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.
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.722Medium
Which generic wildcard usage is INCORRECT?
Answer: D
Nested wildcards like '? extends ? super T' are not allowed. Wildcards cannot be combined or nested.
Q.723Medium
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.724Medium
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.725Hard
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.726Easy
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.727Medium
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.728Hard
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.729Medium
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.730Hard
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.731Easy
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.732Easy
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.733Medium
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.
Q.734Easy
What will be the result of executing: List<Integer> list = new ArrayList<String>();
Answer: A
This causes a compilation error due to type mismatch. List<Integer> cannot hold ArrayList<String>. Generics do not support implicit type conversion.
Q.735Medium
Which statement correctly defines a generic class with multiple type parameters and bounds?
Answer: C
Multiple type parameters with bounds must be comma-separated and each can have its own upper bound. Option C shows correct syntax with recursive bound on T and bound on K.
Q.736Medium
Consider: public static <T> T findMax(T a, T b) { return a; }. What is the issue with calling findMax(5, 3.5)?
Answer: A
The type parameter T cannot simultaneously be int and double. The compiler cannot infer a single type T that satisfies both arguments, resulting in a compilation error.
Q.737Medium
What does List<?> represent in Java generics?
Answer: D
List<?> is an unbounded wildcard that represents a list of unknown type. You can read from it (get Object), but cannot write to it (except null). Both A, B, and C are correct interpretations.
Q.738Medium
Which of the following will NOT compile?
Answer: D
Option D fails because List<Object> cannot reference ArrayList<String>. Generics are invariant. Options A, B, C are valid (covariance with extends, contravariance with super).
Q.739Easy
What is the output of the following code?
List<Integer> list = new ArrayList<Integer>(); list.add(10); Object obj = list.get(0); System.out.println(obj.getClass().getName());
Answer: A
Although obj is declared as Object, it holds an Integer instance. getClass() returns the actual runtime type, which is Integer.
Q.740Medium
Consider a generic interface: interface Comparable<T> { int compareTo(T obj); }. How should a class implement this for String comparison?
Answer: D
Option B directly specifies String, while Option C makes the class generic. Both are valid approaches to implement a generic interface.
