Object 类

本文为书籍《Java编程的逻辑》¹和《剑指Java：核心原理与应用实践》²阅读笔记

java.lang.Object类是类层次结构的根类，每个类（除了Object类本身）都使用Object类作为超类。一个类如果没有显式声明继承另一个类，则相当于默认继承了Object类。换句话说，Object类的变量可以接收任意类型的对象。Java规定Object[]可以接收任意类型对象的数组，但是不能接收基本数据类型的数组。

package com.ieening.learnCommonApi;

public class TestObject {
    public static void main(String[] args) {
        Object foo = new Object();
        Object stringObj = "小姑";
        Object tesObject = new TestObject();
        Object arrayObject = new int[5]; // 编译通过，此时把数组对象当成一个普通对象赋值给arrayObject
        Object[] objects = new Object[3];
        Object[] strings = new String[3];
        Object[] intArray = new int[3]; // 编译报错：Type mismatch: cannot convert from int[] to Object[]
    }
}

Object类是其他类的根父类，因此Object类的所有方法都会继承到子类中，包括数组对象，了解Object类的方法就非常重要。Object类中的主要方法如下所示。

序号	方法名	描述
1	public Object()	无参构造器
2	public final native Class<?> getClass()	返回对象运行时的类型
3	public native int hashCode()	获取对象的 hash 值
4	public boolean equals(Object obj)	比较两个对象的内容是否相等
5	protected native Object clone() throws CloneNotSupportedException	创建并返回对象的一个副本
6	public String toString()	对象字符串表示
7	public final native void notify()	唤醒当前对象监视器下等待的单个线程
8	public final native void notifyAll()	唤醒当前对象监视器下等待的所有线程
9	public final void wait() throws InterruptedException	当前线程等待直到被唤醒
10	public final void wait(long timeoutMillis) throws InterruptedException	当前线程等待直到被唤醒或时间结束
11	public final void wait(long timeoutMillis, int nanos) throws InterruptedException	当前线程等待直到被唤醒或时间结束
12	protected void finalize() throws Throwable	回收当前对象时，做一些清除操作

6.1 public Object()

Object类只有一个默认的空参构造器，所有类的对象创建最终都会通过super()语句调用到Object类的无参构造器中。如果一个类没有显式继承另一个类，那么在它的构造器中出现的super()语句表示调用的就是Object类的无参构造器。

6.2 toString方法

toString方法的作用是返回对象的字符串形式，也就是任意类型对象想转换成String类型，都可以调用toString方法。toString方法的原型返回的是一个类似地址值的字符串，不够简明并且对于开发人员来讲该字符串的信息没有意义，所以建议子类在重写该方法时，返回一个简明易懂的信息表达式，一般为对象的属性信息。

    /**
     * Returns a string representation of the object.
     * @apiNote
     * 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.
     * The string output is not necessarily stable over time or across
     * JVM invocations.
     * @implSpec
     * 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());
    }

6.3 hashCode和equals(Object)

1、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 {@link
     *     #equals(Object) equals} 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 #equals(Object) equals} 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>
     *
     * @implSpec
     * As far as is reasonably practical, the {@code hashCode} method defined
     * by class {@code Object} returns distinct integers for distinct objects.
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.lang.System#identityHashCode
     */
    @IntrinsicCandidate
    public native int hashCode();

hashCode方法的说明有以下几点：

1. hashCode方法用于返回对象的哈希码值。支持此方法是为了提高哈希表（如java.util.Hashtable提供的哈希表）的性能。
2. hashCode在Object类中有native修饰，是本地方法，该方法的方法体不是Java实现的，是由C/C++实现的，最后编译为.dll文件，然后由Java调用。

hashCode方法重写时要满足如下几个要求。

1. 如果两个对象调用equals方法返回true，那么要求这两个对象的hashCode值一定是相等的。
2. 如果两个对象的hashCode值不相等，那么要求这两个对象调用equals方法一定是false。
3. 如果两个对象的hashCode值相等，那么这两个对象调用equals方法可能是true，也可能是false。

2、equals(Object)

判断引用相等使用==，两个对象内容相等使用equals，下面是Object equals代码：

    /**
     * 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>
     * An equivalence relation partitions the elements it operates on
     * into <i>equivalence classes</i>; all the members of an
     * equivalence class are equal to each other. Members of an
     * equivalence class are substitutable for each other, at least
     * for some purposes.
     *
     * @implSpec
     * 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}).
     *
     * In other words, under the reference equality equivalence
     * relation, each equivalence class only has a single element.
     *
     * @apiNote
     * It is generally necessary to override the {@link #hashCode() 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);
    }

从代码可以知道，Object的equals与==的效果是一样的。我们希望判断的是两个对象的属性内容是否相等，所以往往需要重写equals方法。

重写equals方法时，代码主要分为三个方面：

1. 两个对象的地址一样，肯定返回true。
2. 两个对象的类型不一样，肯定返回false。
3. 两个对象被选择比较的属性信息完全一样，肯定返回true，有不一样的则返回false。

关于equals方法的重写，Java规定一定要遵循如下几个原则。

1. 自反性：x.equals(x)返回true。
2. 传递性：x.equals(y)返回true，y.equals(z)返回true，则x.equals(z)也应该返回true。
3. 一致性：只要参与equals方法比较的属性值没有修改，那么无论何时调用equals方法的结果应该都是一致的。
4. 对称性：x.equals(y)与y.equals(x)的结果应该一致。
5. 非空对象.equals(null)的结果一定是false。

关于==和equals方法的区别，总结如下。

• ==可用于判断两个基本数据类型变量，也可以用于判断两个引用类型变量。但都需要保证判断双方的类型一致或兼容，否则编译报错。
• equals方法只能用于判断引用类型的变量，因为只有对象才有方法，默认判断的是对象的内容，如果重写Object类的equals方法，则一般判断的是对象的内容是否相等。

6.4 getClass方法

我们知道对象有静态类型（编译时类型）和动态类型（运行时类型），静态类型和动态类型可能不一样。静态类型比较好判断，就是变量声明时的类型，那么动态类型呢？动态类型需要使用getClass方法。Object getClass代码方法如下：

    /**
     * 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
     */
    @IntrinsicCandidate
    public final native Class<?> getClass();

6.5 clone方法

如果需要复制一个对象，则可以使用Object类提供的clone方法。该方法在Object类中的源码如下所示：

    /**
     * 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.
     *
     * @implSpec
     * 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
     */
    @IntrinsicCandidate
    protected native Object clone() throws CloneNotSupportedException;

调用该方法时可以创建并返回当前对象的一个副本。从源码中可以发现该方法的权限修饰符是protected，说明默认Object类中的clone方法只能在java.lang包或其他包的子类中调用。因此，如果在测试类中要通过自定义类的对象来调用clone方法，则必须重写该方法。这里要注意的是，如果要重写该方法，则子类必须实现java.lang.Cloneable接口，否则会抛出CloneNotSupportedException。

6.6 finalize方法

Object类中finalize方法的源码如下所示：

    /**
     * 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>
     * <b>When running in a Java virtual machine in which finalization has been
     * disabled or removed, the garbage collector will never call
     * {@code finalize()}. In a Java virtual machine in which finalization is
     * enabled, the garbage collector might call {@code finalize} only after an
     * indefinite delay.</b>
     * <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 Finalization is deprecated and subject to removal in a future
     * release. The use of finalization can lead to problems with security,
     * performance, and reliability.
     * See <a href="https://openjdk.org/jeps/421">JEP 421</a> for
     * discussion and alternatives.
     * <p>
     * Subclasses that override {@code finalize} to perform cleanup should use
     * alternative cleanup mechanisms and remove the {@code finalize} method.
     * Use {@link java.lang.ref.Cleaner} and
     * {@link java.lang.ref.PhantomReference} as safer ways to release resources
     * when an object becomes unreachable. Alternatively, add a {@code close}
     * method to explicitly release resources, and implement
     * {@code AutoCloseable} to enable use of the {@code try}-with-resources
     * statement.
     * <p>
     * This method will remain in place until finalizers have been removed from
     * most existing code.
     *
     * @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", forRemoval=true)
    protected void finalize() throws Throwable { }

finalize方法是Object类中的protected方法，子类可以重写该方法以实现资源清理工作，GC在回收对象之前会调用该方法，即该方法不是由开发人员手动调用的。当对象变成不可达时，即对象成为需要被回收的垃圾对象时，GC会判断该对象是否覆盖了finalize方法，若未覆盖，则直接将其回收。若对象未执行过finalize方法，则将其放入F-Queue队列，由一个低优先级线程执行该队列中对象的finalize方法。执行完finalize方法后，GC会再次判断该对象是否可达，若不可达，则进行回收，否则对象复活，复活后的对象下次回收时，将不再放入F-Queue队列，即不再执行其finalize方法。Java语言规范并不能保证finalize方法会被及时执行，而且根本不能保证它们会被执行。所以不建议用finalize方法完成非内存资源清理工作以外的任务。

---

1. 马俊昌.Java编程的逻辑[M].北京:机械工业出版社,2018. ↩︎

2. 尚硅谷教育.剑指Java：核心原理与应用实践[M].北京:电子工业出版社,2023. ↩︎