Java Programming — Generics Questions & Answers

Q.61 Medium Generics

Consider the declaration: List

A List of Integer and its subclasses

B List of Integer and its superclasses

C List of any type

D List of Number and Integer only

Correct Answer: B. List of Integer and its superclasses

EXPLANATION

'? super Integer' is a lower bounded wildcard that accepts Integer and all its superclasses like Number, Object. This allows write operations with Integer values.

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Q.62 Easy Generics

What is the primary purpose of bounded type parameters in generics?

A To restrict type arguments to specific types or their subtypes

B To improve code readability only

C To increase runtime performance

D To create multiple inheritance in Java

Correct Answer: A. To restrict type arguments to specific types or their subtypes

EXPLANATION

Bounded type parameters like <T extends Number> restrict the types that can be passed as type arguments, ensuring type safety and enabling access to specific methods.

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Q.63 Easy Generics

Which of the following is a valid generic method declaration in Java?

A public static void display(T data) { }

B public static void display(T data) { }

C public static void display(T data) { }

D public static void display(T data) { }

Correct Answer: A. public static void display(T data) { }

EXPLANATION

Type parameters in Java must be declared before the return type using angle brackets. Option A has correct syntax: <T> comes before return type void.

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Q.64 Hard Generics

What is the key difference between '? extends T' and '? super T' in practical usage?

A Extends is for reading data safely, Super is for writing data safely

B Extends is for writing, Super is for reading

C They have no practical difference

D Extends is for classes, Super is for interfaces

Correct Answer: A. Extends is for reading data safely, Super is for writing data safely

EXPLANATION

Upper bounds (? extends T) are safe for reading because the compiler knows it's at least T. Lower bounds (? super T) are safe for writing because you can pass T or any superclass.

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Q.65 Medium Generics

Which statement about generic type parameters is TRUE?

A Type parameters must be uppercase single letters (T, U, V, etc.)

B Type parameters can only be used in class declarations, not in method declarations

C Type parameters can be any valid Java identifier

D Type parameters must match the actual type used at instantiation

Correct Answer: C. Type parameters can be any valid Java identifier

EXPLANATION

While convention uses single uppercase letters, type parameters can be any valid identifier. Statements B and D are also incorrect.

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Q.66 Hard Generics

Consider: Map

A A List of unknown element type

B A List

C Any List type

D Nothing can be safely retrieved

Correct Answer: A. A List of unknown element type

EXPLANATION

The value type is '? extends List<?>', so you get a List with unknown element type. The exact subtype is unknown at compile-time.

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Q.67 Medium Generics

What is the significance of the '&' operator in generic bounds like ''?

A T must extend both A and B (multiple bounds)

B T must extend A or B

C It's a bitwise AND operation

D T extends A with B as an alternative

Correct Answer: A. T must extend both A and B (multiple bounds)

EXPLANATION

Multiple bounds (using &) mean T must satisfy all constraints. This is useful when a type needs to implement multiple interfaces.

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Q.68 Easy Generics

In the interface declaration 'interface Pair', how many type parameters does it have?

A One

B Two

C Variable depending on implementation

D None, they are method parameters

Correct Answer: B. Two

EXPLANATION

The interface has two type parameters: K and V, making it a generic interface that can be implemented with different type combinations.

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Q.69 Hard Generics

What will happen when you try to create an array of generic types like 'new ArrayList[10]'?

A Compilation error because arrays of generic types are not allowed

B Runtime error after successful compilation

C It will compile and run successfully

D Warning but no compilation error

Correct Answer: A. Compilation error because arrays of generic types are not allowed

EXPLANATION

Arrays of generic types are not allowed in Java due to type erasure and heap pollution prevention.

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Q.70 Medium Generics

Consider the method: public static T getFirst(List list) { return list.get(0); }

How would you call this method for a List?

A String result = Main.getFirst(stringList);

B String result = getFirst(stringList);

C String result = Main.getFirst(stringList);

D Both A and B are correct

Correct Answer: D. Both A and B are correct

EXPLANATION

Type inference in Java allows the compiler to deduce T from the argument type, so both explicit and implicit type specification work.

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