java

Question 1

What are the main methods of iterator?

Answer 1

next, hasNext and remove

Question 2

is Iterator an interface?

Answer 2

yes

Question 3

What is a Stream?

Answer 3

Sequence of elements supporting sequential and

parallel aggregate operations.

Question 4

How to implement equals correctly?

Answer 4

1. create a protected boolean eq(object o).
   
   1. 1 check if o is instance of class
   2. 2 check condition
2. check if both objects agree on eq.

example:
class Point {
protected boolean eq(Object o) {
if (!(o instanceof Point))
return false;
return ((Point)o).x == x && ((Point)o).y == y;
}
public boolean equals(Object o) {
return (this.eq(o) && ((Point)o).eq(this));
}
}

Question 5

If you override equals what other method should you override?

Answer 5

hashCode

Question 6

What does compareTo?

Answer 6

compares to objects and returns an int indicating which one is bigger.
if return > 0 then this is bigger
otherwise parameter is bigger.

Question 7

What’s the use of Comparator?

Answer 7

comparing objects that doesn’t implement Comparable interface (don’t have compareTo method).

Question 8

What’s a tagging interface?

Answer 8

an interface without methods, used for specifying a purpose.

Question 9

Why not make all classes Cloneable?

Answer 9

because then we can’t have singletons.

Question 10

What are the pitfalls of clone?

Answer 10

Don't use constructors inside clone. 
because the dynamic type might be different from static type. 
for example field A a, actually holds an object of type B.
therefore if we copy a using new A(); we will get wrong behaviour.

Question 11

What are the 4 access modifiers in Java?

Answer 11

1. public - everyone
2. protected - only inheriting classes and same package
3. default - same package
4. private - same (toplevel) class. (if nested classes are involved).

Question 12

What are static nested classes?

Answer 12

defined as member classes, can only access static members.

Question 13

What are inner classes?

Answer 13

non-static nested classes. there are 3 types:

1. non static member classes, regular
2. local class
3. anonymous class

Question 14

What are local classes?

Answer 14

classes defined in a method.

can access parameters as well as local (final!) vars declared in the same method before the class

Question 15

What are anonymous classes?

Answer 15

created inside a method.

definition is also invocation.

Question 16

anonymous class syntax

Answer 16

new () {

}

examle:
interface A {
     void g();
}
class B {
    void f() {
    A a = new A() {
    @Override public void g() {}
     };
      a.g();
  }
}

Question 17

Is it possible to inherit from an inner class?

Answer 17

yes. as long as the inheriting class can be associated with a subtype of the outer class.

Question 18

What are the advantages of java enums over pre c++11 enums?

Answer 18

1. Enum declaration defines a class (type safety)
2. Body can include methods and fields
3. Values may be objects
4. Support for iteration, naming

Question 19

Can an enum type inherit?

Answer 19

No. all enum types implicitly inherit from Class Enum.

Can implement interfaces.

Question 20

Can you extend enum classes?

Answer 20

No. enums are implicitly final.

Question 21

What’s the best way to implement a singleton in Java?

Answer 21

Enums.
public enum MySingleton {
INSTANCE;

advantage:
no need for lazy instantiation.

Question 22

Do annotations affect the code?

Answer 22

mo. but can effect things that use the code: tools, code generation etc..

Question 23

What is the annotation @FunctionalInterface?

Answer 23

asserts that the tagged interface has one and only one
method that is without implementation. So we can be
sure that it can be replaced with lambda.

Question 24

What are the 3 types of annotations?

Answer 24

Marker annotations
– Have no attributes
– @Override, @Deprecated

Single value annotations
– Provide a single piece of data.
– Attribute type can be
 primitive, string, Enum, or array of the previous.
• @Author(“Pazit”)
• @SuppressWarnings({“unchecked”,
“deprecation”})

Multi valued annotations
– @MethodKind(composite=true, mutator=false)

Question 25

What are meta-annotaitions?

Answer 25

* Annotations that annotate annotations | * Specify how the annotation should be used

Question 26

What are the 4 meta-annotaitions we learned about?

Answer 26

@Inherited • Does the annotation get applied to subclasses @Target • Where the annotation can be used (source elements) • Default is all @Retention • Where is the annotation retained (where it “lives”) • Possible values: runtime, class, source @Repeatable – Specifies that the tagged annotation may appear over an element more then once.