Generics

Question 1

1. Generic type declarations can be used as…
2. What are the 4 possible constraints for these declarations?

Answer 1

• Fields
• Parameters
• Local variables
• Return types

1. <Type>
2. <?>
3. <? extends Type>
4. <? super Type>

Question 2

1. What are generics?
2. Can a generic type be passed to another generic type?

Answer 2

1. Generics are parameterized classes, interfaces, or methods
2. Yes

MyGeneric<T> {...}

MyOtherGeneric<U> {...}

MyGeneric<MyOtherGeneric<SomeClass>>

Question 3

1. What is the syntax for a generic constructor?
2. Is the generic constructor type parameter the same as its generic class parameter?
3. Can a generic constructor be used inside a non-generic class?

Answer 3

1. <T> ConstructorName(T param) {...}
2. No
3. Yes

Question 4

Can a non-generic class be a superclass of a generic class?

Answer 4

Yes

Question 5

1. What is the syntax for a generic method?
2. Is the generic method type parameter the same as its generic class parameter?
3. Can a generic method be defined inside a non-generic class?
4. Can a generic method be static or non-static?

Answer 5

<T> T method(T param) {...}

<T> ReturnType method(T param) {...}

2. No
3. Yes
4. Yes

Question 6

What is the syntax for a generic class that implements a generic interface?

Answer 6

interface MyGenericInterface<T> {...}

class MyGenericClass<T> implements MyGenericInterface<T> {...}

Question 7

1. When defining a generic class, what is the syntax to bind a generic type parameter to another class and interfaces?

Answer 7

1. class MyGeneric<T extends ClassName & InterfaceName, InterfaceName2> {...}

ClassName must always be first

Question 8

Legal or illegal?

class A {...}
class B {...}
class MyGeneric<T> {...}

MyGeneric<B> b = new MyGeneric<>();
MyGeneric<A> a = b;

Answer 8

Illegal. Even though B extends A, MyGeneric<A> has no relationship to MyGeneric<B>

Question 9

What is type erasure?

Answer 9

Type erasure is the process whereby the Java compiler erases all of the generic type parameters and replaces them with Object if unbounded or with their bounded type if bounded

Question 10

Legal or illegal?

MyGeneric<T>[] arr;

Answer 10

Illegal

Question 11

Can a generic class define static parameterzed members?

Answer 11

No, but generic methods can be static