method
* unless the subclass embeds non-heap resources that must be cleaned up
* before the instance is collected.
* Finalizer invocations are not automatically chained, unlike constructors.
* If a subclass overrides {@code finalize} it must invoke the superclass
* finalizer explicitly.
* To guard against exceptions prematurely terminating the finalize chain,
* the subclass should use a {@code try-finally} block to ensure
* {@code super.finalize()} is always invoked. For example,
* <pre>{@code @Override
* protected void finalize() throws Throwable {
* try {
* ... // cleanup subclass state
* } finally {
* super.finalize();
* }
* }
* }</pre>
*
* @deprecated The finalization mechanism is inherently problematic.
* Finalization can lead to performance issues, deadlocks, and hangs.
* Errors in finalizers can lead to resource leaks; there is no way to cancel
* finalization if it is no longer necessary; and no ordering is specified
* among calls to {@code finalize} methods of different objects.
* Furthermore, there are no guarantees regarding the timing of finalization.
* The {@code finalize} method might be called on a finalizable object
* only after an indefinite delay, if at all.
*
* Classes whose instances hold non-heap resources should provide a method
* to enable explicit release of those resources, and they should also
* implement {@link AutoCloseable} if appropriate.
* The {@link java.lang.ref.Cleaner} and {@link java.lang.ref.PhantomReference}
* provide more flexible and efficient ways to release resources when an object
* becomes unreachable.
*
* @throws Throwable the {@code Exception} raised by this method
* @see java.lang.ref.WeakReference
* @see java.lang.ref.PhantomReference
* @jls 12.6 Finalization of Class Instances
*/
@Deprecated(since="9")
protected void finalize() throws Throwable { }
}
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