A.Compilation fails.
B.An exception is thrown at runtime.
C.An instance of Forest is serialized.
D.An instance of Forest and an instance of Tree are both serialized.

A.s.defaultReadObject（）；
B.this = s.defaultReadObject（）；
C.y = s.readInt（）； x = s.readInt（）；
D.x = s.readInt（）； y = s.readInt（）；

A.Compilation fails.
B.The file system has a new empty directory named dir.
C.The file system has a new empty directory named newDir.
D.The file system has a directory named dir， containing a file f1.txt.
E.The file system has a directory named newDir， containing a file f1.txt.

A.Closing an open stream.
B.Flushing an open stream.
C.Writing to an open stream.
D.Writing a line separator to an open stream.

A.restore 400
B.restore 403
C.restore 453
D.Compilation fails.
E.An exception is thrown at runtime.

A.The application will crash.
B.The code on line 29 will be executed.
C.The code on line 5 of class A will execute.
D.The code on line 5 of class B will execute.
E.The exception will be propagated back to line 27.

A.assert value == null；
B.assert value != null ： "value is null"；
C.if （value == null）{    throw new AssertionException（"value is null"）；   }
D.if （value == null）{    throw new IllegalArgumentException（"value is null"）；   }

A.A
B.B
C.C
D.D
E.E
F.F

A.X， followed by an Exception.
B.No output， and an Exception is thrown.
C.Compilation fails due to an error on line 14.
D.Compilation fails due to an error on line 16.
E.Compilation fails due to an error on line 17.
F.X， followed by an Exception， followed by B

A.test
B.Exception
C.Compilation fails.
D.NullPointerException