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24  */
25 
26 package java.lang;
27 
28 /**
29  * Thrown when an application attempts to use {@code null} in a
30  * case where an object is required. These include:
31  * <ul>
32  * <li>Calling the instance method of a {@code null} object.
33  * <li>Accessing or modifying the field of a {@code null} object.
34  * <li>Taking the length of {@code null} as if it were an array.
35  * <li>Accessing or modifying the slots of {@code null} as if it
36  *     were an array.
37  * <li>Throwing {@code null} as if it were a {@code Throwable}
38  *     value.
39  * </ul>
40  * <p>
41  * Applications should throw instances of this class to indicate
42  * other illegal uses of the {@code null} object.
43  *
44  * {@code NullPointerException} objects may be constructed by the
45  * virtual machine as if {@linkplain Throwable#Throwable(String,
46  * Throwable, boolean, boolean) suppression were disabled and/or the
47  * stack trace was not writable}.
48  *
49  * @since   1.0
50  */
51 public class NullPointerException extends RuntimeException {
52     @java.io.Serial
53     private static final long serialVersionUID = 5162710183389028792L;
54 
55     /**
56      * Constructs a {@code NullPointerException} with no detail message.
57      */
NullPointerException()58     public NullPointerException() {
59         super();
60     }
61 
62     /**
63      * Constructs a {@code NullPointerException} with the specified
64      * detail message.
65      *
66      * @param   s   the detail message.
67      */
NullPointerException(String s)68     public NullPointerException(String s) {
69         super(s);
70     }
71 
72     // 0: no backtrace filled in, no message computed.
73     // 1: backtrace filled in, no message computed.
74     // 2: message computed
75     private transient int extendedMessageState;
76     private transient String extendedMessage;
77 
78     /**
79      * {@inheritDoc}
80      */
fillInStackTrace()81     public synchronized Throwable fillInStackTrace() {
82         // If the stack trace is changed the extended NPE algorithm
83         // will compute a wrong message. So compute it beforehand.
84         if (extendedMessageState == 0) {
85             extendedMessageState = 1;
86         } else if (extendedMessageState == 1) {
87             extendedMessage = getExtendedNPEMessage();
88             extendedMessageState = 2;
89         }
90         return super.fillInStackTrace();
91     }
92 
93     /**
94      * Returns the detail message string of this throwable.
95      *
96      * <p> If a non-null message was supplied in a constructor it is
97      * returned. Otherwise, an implementation specific message or
98      * {@code null} is returned.
99      *
100      * @implNote
101      * If no explicit message was passed to the constructor, and as
102      * long as certain internal information is available, a verbose
103      * description of the null reference is returned.
104      * The internal information is not available in deserialized
105      * NullPointerExceptions.
106      *
107      * @return the detail message string, which may be {@code null}.
108      */
getMessage()109     public String getMessage() {
110         String message = super.getMessage();
111         if (message == null) {
112             synchronized(this) {
113                 if (extendedMessageState == 1) {
114                     // Only the original stack trace was filled in. Message will
115                     // compute correctly.
116                     extendedMessage = getExtendedNPEMessage();
117                     extendedMessageState = 2;
118                 }
119                 return extendedMessage;
120             }
121         }
122         return message;
123     }
124 
125     /**
126      * Get an extended exception message. This returns a string describing
127      * the location and cause of the exception. It returns null for
128      * exceptions where this is not applicable.
129      */
getExtendedNPEMessage()130     private native String getExtendedNPEMessage();
131 }
132