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| No. | Answer | Remark | |
|---|---|---|---|
| 1 | c | Prints: 0 |
If
NaN
is the argument passed to
|
| 2 | c | Compile-time error | A compile-time error is generated, because the Byte class does not have a charValue method. The Byte class extends the Number class and implements all six of the methods declared in Number. |
| 3 | c | Compile-time error | The Integer class has only two constructors: one accepts a primitive int, and the other accepts a String. There is no constructor that accepts an argument that is an instance of type Integer. |
| 4 | d | Compile-time error | Long is a subclass of the abstract class Number, and Long implements all of the methods of Number: byteValue, shortValue, intValue, longValue, floatValue and doubleValue. The attempt to invoke the charValue method on an instance of Long generates a compile-time error, because there is no charValue method. |
| 5 | d | Prints: -1,-1,true |
Double
is a subclass of the abstract class
Number,
and implements all of the methods of
Number
such as
byteValue,
shortValue,
floatValue,
etc.
In this case, the
Double
instance contains the value
0xFFFF.
When that value is converted to type
byte
the result is
0xFF
which is also the two's complement representation of the
byte
value -1. Similarly,
0xFFFF
is the two's complement representation of the
short
value -1.
Please note there is no
|
| 6 | b d f | new Float("A") new Float("1L") new Float("0x10") | The Float constructor is overloaded: one version accepts a primitive of type float; one accepts a primitive of type double; one accepts a String representation of a floating-point literal. The primitive char literal 'A' is converted to a float, and is accepted by the constructor that declares a parameter of type float. The String literals "NaN" and "Infinity" are accepted by the Float constructor. A sign (+ or -) is optional. The API specification states that any other String must represent a floating-point value; however, a little experimentation proves that a String is acceptable if it can be parsed as a decimal integer value. The leading 0 of an octal value is ignored, and the String is parsed as a decimal value. A String representation of a hexadecimal value is not acceptable. The String "A" does not represent a floating-point literal value; therefore, a NumberFormatException is thrown. Arguments of type String can not contain an integer type suffix, L or l. A floating-point suffix, F, f, D or d, is acceptable, but the suffix has no impact on the result. |
| 7 | h | Prints: true,true,true |
The
Boolean
class contains two
public static final Boolean
instances:
|
| 8 | c d e | new Short("+1") new Short("1.0") new Short("0x1") | The Short class has only two constructors: one accepts a primitive short; the other accepts a String. A String argument must represent an integral primitive type. A leading minus sign can be added to indicate a negative value. A leading plus sign generates a run-time error. The constructor is not able to determine the radix of the String value by examing a prefix such as 0 or 0x. The 0 prefix used to identify octal values is accepted, but the String is parsed as a decimal value. The prefix 0x generates a run-time error. A run-time error is generated if the String argument is not formatted as a decimal integer. A floating-point format results in a run-time error. |
| 9 | a c d | append delete insert | The StringBuffer class has methods named append, delete and insert, but the String class does not. A typical trick question will attempt to invoke StringBuffer methods on a String instance. |
| 10 | d | Prints: 1.0,2.0 |
The
|
| 11 | b | Prints: false,true | The expression b1==b2 compares the references of two instances of Byte. The result is false, because the instances are distinct. The expression b1.equals(b2) compares the contents of two instances of Byte. The result is true, because the two instances contain the same value. |
| 12 | k | None of the above | The binary representation of 256 is one bit that is set to one followed by eight bits that are set to zero. When 256 is converted to an eight bit byte value, the bit that is set to one is lost and only the bits that are set to zero remain. When 256 is converted to a short, no information is lost; so the value remains 256. |
| 13 | a | Prints: long | The Long.parseLong method returns a primitive long. |
| 14 | h | Prints: true,true,true |
NaN
is the only value that is not equal to itself. The
|
| 15 | a | Compile-time error at 1 |
The
|
| 16 | e | Prints: true,false,false | The Boolean constructor is overloaded: one version accepts a primitive boolean argument; the other accepts a String. If the String value is the word true, then the new Boolean instance will contain the value true. Both upper and lower case letters are acceptable. If the String contains any word other than true or if the reference is null, then the new instance will contain the value false. |
| 17 | d | false,true,true | The equality expression s1==s2 compares the reference values of two distinct instances of type Short. Since the instance are distinct, the equality expression is false. The expression s1.equals(s2) compares the values of two instances of type Short. Since both instances contain the value 1, the returned value is true. The expression a1==b1 compares the hash codes of two instances of Short. The result is true, because the two instances contain the same value. |
| 18 | c | Compile-time error | A compile-time error is generated due to the attempt to access the length method of the String class as though it were a variable. |
| 19 | a | Prints: false,false |
|
| 20 | c | Prints: true,false |
The
StringBuffer
class does not override the
equals
and
hashCode
methods of the
Object
class.
The
|