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public final class NullContentDescriptor implements ContentDescriptor, Externalizable
Content descriptor class which represents missing contents.

It is not recommended to use this class under normal circumstances.

The isChanged(ContentDescriptor) method will always return true.

Constructors
public
Creates a new instance.
Methods
public boolean
Indicates whether some other object is "equal to" this one.
public static NullContentDescriptor
Gets a singleton instance of a NullContentDescriptor.
public int
Returns a hash code value for the object.
public boolean
Detects changes between this content descriptor and the previous one.
public void
The object implements the readExternal method to restore its contents by calling the methods of DataInput for primitive types and readObject for objects, strings and arrays.
public void
The object implements the writeExternal method to save its contents by calling the methods of DataOutput for its primitive values or calling the writeObject method of ObjectOutput for objects, strings, and arrays.
Creates a new instance.
public boolean equals(Object obj)
Overridden from: Object
Indicates whether some other object is "equal to" this one.

The equals method implements an equivalence relation on non-null object references:

  • It is reflexive: for any non-null reference value x, x.equals(x) should return true.
  • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
  • It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
  • It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
  • For any non-null reference value x, x.equals(null) should return false.

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

objthe reference object with which to compare.
true if this object is the same as the obj argument; false otherwise.
Gets a singleton instance of a NullContentDescriptor.
The singleton instance.
public int hashCode()
Overridden from: Object
Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.

The general contract of hashCode is:

  • Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in 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.
  • If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
  • It is not required that if two objects are unequal according to the Object.equals(Object) method, then calling the 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.

As much as is reasonably practical, the hashCode method defined by class 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.)

a hash code value for this object.
public boolean isChanged(ContentDescriptor ref)
Overridden from: ContentDescriptor
Detects changes between this content descriptor and the previous one.

The default implementation compares the two objects using ContentDescriptor.equals(Object).

Implementations are encouraged to check the type of the parameter using instanceof before casting them. It is further encouraged to check the content differences against interfaces instead of direct class implementations.

One can think about this function as this is the expected content of a file, and the parameter is the actual contents. If this method returns true then the file associated with this should replace the file associated with the parameter contents. This is mainly used when synchronizing files in memory to the filesystem.

refThe previous content to examine. Might be null to represent that the previous content doesn't exist.
true if the contents described by this descriptor differs from the parameter.
Overridden from: Externalizable
The object implements the readExternal method to restore its contents by calling the methods of DataInput for primitive types and readObject for objects, strings and arrays. The readExternal method must read the values in the same sequence and with the same types as were written by writeExternal.
inthe stream to read data from in order to restore the object
IOExceptionif I/O errors occur
ClassNotFoundExceptionIf the class for an object being restored cannot be found.
public void writeExternal(ObjectOutput out) throws IOException
Overridden from: Externalizable
The object implements the writeExternal method to save its contents by calling the methods of DataOutput for its primitive values or calling the writeObject method of ObjectOutput for objects, strings, and arrays.
outthe stream to write the object to
IOExceptionIncludes any I/O exceptions that may occur