Chapter 2

Question 1

Which of the following are valid java identifiers?

A. _
B. _helloWorld$
C. true
D. java.lang
E. Public
F. 1980_s
G. _Q2_

Answer 1

B, E, G

• Single underscore is not allowed.
• It cannot be a java keyword (case sensitive)
• Dot is not allowed
• It cannot start with a number

Question 2

What is an instance initializer?

Answer 2

A code block that is inside a class but outside a method.

Question 3

What is the output?

public class Egg {
    public Egg() {
        number = 5;
    }
    public static void main(String[] args) {
        Egg egg = new Egg();
        System.out.println(egg.number);
    }
    private int number = 3;
    { number = 4; }
}

Answer 3

The output is 5.

Fields and instance initializers are run in the order in which they appear in the file.
The constructor runs after all fields and instance initializer blocks have run.

Question 4

Java has 8 built in data types. Which are they?

Answer 4

boolean, byte, short, int, long, float, double, char

Question 5

What is the size of the primitive type ‘boolean’?

Answer 5

1 bit

Question 6

What is the size of the primitive type ‘byte’?

Answer 6

8 bits / 1 byte

Question 7

What is the size of the primitive type ‘short’?

Answer 7

16 bits / 2 bytes

Question 8

What is the size of the primitive type ‘int’?

Answer 8

32 bits / 4 bytes

Question 9

What is the size of the primitive type ‘long’?

Answer 9

64 bits / 8 bytes

Question 10

What is the size of the primitive type ‘float’?

Answer 10

32 bit / 4 bytes

Question 11

What is the size of the primitive type ‘double’?

Answer 11

64 bits / 8 bytes

Question 12

What is the size of the primitive type ‘char’?

Answer 12

16 bits / 2 bytes

Question 13

True or false

A float does not require the letter ‘f’ following the number in Java.

Answer 13

False.

The letter ‘f’ is required so Java knows its a float.

Question 14

A byte can hold a value from … to …

Answer 14

-128 to 127

Question 15

What is the difference between a short and a char?

Answer 15

Short and char are closely related, as both as stored as integral types with 16 bit length. The difference is that short is signed (which means it can hold positive and negative numbers) and char is unsigned (which means it can only hold positive numbers, including 0).

Question 16

When writing a numeric literal, what data type does java assume you are writing?

Answer 16

An int. This is why we need to specify an ‘L’ after the number if it should be a long.

Question 17

Java allows other formats other than the decimal system to write numbers. Name the following formats:

1. 017
2. 0xFF
3. 0b10

Answer 17

1. Octal
2. Hexadecimal
3. Binary

Question 18

Explain why the following does or does not compile:

1. 1000_.00
2. _1000.00
3. 1_00_0.0_0
4. 1000.00_
5. 1_____2

Answer 18

1. Does not compile. Underscore cannot be next to decimal point.
2. Does not compile. Underscore cannot be at the start.
3. Compiles fine.
4. Does not compile. Underscore cannot be at the end.
5. Compiles fine.

Question 19

Which if these can compile?

• int value = null;
• String value = null;

Answer 19

String value = null compiles fine.

A primitive data type cannot be assigned null.

Question 20

Does this code compile or not? Why?

String reference = “Hello”;
int len = reference.length();
int len2 = len.length();

Answer 20

Reference types can be used to call methods but primitives to not have methods declared on them. Thus len.length() is not valid.

Question 21

An identifier is the name of a variable, method, class, interface, or package. There are 4 rules for legal identifiers. What are they?

Answer 21

• Identifiers must begin with a letter, a $ symbol, or a _ symbol.
• Identifiers can include numbers but not start with them.
• A single underscore _ is not allowed
• A java reserved word is not allowed (case sensitive)

Question 22

Whcih of these identifiers are legal?

1. someidentifier
2. hollywood@vine
3. __SomeIdentifier$
4. 3DPointClass

Answer 22

1 and 3.

@ is not allowed and an identifier cannot start with a number.

Question 23

Which of the following are legal/illegal declarations? why?

1. boolean b1, b2;
2. String s1 = “1”, s2;
3. double d1, double d2;
4. int i1; int i2;
5. int i3; i4;

Answer 23

1. legal. It declares two variables without initializing them.
2. legal. It declares two variables and initailized only one of them.
3. not legal. they must share the same type declaration and not repeat it: ‘double d1, d2’ is legal.
4. legal. Although they are seperate statements (;).
5. not legal. The second statement doesn’t have a type.

Question 24

What is a local variable?

Answer 24

A variable defined in a constructor, method or initializer block.

Question 25

Why do local variables need to be initialized before use?

Answer 25

They don't have a default value.

Question 26

True or false: the following code is invalid. public void findAnswer( boolean check) { } public void checkAnswer() { boolean value; findAnswer(value); }

Answer 26

True. It is invalid because it tries to use a variable that is not initialized. Variables passed to a constructor or method need to be initialized.

Question 27

Of the following variable types, which ones do or do not require to be initalized by the programmer before use? - local variables - instance variables - class variables (static variables)

Answer 27

- Local variables need to be initialized by the programmer | - Instance and class variables are initialized implicitely.

Question 28

What is the default initialization value of a boolean?

Answer 28

false

Question 29

What is the default initialization value of a byte/short/int/long?

Answer 29

0 (zero)

Question 30

What is the default initialization value of a float/double?

Answer 30

0.0 (zero dot zero)

Question 31

What is the default initialization value of a char?

Answer 31

'\u0000' (NUL)

Question 32

What is the default initialization value of an object reference?

Answer 32

null

Question 33

Does the following code compile? ``` public class VarKeyword { var greeting = "hello"; } ```

Answer 33

No. Var can only be used for local variables. The variable greeting is an instance variable.

Question 34

Why does the following code not compile? ``` public void test() { var number = 7; number = 4; number = "five"; } ```

Answer 34

On the line with the var keyword, the compiler determines that we want an int variable, because it is assigned a number. Assigning a string to an int variable causes a compiler error.

Question 35

Why does the following code not compile? ``` public void test() { var number = (short) 10; number = (byte) 5; number = 1_000_000; } ```

Answer 35

The last line does not compile because the type of var is short. a byte fits inside a short so that line does compile.

Question 36

Does the following code compile? ``` public void compileTest(boolean check) { var question; question = 1; var answer; if (check) { answer = 2; else { answer = 3; } System.out.println(answer); } ```

Answer 36

It does not compile because for a local variable type inference (var), the compiler looks only at the line with the decleration to determine the type of var. Since question and answer are not assigned values on the same line where they are defined, the compiler does not know what to make of them.

Question 37

Does Java allow var in multiple variable declarations? example: ``` public void test() { var a = 2, b = 3; } ```

Answer 37

Java does not allow var in multiple declarations.

Question 38

Does the following local variable compile? var test = null;

Answer 38

No, because the compiler cannot determine the type in the assignment.

Question 39

Does the following code compile? ``` public void test() { var n = "hello"; n = null; var m = 1; m = null; } ```

Answer 39

It does not compile because the type for m is an int and a primitive int cannot be assigned null. n = null compiles fine.

Question 40

Does the following local variable compile? var test = (String) null;

Answer 40

Since the type is provided, this line compiles fine. The variable test will be of type String.

Question 41

Does the following code compile? public int addition(var a, var b) { return a + b; }

Answer 41

It does not compile because a and b are method parameters. Var can only be used as local variables.

Question 42

Is var a reserved word within Java?

Answer 42

No. Var is not a reserved word and allowed to be used as an identifier. It is considered to be a reserved type name, meaning it cannot be used to define a type, such as a class, interface or enum.

Question 43

What does the command System.gc() do?

Answer 43

It suggests that the JVM kick off garbage collection. The JVM may perform garbage collection at that moment, or it might be busy and decide not to. The command is not guaranteed to make the garbage collection do anything.

Question 44

Which of the following object(s) are eligible for garbage collection? ``` public static void main(String[] args) { String one, two; one = new String("a"); two = new String("b"); one = two; String three = one; one = null; } ```

Answer 44

The String object "a" is eligible for garbage collection because no variable is pointing to that object.