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<title>The Java Language Specification The Package java.lang </title>
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<center><h1>20.1 The Class <code>java.lang.Object</code></h1></center>
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The class <code>Object</code> is the single root of the class hierarchy. All objects, including
arrays, implement the methods of this class.
<p><pre><a name="14341"></a>public class <code><b>Object</b></code> {
<a name="1546"></a> public final Class <code><b>getClass</b></code>();
<a name="1551"></a> public String <code><b>toString</b></code>();
<a name="1556"></a> public boolean <code><b>equals</b></code>(Object obj);
<a name="1560"></a> public int <code><b>hashCode</b></code>();
<a name="1570"></a> protected Object <code><b>clone</b></code>()
<a name="30882"></a> throws CloneNotSupportedException;
<a name="31617"></a> public final void <code><b>wait</b></code>()<br>
throws IllegalMonitorStateException,<br>
InterruptedException;
<a name="31619"></a> public final void <code><b>wait</b></code>(long millis)<br>
throws IllegalMonitorStateException,<br>
InterruptedException;
<a name="1573"></a> public final void <code><b>wait</b></code>(long millis, int nanos)<br>
throws IllegalMonitorStateException,
<a name="28224"></a> InterruptedException;
<a name="1574"></a> public final void <code><b>notify</b></code>()
<a name="28242"></a> throws IllegalMonitorStateException;
<a name="1575"></a> public final void <code><b>notifyAll</b></code>()
<a name="28235"></a> throws IllegalMonitorStateException;
<a name="27958"></a> protected void <code><b>finalize</b></code>()<br>
throws Throwable;
<a name="1581"></a>}
</pre><a name="13783"></a>
<p><font size=+1><strong>20.1.1 </strong> <code>public final Class <code><b>getClass</b></code>()</code></font>
<p>
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This method returns a reference to the unique object of type <code>Class</code> <a href="javalang.doc2.html#14342">(§20.3)</a> that
represents the class of this object. That <code>Class</code> object is the object that is locked by
<code>static synchronized </code>methods of the represented class.
<p><a name="1152"></a>
<p><font size=+1><strong>20.1.2 </strong> <code>public String <code><b>toString</b></code>()</code></font>
<p>
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The general contract of <code>toString</code> is that it returns a string that "textually represents"
this object. The idea is to provide a concise but informative representation
that will be useful to a person reading it.
<p><a name="30954"></a>
The <code>toString</code> method defined by class <code>Object</code> returns a string consisting of the name of the class of which the object is an instance, a commercial at character <code>'@'</code>, and the unsigned hexadecimal representation of the hashcode of the object. In other words, this method returns a string equal to the value of:<p>
<pre><a name="30955"></a>getClass().getName() + <code>'</code>@<code>'</code> + Integer.toHexString(hashCode())
</pre><a name="30965"></a>
Overridden by <code>Class</code> <a href="javalang.doc2.html#14342">(§20.3)</a>, <code>Boolean</code> <a href="javalang.doc3.html#14344">(§20.4)</a>, <code>Character</code> <a href="javalang.doc4.html#14345">(§20.5)</a>, <code>Integer</code> <a href="javalang.doc6.html#14348">(§20.7)</a>, <code>Long</code> <a href="javalang.doc7.html#46750">(§20.8)</a>, <code>Float</code> <a href="javalang.doc8.html#14394">(§20.9)</a>, <code>Double</code> <a href="javalang.doc9.html#14390">(§20.10)</a>, <code>String</code> <a href="javalang.doc11.html#14460">(§20.12)</a>, <code>StringBuffer</code> <a href="javalang.doc12.html#14461">(§20.13)</a>, <code>Thread</code> <a href="javalang.doc18.html#2658">(§20.20)</a>, <code>ThreadGroup</code> <a href="javalang.doc19.html#14469">(§20.21)</a>, <code>Throwable</code> <a href="javalang.doc20.html#3276">(§20.22.4)</a>, and <code>Bitset</code> <a href="javautil.doc1.html#7404">(§21.2)</a>.<p>
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<p><font size=+1><strong>20.1.3 </strong> <code>public boolean <code><b>equals</b></code>(Object obj)</code></font>
<p>
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This method indicates whether some other object is "equal to" this one.
<p><a name="14922"></a>
The general contract of <code>equals</code> is that it implements an equivalence relation:<p>
<ul><a name="14893"></a>
<li>It is <i>reflexive</i>: for any reference value <code>x</code>, <code>x.equals(x)</code> should return <code>true</code>.
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<li>It is <i>symmetric</i>: for any reference values <code>x</code> and <code>y</code>, <code>x.equals(y)</code> should return <code>true</code> if and only if <code>y.equals(x)</code> returns <code>true</code>.
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<li>It is <i>transitive</i>: for any reference values <code>x</code>, <code>y</code>, and <code>z</code>, if <code>x.equals(y)</code> returns <code>true</code> and <code>y.equals(z)</code> returns <code>true</code>, then <code>x.equals(z)</code> should return <code>true</code>.
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<li>It is <i>consistent</i>: for any reference values <code>x</code> and <code>y</code>, multiple invocations of <code>x.equals(y)</code> consistently return <code>true</code> or consistently return <code>false</code>, provided no information used by <code>x</code> and <code>y</code> in <code>equals</code> comparisons is modified.
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<li>For any non-null reference value <code>x</code>, <code>x.equals(null)</code> should return <code>false</code>.
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The <code>equals</code> method defined by class <code>Object</code> implements the most discriminating possible equivalence relation on objects; that is, for any reference values <code>x</code> and <code>y</code>, <code>((Object)x).equals(y)</code> returns <code>true</code> if and only if <code>x</code> and <code>y</code> refer to the same object.<p>
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Overridden by <code>Boolean</code> <a href="javalang.doc3.html#14344">(§20.4)</a>, <code>Character</code> <a href="javalang.doc4.html#14345">(§20.5)</a>, <code>Integer</code> <a href="javalang.doc6.html#14348">(§20.7)</a>, <code>Long</code> <a href="javalang.doc7.html#46750">(§20.8)</a>, <code>Float</code> <a href="javalang.doc8.html#14394">(§20.9)</a>, <code>Double</code> <a href="javalang.doc9.html#14390">(§20.10)</a>, <code>String</code> <a href="javalang.doc11.html#14460">(§20.12)</a>, and <code>Bitset</code> <a href="javautil.doc1.html#7404">(§21.2)</a>.<p>
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<p><font size=+1><strong>20.1.4 </strong> <code>public int <code><b>hashCode</b></code>()</code></font>
<p>
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This method is supported principally for the benefit of hash tables such as those
provided by the Java library class <code>java.util.Hashtable</code> <a href="javautil.doc4.html#23193">(§21.5)</a>.
<p><a name="34702"></a>
The general contract of <code>hashCode</code> is as follows:<p>
<ul><a name="14763"></a>
<li>Whenever it is invoked on the same object more than once during an execution of a Java application, <code>hashCode</code> must consistently return the same integer.  The integer may be positive, negative, or zero. This integer does not, however, have to remain consistent from one Java application to another, or from one execution of an application to another execution of the same application.
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<li>If two objects are equal according to the <code>equals</code> method <a href="javalang.doc1.html#14865">(§20.1.3)</a>, then calling the <code>hashCode</code> method on each of the two objects must produce the same integer result.
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<li>It is <i>not</i> required that if two objects are unequal according to the <code>equals</code> method <a href="javalang.doc1.html#14865">(§20.1.3)</a>, then calling the <code>hashCode</code> 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 hashtables.
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As much as is reasonably practical, the <code>hashCode</code> method defined by class <code>Object</code> 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 language.)<p>
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Overridden by <code>Boolean</code> <a href="javalang.doc3.html#14344">(§20.4)</a>, <code>Character</code> <a href="javalang.doc4.html#14345">(§20.5)</a>, <code>Integer</code> <a href="javalang.doc6.html#14348">(§20.7)</a>, <code>Long</code> <a href="javalang.doc7.html#46750">(§20.8)</a>, <code>Float</code> <a href="javalang.doc8.html#14394">(§20.9)</a>, <code>Double</code> <a href="javalang.doc9.html#14390">(§20.10)</a>, <code>String</code> <a href="javalang.doc11.html#14460">(§20.12)</a>, and <code>Bitset</code> <a href="javautil.doc1.html#7404">(§21.2)</a>.<p>
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<p><font size=+1><strong>20.1.5 </strong> <code>protected Object <code><b>clone</b></code>()<br>throws CloneNotSupportedException</code></font>
<p>
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The general contract of clone is that it 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</code>, the expression:
<p><pre><a name="28430"></a>x.clone() != x
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will be <code>true</code>, and that the expression:
<p><pre><a name="28454"></a>x.clone.getClass() == x.getClass()
</pre><a name="28455"></a>
will be <code>true</code>, but these are not absolute requirements. While it is typically the case
that:
<p><pre><a name="28448"></a>x.clone.equals(x)
</pre><a name="28449"></a>
will be <code>true</code>, this is not an absolute requirement. Copying an object will typically
entail creating a new instance of its class, but it also may require copying of internal
data structures as well.
<p><a name="28465"></a>
The method <code>clone</code> for class <code>Object</code> performs a specific cloning operation. First, if the class of this object does not implement the interface <code>Cloneable</code>, then a <code>CloneNotSupportedException</code> is thrown. Note that all arrays are considered to implement the interface <code>Cloneable</code>. 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>
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The class <code>Object</code> does <i>not</i> itself implement the interface <code>Cloneable</code>, so calling the <code>clone</code> method on an object whose class is <code>Object</code> will result in throwing an exception at run time. The <code>clone</code> method is implemented by the class <code>Object</code> as a convenient, general utility for subclasses that implement the interface <code>Cloneable</code>,  possibly also overriding the <code>clone</code> method, in which case the overriding definition can refer to this utility definition by the call:<p>
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