如果餓了就吃，困了就睡，渴了就喝，人生就太無趣了
1. 介紹
類Object 是類層次結構的根類。每個類都使用 Object 作爲超類。所有對象（包括數組）都實現這個類的方法。
簡單來說就是，Object類是所有類的父類，包括我們所寫的類，我們在使用類的時候就會利用Object類中的方法
2.常用方法
Object類中的大部分方法都是native方法，用此關鍵字修飾的方法是Java中的本地方法，一般是用C/C++語言來實現。
2.1 類圖
2.2 getClass()
/**
* 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();
- 該類使用了
final關鍵字，表示此方法不能重寫 - 此方法會返回一個類運行時類型。
2.3 hashCode()
/**
* 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();
- 返回對象的哈希值，返回一個整數
- 此方法遵循三個規則：
- 在java程序運行期間，若用於equals方法的信息或者數據沒有修改，name同一個對象多次調用此方法，返回的哈希碼是相同的。而在兩次獨立的運行java程序時，對於同一對象，不需要返回的哈希碼相同
- 如果根據equals方法，兩個對象相同，則這兩個對象的哈希碼一定相同
- 假如兩個對象通過equals方法比較不相同，那麼這兩個對象調用hashCode也不是要一定不同，相同也是可以的。但是使用者應該知道對不同的對象產生不同的
hashCode是可以提高hash tables的性能的。
2.4 equals()
/**
* 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);
}
- 主要是比較兩個對象是否相同，Object中的equals方法比較的是對象的地址是否相同,跟String.equals方法不同,它比較的只是對象的值
- 非Null值的傳遞規則
- 自反性：任何非null值x，
x.equals(x)返回true - 對稱性：任何非null值x，y，當且僅當
x.equals(y),返回true，y.equals(x)才返回true。 - 傳遞性：任何非null值x，y，z，如果
x.equals(y),返回true，y.equals(z)返回true，則x.equals(z),返回true。 - 一致性：任何非null值x，y，如果未修改對象的equals比較中使用的信息，則多次調用
x.equals（y）始終返回true或始終返回false。 - 對於任何非null值x，
x.equals(null)返回false.
例子：
public class People {
// other method
public static void main(String[] args) {
People a=new People();
People b=new People();
People c=a;
System.out.println("a.equals(b) 返回"+a.equals(b));
System.out.println("a.equals(c) 返回"+a.equals(c));
}
}
返回結果：
2.5 clone()
/**
* 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;
- 本地CLONE方法，用於對象的複製
- 克隆分爲淺克隆和深克隆，本人在這篇博客中已經詳細講述：https://blog.csdn.net/weixin_41938180/article/details/105246940
2.6 toString()
/**
* 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());
}
- 用簡潔的文本來描述對象，建議所有子類都重寫此方法
- Object類的toString方法返回一個字符串，該字符串包括該對象是其實例的類的名稱，
@符號，和哈希值
2.7 notify()
/**
* 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()
*/
public final native void notify();
通知可能等待該對象的對象鎖的其他線程。由JVM(與優先級無關)隨機挑選一個處於wait狀態的線程。
- 在調用notify()之前，線程必須獲得該對象的對象級別鎖
- 執行完notify()方法後，不會馬上釋放鎖，要直到退出synchronized代碼塊，當前線程纔會釋放鎖
- notify()一次只隨機通知一個線程進行喚醒
2.8 notifyAll()
/**
* 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()
*/
public final native void notifyAll();
和notify()差不多，只不過是使所有正在等待池中等待同一共享資源的全部線程從等待狀態退出，進入可運行狀態
讓它們競爭對象的鎖，只有獲得鎖的線程才能進入就緒狀態
每個鎖對象有兩個隊列：就緒隊列和阻塞隊列
- 就緒隊列：存儲將要獲得鎖的線程
- 阻塞隊列：存儲被阻塞的線程
2.9 wait()
存在三種wait()方法:
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);
}
可見wait()和wait(long timeout, int nanos)都在在內部調用了wait(long timeout)方法。(此處對線程方面不是很瞭解，全面瞭解後會專門去說，此時只是將註釋簡單翻譯，望諒解)
- wait方法會引起當前線程阻塞，直到另外一個線程在對應的對象上調用notify或者notifyAll()方法，或者達到了方法參數中指定的時間。
- 調用wait方法的當前線程一定要擁有對象的監視器鎖。
- wait方法會把當前線程T放置在對應的object上的等到隊列中，在這個對象上的所有同步請求都不會得到響應。線程調度將不會調用線程T，在以下四件事發生之前，線程T一直處於休眠狀態（線程T是在其代碼中調用wait方法的那個線程）
- 當其他的線程在對應的對象上調用notify方法，而在此對象的對應的等待隊列中將會任意選擇一個線程進行喚醒。
- 其他一些線程爲此對象調用notifyAll方法
- 其他的線程調用了interrupt方法來中斷線程T
- 等待的時間已經超過了wait中指定的時間。如果參數timeout的值爲0，不是指真實的等待時間是0，而是線程等待直到被另外一個線程喚醒。
2.10 finalize()
/**
* 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.
*
* @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
*/
protected void finalize() throws Throwable { }
- 當垃圾回收期確定不存在對該對象的更多引用時,由對象的垃圾回收器調用此方法
- 子類覆蓋finalize方法以處置系統資源或執行其他清除