Generics
What is the difference between List and List?
A
List is read-only while List allows adding any object type
B
List is read-only while List allows adding any type
C
Both are equivalent
D
List uses type erasure while List does not
Correct Answer:
A. List is read-only while List allows adding any object type
List<?> accepts any type but is read-only (can't add). List<Object> explicitly accepts Object type and allows adding objects.
Which of the following represents a bounded type parameter?
A
class Box { }
B
class Box { }
C
class Box
D
class Box { }
Correct Answer:
A. class Box { }
Bounded type parameters restrict the types that can be used as arguments. T extends Number ensures T is Number or its subclass.
Consider the code:
A
No, compilation error - cannot add to ? extends
B
Yes, it compiles successfully
C
Only if Integer is used
D
Only at runtime
Correct Answer:
A. No, compilation error - cannot add to ? extends
Upper bounded wildcards (? extends Number) are read-only. You cannot add elements because the compiler doesn't know the exact subtype.
What will happen when you try to create an array of generics?
A
Compilation error
B
Runtime exception
C
Code executes successfully
D
Creates an unchecked warning only
Correct Answer:
A. Compilation error
You cannot create arrays of generic types because of type erasure. The compiler prevents this with a compilation error.
Which statement about wildcard generics (?) is correct?
A
? represents an unknown type and can be used with upper bounds (? extends T) or lower bounds (? super T)
B
? can only be used with extends keyword
C
? is used to create generic classes
D
? eliminates the need for type parameters
Correct Answer:
A. ? represents an unknown type and can be used with upper bounds (? extends T) or lower bounds (? super T)
Wildcards (?) represent an unknown type. Upper bound (? extends T) accepts T or its subtypes. Lower bound (? super T) accepts T or its supertypes.
What is type erasure in Java Generics?
A
The process by which generic type information is removed at runtime
B
Automatic deletion of generic collections
C
Compiler optimization of generic code
D
Runtime type checking for generics
Correct Answer:
A. The process by which generic type information is removed at runtime
Type erasure is a mechanism where the Java compiler removes all generic type information during compilation, converting generic code to its raw type equivalent.
Which of the following is a valid generic method declaration?
A
public T getValue(T value) { return value; }
B
public T getValue(T value) { return value; }
C
public getValue(T value) { return value; }
D
public getValue(T value) { return value; }
Correct Answer:
A. public T getValue(T value) { return value; }
The correct syntax for a generic method is: access_modifier <T> returnType methodName(T parameter). The type parameter <T> must come before the return type.
What will be the output of the following code?
A
4
B
5
C
Compilation error
D
Runtime exception
"Java" has 4 characters. The generic type ensures the element is a String, so .length() method is available without casting.
Which keyword is used to declare a generic class in Java?
A
B
generic
C
template
D
type
<T> is the standard syntax for declaring a generic type parameter in Java classes.
What is the primary purpose of Generics in Java?
A
To provide type safety and eliminate type casting
B
To increase memory allocation
C
To reduce code compilation time
D
To automatically manage garbage collection
Correct Answer:
A. To provide type safety and eliminate type casting
Generics enable type safety at compile time and eliminate the need for explicit type casting, reducing runtime errors.
