Java interviews and campus tests lean heavily on object oriented behaviour rather than API recall. This set covers inheritance and polymorphism, interfaces and abstract classes, the collections framework, exception handling, multithreading, string handling, and JVM basics such as garbage collection. Output based questions are included because they expose the gaps that theory questions hide.
What is the primary advantage of using generics in Java?
Answer: A
Generics provide compile-time type checking, preventing ClassCastException and eliminating the need for explicit casting at runtime.
Q.22Medium
In the declaration 'List<? extends Number> list', what types can be added to this list at runtime?
Answer: C
Upper-bounded wildcards (? extends T) are used for reading data. Adding elements is unsafe because the compiler doesn't know the exact type.
Q.23Medium
Which statement correctly uses the lower-bounded wildcard in generics?
Answer: A
Lower-bounded wildcards (? super T) allow superclasses of T. This assignment is valid because ArrayList<Number> fits '? super Integer'.
Q.24Medium
Which generic declaration would allow storing both String and Integer in the same collection?
Answer: D
All three allow storing multiple types because they use unbounded or Object-level wildcards, though List<?> is most restrictive for additions.
Q.25Medium
What does the PECS principle stand for in generics context?
Answer: A
PECS guides when to use upper bounds (? extends) for producers/readers and lower bounds (? super) for consumers/writers.
Q.26Easy
Which of the following will compile successfully?
Answer: B
Generics only work with reference types. 'int' is a primitive type, so 'Integer' wrapper class must be used instead.
Q.27Medium
In a generic class 'class Box<T extends Comparable<T>>', what constraint is placed on T?
Answer: A
The bound 'T extends Comparable<T>' ensures T is comparable to itself, enabling type-safe comparison operations.
Q.28Easy
What is the output of this code snippet?
ArrayList<String> list = new ArrayList<>(); list.add("Java"); Object obj = list.get(0); String str = (String) obj; System.out.println(str.length());
Answer: A
The code compiles and runs correctly. 'Java' has 4 characters. The explicit cast is necessary because get() returns Object type.
Q.29Medium
Which generic wildcard usage is INCORRECT?
Answer: D
Nested wildcards like '? extends ? super T' are not allowed. Wildcards cannot be combined or nested.
Q.30Medium
What is the relationship between raw types and generics in Java?
Answer: A
Raw types (like 'List' without type parameters) are legacy and should be avoided as they bypass compile-time type safety.
Q.31Medium
Consider the method: public static <T> T getFirst(List<T> list) { return list.get(0); }
How would you call this method for a List<String>?
Answer: D
Type inference in Java allows the compiler to deduce T from the argument type, so both explicit and implicit type specification work.
Q.32Hard
What will happen when you try to create an array of generic types like 'new ArrayList<String>[10]'?
Answer: A
Arrays of generic types are not allowed in Java due to type erasure and heap pollution prevention.
Q.33Easy
In the interface declaration 'interface Pair<K, V>', how many type parameters does it have?
Answer: B
The interface has two type parameters: K and V, making it a generic interface that can be implemented with different type combinations.
Q.34Medium
What is the significance of the '&' operator in generic bounds like '<T extends A & B>'?
Answer: A
Multiple bounds (using &) mean T must satisfy all constraints. This is useful when a type needs to implement multiple interfaces.
Q.35Hard
Consider: Map<String, ? extends List<?>> map = new HashMap<>();
What can you safely retrieve from this map?
Answer: A
The value type is '? extends List<?>', so you get a List with unknown element type. The exact subtype is unknown at compile-time.
Q.36Medium
Which statement about generic type parameters is TRUE?
Answer: C
While convention uses single uppercase letters, type parameters can be any valid identifier. Statements B and D are also incorrect.
Q.37Hard
What is the key difference between '? extends T' and '? super T' in practical usage?
Answer: A
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.
Q.38Easy
Which of the following is a valid generic method declaration in Java?
Answer: A
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.
Q.39Easy
What is the primary purpose of bounded type parameters in generics?
Answer: A
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.
Q.40Medium
Consider the declaration: List<? super Integer> list. What does this wildcard represent?
Answer: B
'? super Integer' is a lower bounded wildcard that accepts Integer and all its superclasses like Number, Object. This allows write operations with Integer values.
