Language Fundamentals

Which three are valid declarations of a float?

float f1 = -343;
float f2 = 3.14;
float f3 = 0x12345;
float f4 = 42e7;
float f5 = 2001.0D;
float f6 = 2.81F;

A:

1, 2, 4

B:

2, 3, 5

C:

1, 3, 6

D:

2, 4, 6

Answer: C

(1) and (3) are integer literals (32 bits), and integers can be legally assigned to floats (also 32 bits). (6) is correct because (F) is appended to the literal, declaring it as a float rather than a double (the default for floating point literals).

(2), (4),and (5) are all doubles.

📖

Which one is a valid declaration of a boolean?

A:

boolean b1 = 0;

B:

boolean b2 = 'false';

C:

boolean b3 = false;

D:

boolean b4 = Boolean.false();

Answer: C

A boolean can only be assigned the literal true or false.

📖

Which is the valid declarations within an interface definition?

A:

public double methoda();

B:

public final double methoda();

C:

static void methoda(double d1);

D:

protected void methoda(double d1);

Answer: A

Option A is correct. A public access modifier is acceptable. The method prototypes in an interface are all abstract by virtue of their declaration, and should not be declared abstract.

Option B is wrong. The final modifier means that this method cannot be constructed in a subclass. A final method cannot be abstract.

Option C is wrong. static is concerned with the class and not an instance.

Option D is wrong. protected is not permitted when declaring a method of an interface. See information below.

Member declarations in an interface disallow the use of some declaration modifiers; you cannot use transient, volatile, or synchronized in a member declaration in an interface. Also, you may not use the private and protected specifiers when declaring members of an interface.

📖

Which three are valid declarations of a char?

char c1 = 064770;
char c2 = 'face';
char c3 = 0xbeef;
char c4 = \u0022;
char c5 = '\iface';
char c6 = '\uface';

A:

1, 2, 4

B:

1, 3, 6

C:

3, 5

D:

5 only

Answer: B

(1), (3), and (6) are correct. char c1 = 064770; is an octal representation of the integer value 27128, which is legal because it fits into an unsigned 16-bit integer. char c3 = 0xbeef; is a hexadecimal representation of the integer value 48879, which fits into an unsigned 16-bit integer. char c6 = '\uface'; is a Unicode representation of a character.

char c2 = 'face'; is wrong because you can't put more than one character in a char literal. The only other acceptable char literal that can go between single quotes is a Unicode value, and Unicode literals must always start with a '\u'.

char c4 = \u0022; is wrong because the single quotes are missing.

char c5 = '\iface'; is wrong because it appears to be a Unicode representation (notice the backslash), but starts with '\i' rather than '\u'.

📖

Which one of the following will declare an array and initialize it with five numbers?

A:

Array a = new Array(5);

B:

int [] a = {23,22,21,20,19};

C:

int a [] = new int[5];

D:

int [5] array;

Answer: B

Option B is the legal way to declare and initialize an array with five elements.

Option A is wrong because it shows an example of instantiating a class named Array, passing the integer value 5 to the object's constructor. If you don't see the brackets, you can be certain there is no actual array object! In other words, an Array object (instance of class Array) is not the same as an array object.

Option C is wrong because it shows a legal array declaration, but with no initialization.

Option D is wrong (and will not compile) because it declares an array with a size. Arrays must never be given a size when declared.

📖

Questions for: Language Fundamentals