Chapter 4: Operators

Question 1

What are the compound assignments you need to know?

Answer 1

-= - extract and assign;
\+= - add and assign;
*= - multiply and assign;
/= - divide and assign;

y = y - 5 -> y -= 5;
y = y * 5 -> y *= 5;

Question 2

What is the result of the following:
int y = 5;
y *= 1 + 3;

Answer 2

20

When using compound assignments the part after the = will be resolved first.

Question 3

What are the relational operators?

Answer 3

== equal
!= not equal
> greater than
>= greater than or equal
< less than
<= less than or equal

Question 4

What is the result of the following instanceof comparisons:
class A implements Foo {}
class B extends A {}
class C extends A {}

A a = new A(); B b = new B(); C c = new C();

a instanceof A // 1
b instanceof A // 2
b instanceof Foo // 3
c instanceof B // 4
B instanceof B // 5
null instanceof String // 6

Answer 4

1. True
2. True
3. True, indirect implementation of interface Foo
4. Compiler Error wrong hierachy tree.
5. Compiler error, instanceof only works on instances not class refs.
6. False, always false

Question 5

What is the result?
int result = 1 + 5 * 2 / 2 % 2 + (5 + 5);
System.out.println(result);

Answer 5

12

Left to right. Parantheses first. Also remember that *, / and % have a higher precedence than the + and - operators.

Question 6

What is the result:
int players = 0;
        System.out.println(players++);
        System.out.println(players);
        System.out.println(++players);

Answer 6

0
1
2

Postfix ++ or –, the value is modified after the variable is used.
Prefix ++ or –., the value is modified before the variable is used.

Question 7

What is the the value of z?
int z = 5;
if(++z > 5 || ++z > 6) z++

Answer 7

z = 7.

|| is a short circuit operator, if the first one matches to true the second expression will not be evaluated.

Question 8

What is the the value of z?
int z = 5;
if(++z > 5 | ++z > 6) z++

Answer 8

z = 8.
| is a non-short circuit operator, regardless of the the result of the first expression the JVM will still evaluate the second expression.

Question 9

What does the ^ operator do?

Answer 9

exclusive-OR operator.

if((1 == 1) ^ (2 == 2)) -> wil result in false. Only one of the expressions should result in true.

Question 10

Which operators will always evaluate all the operands?
&&
|
||
? :
%

Answer 10

All mathematical operators evaluate all the operands.

and %,

Question 11

Integer i = new Integer(42);
Long ln = new Long(42);
Double d = new Double(42.0);

Which of the following options are valid code fragments?

i == ln; 
ln == d; 
i.equals(d);
d.equals(ln);
ln.equals(42);

Answer 11

Valid

i. equals(d);
d. equals(ln);
ln. equals(42); // Autobox to Int.

Invalid
i == ln; // Will give compile errors.
ln == d; // Will give compile errors.

Question 12

class Test
{
   public static void main(String[] args)
   {
      int k = 1;
      int[] a = { 1 };
      k += (k = 4) * (k + 2);
      a[0] += (a[0] = 4) * (a[0] + 2);
      System.out.println( k + " , " + a[0]);
   }
}

Answer 12

25, 25
The value 1 of k is saved by the compound assignment operator += before its right-hand operand (k = 4) * (k + 2) is evaluated. Evaluation of this right-hand operand then assigns 4 to k, calculates the value 6 for k + 2, and then multiplies 4 by 6 to get 24. This is added to the saved value 1 to get 25, which is then stored into k by the += operator. An identical analysis applies to the case that uses a[0].

Question 13

Which of the following expressions will evaluate to true if preceded by the following code?

    String a = "java";
    char[] b = { 'j', 'a', 'v', 'a' };
    String c = new String(b);
    String d = a;

(a == d)
(b == d)
(a == “java”)
a.equals(c)

Answer 13

(a == d)
(a == “java”)
a.equals(c)

Question 14

Which of the following code snippets will print exactly 10?

1.	Object t = new Integer(106);
	int k = ((Integer) t).intValue()/10;
	System.out.println(k);
2.	System.out.println(100/9.9);
3.	System.out.println(100/10.0);
4.	System.out.println(100/10);
5. 	System.out.println(3 + 100/10*2-13);

Answer 14

1, 4, 5

3 wil result in 10.0.

Question 15

Which of the following statements will compile without any error?

1. System.out.println(“a”+’b’+63);
2. System.out.println(“a”+63);
3. System.out.println(‘b’+new Integer(63));
4. String s = ‘b’+63+”a”;
5. String s = 63 + new Integer(10);

Answer 15

1,2,3,4

1. Since the first operand is a String all others (one by one) will be converted to String.”ab” + 63 => “ab63”
2. Since the first operand is a String all others (one by one) will be converted to String.”a” + ‘b’ => “a63”
3. Since the first operand of + one is of numeric type, its numeric value of 98 will be used. Integer 63 will be unboxed and added to 98. Therefore, the final value will be int 161.
4. Since the first one is numeric type so, ‘b’+63 = 161, 161+”a” = 161a.
5. Since none of ‘+’ the operands is a String, the + operator will not generate a String. However, due to auto-unboxing, it will generate an int value of 73.

Question 16

Which of the following statements are true?

1. System.out.println(1 + 2 + “3”); would print 33.
2. System.out.println(“1” + 2 + 3); would print 15.
3. System.out.println(4 + 1.0f); would print 5.0
4. System.out.println(5/4); would print 1.25
5. System.out.println(‘a’ + 1 ); would print b.

Answer 16

1 & 3

1. operator + is left associative so evaluation of (1 + 2 + “3” ) is as follows: ( 1 + 2 ) + “3” -> 3 + “3” -> “33”.
2. evaluation of (“1” + 2 + 3) is as follows : (“1” + 2) + 3 -> “12” + 3 -> “123”.
3. (4 + 1.0f ) evaluates as 4.0f + 1.0f ->5.0f -> 5.0
4. (5/4) performs integer division because both 5 and 4 are integers, resulting in the value 1.
5. Both operands in the expression ( ‘a’ + 1 ) will be promoted to int => 97 + 1 = 98