Summary
-------

The `compact()`, `duplicate()`, and `slice()` methods of `java.nio.MappedByteBuffer` are changed to covariant overrides returning a `MappedByteBuffer` instead of a `ByteBuffer`.

Problem
-------

The `compact()`, `duplicate()`, and `slice()` methods of `MappedByteBuffer` return a `ByteBuffer` thereby precluding the ability to use these methods in chain invocations except as the terminal invocation.

Solution
--------

Provide covariant overrides for the `MappedByteBuffer` methods `compact()`, `duplicate()`, and `slice()`.

Specification
-------------

    --- a/src/java.base/share/classes/java/nio/MappedByteBuffer.java
    +++ b/src/java.base/share/classes/java/nio/MappedByteBuffer.java
    @@ -205,7 +215,10 @@ public abstract class MappedByteBuffer
     
         /**
          * Forces any changes made to this buffer's content to be written to the
    -     * storage device containing the mapped file.
    +     * storage device containing the mapped file.  The region starts at index
    +     * zero in this buffer and is {@code capacity()} bytes.  An invocation of
    +     * this method behaves in exactly the same way as the invocation
    +     * {@link force(int,int) force(0,capacity())}.
          *
          * <p> If the file mapped into this buffer resides on a local storage
          * device then when this method returns it is guaranteed that all changes
    
    @@ -362,4 +375,41 @@ public abstract class MappedByteBuffer
             super.rewind();
             return this;
         }
    +
    +    /**
    +     * {@inheritDoc}
    +     *
    +     * <p> Reading bytes into physical memory by invoking {@code load()} on the
    +     * returned buffer, or writing bytes to the storage device by invoking
    +     * {@code force()} on the returned buffer, will only act on the sub-range
    +     * of this buffer that the returned buffer represents, namely
    +     * {@code [position(),limit())}.
    +     */
    +    @Override
    +    public abstract MappedByteBuffer slice();
    +
    +    /**
    +     * {@inheritDoc}
    +     *
    +     * <p> Reading bytes into physical memory by invoking {@code load()} on the
    +     * returned buffer, or writing bytes to the storage device by invoking
    +     * {@code force()} on the returned buffer, will only act on the sub-range
    +     * of this buffer that the returned buffer represents, namely
    +     * {@code [index,index+length)}, where {@code index} and {@code length} are
    +     * assumed to satisfy the preconditions.
    +     */
    +    @Override
    +    public abstract MappedByteBuffer slice(int index, int length);
    +
    +    /**
    +     * {@inheritDoc}
    +     */
    +    @Override
    +    public abstract MappedByteBuffer duplicate();
    +
    +    /**
    +     * {@inheritDoc}
    +     */
    +    @Override
    +    public abstract MappedByteBuffer compact();
     }