Methods

Question 1

What about methods with same name, but they have wrapping class and primitive as only parameters?

Answer 1

It is allowed for them to exist in same class:

public void method(int a) {}
public void method(Integer a) {}

public void method(Integer[] a) {}
public void method(int[] a) {}

Same goes for arrays, but not varargs and arrays

Question 2

What happens when we set a value to autobox and numeric promote?

Answer 2

Java wont do both at the same time, it can ether autobox or numeric promote smaller value to bigger. Eg.
Long l = 9 wont compile.
long l = 9 will.

Same applies for methods:
method(Long a) <- method(9) wont compile

Question 3

What is alternative to autoboxing and unboxing?

Answer 3

Depending on value type:

Integer i1 = Integer.valueOf(int) is autoboxing
int i2 = i1.intValue() is unboxing

Question 4

How can we get NullpointerException while unboxing?

Answer 4

When we try to unbox a wrapper class that is null to primitive

Question 5

Are varargs and arrays interchangeable?

Answer 5

Yes.
new boolean[2] -> boolean…

method(new int[] { 1, 2, 3 }) is also valid.

Question 6

What is position where varargs need to be when used as parameters?

Answer 6

At the end

Question 7

Can varargs have … before name of the varargs?

Answer 7

No, they must be after the name.

args… O
…args X

Question 8

What is rule for initializing static final class member?

Answer 8

It can be initialized only once, second time will throw compile error. So they can be inited directly inline or with static blocks, but only once.

Question 9

What are rules for package access methods?

Answer 9

If class is public, then it can be initialized from anywhere, but if it contains package access protected methods, they can be only ran from inside the same package.

This access type is more restrictive than protected.

Question 10

What modifiers can instance variable have?

Answer 10

static
transient
volatile
final

Question 11

What access modifiers are there?

Answer 11

public
protected
package protected
private

Question 12

What optional modifiers can methods have?

Answer 12

synchronized
final
static
abstract
default

Question 13

What do we need to watch out when using final with instance variables?

Answer 13

We must initialize its value before it is used, in instance variable case, before constructor call ends. In static class members, before usage or end of the class.

Question 14

What do we need to watch out regarding modifiers?

Answer 14

Optional modifiers can come before other modifiers

Question 15

Can “final” and “var” be used together?

Answer 15

Yes, but only in method bodies.

Question 16

What can you tell about static members of a class and their initialization?

Answer 16

They auto. initialize. eg. int to 0 and String to null

Question 17

What happens when you null a reference that contains static member?

Answer 17

You can still access static member trough that nulled reference.

Question 18

How does “package access” modifier work?

Answer 18

If defined on class, only classes that are instantiated in the same package (not even subpackage) can instantiate it. If extended, class members are only available to child methods.
If defined on class members, only classes that are in the same package can use those members, but class can be instantiated if public.

Question 19

How does “protected” modifier work?

Answer 19

Cannot be defined on a class.
When defined on class members, only classes that are in the same package can use those members. If extended, class members are only available to child methods or classes that are instantiated in the same package.

Question 20

Is there numeric promotion between wrapping classes?

Answer 20

No, those classes need to be unboxed and then numberic promotion can happen

Question 21

What is set when you instantiate “new boolean[2]”

Answer 21

Values of the array are auto. set to false