3. Types, variables and expressions

Question 1

What principles do we apply to keep out source lines short and readable?

Answer 1

We keep our source code lines shorter than 80 characters, and use indentation.

Question 2

What are two reasons for hard coding data?

Answer 2

When we are developing a program, and we haven’t yet written the code that obtains the data from the appropriate place.

The other case would be if the data only rarely changes, then it’s fine to hard code it.

Question 3

Name four binary infix operators and give their Java spellings. Two of these can also be used in another way - what do we call that sort of operator?

Answer 3

addition (+)
subtraction (-)
multiplication (*)
division (/)

Question 4

What is the value of -15 / 4 (in Java!)?

Answer 4

-3

Question 5

What is the value of 9 * 4 / 2 - 9 / 2 * 4 (in Java!)?

Answer 5

2

Question 6

What is the value and type of 9.0 / 2 (in Java!)?

Answer 6

4.5

Question 7

Why is the value of 10 - 6 / 2 not 2?

Answer 7

Because the / operator has higher presence.

Question 8

In general, operators could have right or left associativity. Which would / need to have in order for 16 / 4 / 2 to equal 8? What value does that expression have in Java?

Answer 8

Left associativity. This expression has a value of 2 in Java.

Question 9

What does Double.parseDouble() do?

Answer 9

It turns a text data string, into the real (fractional decimal number) it represents.

Question 10

What does Integer.parseInt() do?

Answer 10

It turns a text data string, into the integer (whole number) it represents.

Question 11

What property must some value have in order for it to be assignable to a particular variable?

Answer 11

It must have the same type as the variable.

Question 12

What is a literal value? Give an example of an integer literal.

Answer 12

Literal value is a constant, that we can add to a variable.

E.g noOfPeopleLivingInMyStreet = 47;

Question 13

Apart from bits of syntax, what two main parts does an assignment statement consist of?

Answer 13

The variable, and the value of an expression being assigned to it.

Question 14

Name three kinds of data.

Answer 14

Numbers, text data, images.

Question 15

What is the difference between 51 and “51”?

Answer 15

The integer literal 51 is an int, a number, whereas the string literal “51” is a text data string - a string of 2 separate characters.

Question 16

What is the value of 1234.0 + 0.5 + “”? What about “” + 1234.0 + 0.5?

Answer 16

The first one is treated as (1234.0 + 0.5) + “” aka its output is 1234.5. The second output, however, is 1234.00.5.

Question 17

What is an escape sequence?

Answer 17

It’s the \ symbol, that has a special meaning to the Java compiler.

Question 18

How do we include a double quote in a string literal?

Answer 18

"

Question 19

Give three examples of an int value?

Answer 19

5
300
31542

Question 20

Why is this double type called double?

Answer 20

It’s called double because it uses a means of storing the numbers called double precision.

Question 21

Why does:
System.out.println(0.7 + “ is equal to “
+ (1 - 0.1 - 0.1 - 0.1));
print 0.7 is equal to 0.700000000000001?

Answer 21

Because even though the type double provides a really precise answer, it can not go on long enough, to eventually understand, that it’s 0.7.

Question 22

What three things does a variable have?

Answer 22

A name.
A value.
A type.

Question 23

When we declare a variable, what two things do we write, and in what order do we write them?

Answer 23

We must first declare the variable by stating the type and then the name of the variable.

Then we have to initialize is by giving it a value.

Question 24

Write a single line of Java that declares a variable, x, which is a double, and gives it the value 123.4

Answer 24

double x = 123.4;

Question 25

If the double variable x has the value 10.0, what is the value of x / 2? How would this be different if x was an int?

Answer 25

The value will be 5.0. If x was an int, the value would have been 5.