ch3

Question 1

Can a numeric literal contain a comma?

Answer 1

no

Question 2

Which object does a variable with the value null refer to?

Answer 2

To no object at all. There is no null singleton or sth like that in Java.

Question 3

What is the scope of static variables?

Answer 3

Created: when the class is loaded. Stay: as long as the class stays in the JVM.

Question 4

What are the possible scopes of a variable?

Answer 4

static, instance / member, local / method / stack (temporary, automatic??), block (Remember: no life outside a class)

Question 5

What about the variables of the method which has invoked the current method?

Answer 5

They are alive, but out of scope. (They are on the stack, but not in the top étage.)

Question 6

When do instance variables get a value?

Answer 6

When the object gets created. If no explicit value is defined, they get initialized with a default value.

Question 7

Is an array an object?

Answer 7

Yes.

Question 8

int [] numbers = new int[10]; What does this array contain?

Answer 8

10 int elements with the value 0.

Question 9

Do local variables have a default value?

Answer 9

No.

Question 10

What happens, if you use a local variable which has not been initialized?

Answer 10

compiler error: might not have been initialized

Question 11

Reference variables referring to such objects behave a bit different.

Answer 11

String

Question 12

What does variable shadowing mean?

Answer 12

Reusing a variable name which has already been declared somewhere else.

Question 13

Which part of the memory is involved in the garbage collection process?

Answer 13

heap

Question 14

What does the garbage collection do?

Answer 14

1. search for objects eligible for garbage collection: No live thread can reach it. 2. evtl delete them

Question 15

Who starts the garbage collection?

Answer 15

The JVM. The program code can request a run, but there is no guarantee, it will run indeed.

Question 16

Is the following statement true: If there is a reference to an object, it is not eligible for garbage collection ?

Answer 16

No. If the object belongs to an ‘island of isolation’, it is eligible for garbage collection.

Question 17

How can you request a garbage collection?

Answer 17

System.gc() Runtime.getRuntime().gc()

Question 18

What is Runtime?

Answer 18

a class. It has a single instance (a singleton) for each main program. It provides a mechanism to communicate directly with the JVM.

Question 19

How can you communicate directly with the JVM in your code?

Answer 19

via the singleton instance of the Runtime class: Runtime.getRuntime()

Question 20

Can you get an OutOfMemoryException, if the garbage collector has never run?

Answer 20

No.

Question 21

What are the drawbacks of the finalize method?

Answer 21

It is not deterministic when the garbage collection runs, thus it is not deterministic when finalize runs. If the code in finalize saves the object from deletion, but it gets eligible for garbage collection again later, the finalize method will not run before the actual deletion.