InterviewSolution

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This section includes InterviewSolutions, each offering curated multiple-choice questions to sharpen your knowledge and support exam preparation. Choose a topic below to get started.

1.	which two code fragments inserted at end of the program, will allow to compile?
Answer» (3) are (5) are correct because interfaces and abstract classes do not need to fully implement the interfaces they extend or implement (respectively). (1) is incorrect because a class cannot extend an interface. (2) is incorrect because an interface cannot implement anything. (4) is incorrect because the method being implemented is from the wrong interface.

2.	Which two statements are true for any concrete class implementing the java.lang.Runnable interface?
Answer» (2) and (6). When a thread's run() method completes, the thread will die. The run() method must be declared public void and not take any arguments. (1) is incorrect because classes can never extend interfaces. (3) is incorrect because the run() method is typically not empty; if it were, the thread would do nothing. (4) is incorrect because the mandatory method is run(). (5) is incorrect because the class implements Runnable.

3.	which two statements, added independently at beginning of the program, allow the code to compile?
Answer» (2) and (5). TreeMap is the only class that must be imported. TreeSet does not need an import statement because it is described with a fully qualified name. (1) is incorrect because TreeMap must be imported. (3) is incorrect syntax for an import statement. (4) is incorrect because it will not import TreeMap, which is required.

4.	Which three statements are true?
Answer» (2) sounds correct as in the example below class CoffeeCup { private int innerCoffee; public CoffeeCup() { } public void add(int amount) { innerCoffee += amount; } //... } The compiler gives default constructors the same access level as their class. In the example above, class CoffeeCup is public, so the default constructor is public. If CoffeeCup had been given package access, the default constructor would be given package access as well. (3) is correct. The Java compiler generates at least one instance initialisation method for every class it compiles. In the Java class file, the instance initialisation method is named "<init>." For each constructor in the source code of a class, the Java compiler generates one <init>() method. If the class declares no constructors explicitly, the compiler generates a default no-arg constructor that just invokes the superclass's no-arg constructor. As with any other constructor, the compiler creates an <init>() method in the class file that corresponds to this default constructor. (5) is correct. The compiler creates a default constructor if you do not declare any constructors in your class.

5.	which statement is true?
Answer» The ParentUtil instance p cannot be used to access the doStuff() method. Because doStuff() has protected access, and the ChildUtil class is not in the same package as the ParentUtil class, doStuff() can be accessed only by instances of the ChildUtil class (a subclass of ParentUtil). Option A, B and D are incorrect because of the access rules described previously.

6.	You want subclasses in any package to have access to members of a superclass. Which is the most restrictive access that accomplishes this objective?
Answer» Access modifiers dictate which classes, not which instances, may access features. Methods and variables are collectively known as members. Method and variable members are given access control in exactly the same way. private makes a member accessible only from within its own class protected makes a member accessible only to classes in the same package or subclass of the class default access is very similar to protected (make sure you spot the difference) default access makes a member accessible only to classes in the same package. public means that all other classes regardless of the package that they belong to, can access the member (assuming the class itself is visible) final makes it impossible to extend a class, when applied to a method it prevents a method from being overridden in a subclass, when applied to a variable it makes it impossible to reinitialise a variable once it has been initialised abstract declares a method that has not been implemented. transient indicates that a variable is not part of the persistent state of an object. volatile indicates that a thread must reconcile its working copy of the field with the master copy every time it accesses the variable. After examining the above it should be obvious that the access modifier that provides the most restrictions for methods to be accessed from the subclasses of the class from another package is C - protected. A is also a contender but C is more restrictive, B would be the answer if the constraint was the "same package" instead of "any package" in other words the subclasses clause in the question eliminates default.

7.	What is the widest valid returnType for methodA in line 3?
Answer» However A, B and C are all wrong. Each of these would result in a narrowing conversion. Whereas we want a widening conversion, therefore the only correct answer is D. Don't be put off by the long cast, this applies only to the variable x and not the rest of the expression. It is the variable y (of type double) that forces the widening conversion to double. Java's widening conversions are: - From a byte to a short, an int, a long, a float, or a double. - From a short, an int, a long, a float, or a double. - From a char to an int, a long, a float, or a double. - From an int to a long, a float, or a double. - From a long to a float, or a double. - From a float to a double.

8.	Which is valid in a class that extends ?
Answer» Option A is correct - because the class that extends A is just simply overriding method1. Option B is wrong - because it can't override as there are less access privileges in the subclass method1. Option C is wrong - because to override it, the return type needs to be an integer. The different return type means that the method is not overriding but the same argument list means that the method is not overloading. Conflict - compile time error. Option D is wrong - because you can't override a method and make it a class method i.e. using static.

9.	Which one creates an instance of an array?
Answer» Option A is correct. It uses correct array declaration and correct array construction. Option B is incorrect. It generates a compiler error: incompatible types because the array variable declaration is not correct. The array construction expects a reference type, but it is supplied with a primitive type in the declaration. Option C is incorrect. It generates a compiler error: incompatible types because a string literal is not assignable to a character type variable. Option D is wrong, it generates a compiler error <identifier> expected. The compiler thinks that you are trying to create two arrays because there are two array initialisers to the right of the equals, whereas your intention was to create a 3 x 3 two-dimensional array.

10.	Which two cause a compiler error?
Answer» (1) causes two compiler errors ( '[' expected and illegal start of expression) because the wrong type of bracket is used, ( ) instead of [ ]. The following is the correct syntax: float[ ] f = new float[3]; (2) causes a compiler error ( '{' expected ) because the array constructor does not specify the number of elements in the array. The following is the correct syntax: float f2[ ] = new float[3]; (3), (4), and (5) compile without error.

11.	Given a method in a protected class, what access modifier do you use to restrict access to that method to only the other members of the same class?
Answer» The private access modifier limits access to members of the same class. Option A, B, D, and E are wrong because protected are the wrong access modifiers, and final, static, and volatile are modifiers but not access modifiers.

12.	Which is a valid declaration within an interface?
Answer» (A) is valid interface declarations. (B) and (C) are incorrect because interface variables cannot be either protected or transient. (D) is incorrect because interface methods cannot be final or static.

13.	What is the most restrictive access modifier that will allow members of one class to have access to members of another class in the same package?
Answer» default access is the "package oriented" access modifier. Option A and C are wrong because public and protected are less restrictive. Option B and D are wrong because abstract and synchronized are not access modifiers.

14.	Which cause a compiler error?
Answer» Option B generates a compiler error: <identifier> expected. The compiler thinks you are trying to create two arrays because there are two array initialisers to the right of the equals, whereas your intention was to create one 3 x 3 two-dimensional array. To correct the problem and make option B compile you need to add an extra pair of curly brackets: int [ ] [ ] scores = { {2,7,6}, {9,3,45} };

15.	Which three are valid method signatures in an interface?
Answer» (2), (3), and (5). These are all valid interface method signatures. (1), is incorrect because an interface method must be public; if it is not explicitly declared public it will be made public implicitly. (4) is incorrect because interface methods cannot be static.

16.	You want a class to have access to members of another class in the same package. Which is the most restrictive access that accomplishes this objective?
Answer» The only two real contenders are C and D. Protected access Option C makes a member accessible only to classes in the same package or subclass of the class. While default access Option D makes a member accessible only to classes in the same package.

17.	which two code fragments will compile?
Answer» (3) is correct because an abstract class doesn't have to implement any or all of its interface's methods. (4) is correct because the method is correctly implemented ((7 > 4) is a boolean). (1) is incorrect because interfaces don't implement anything. (2) is incorrect because classes don't extend interfaces. (5) is incorrect because interface methods are implicitly public, so the methods being implemented must be public.

18.	Which three form part of correct array declarations?
Answer» (1), (2) and (6) are valid array declarations. Option (3) is not a correct array declaration. The compiler complains with: illegal start of type. The brackets are in the wrong place. The following would work: public int[ ] a Option (4) is not a correct array declaration. The compiler complains with: ']' expected. A closing bracket is expected in place of the 3. The following works: private int a [] Option (5) is not a correct array declaration. The compiler complains with 2 errors: ']' expected. A closing bracket is expected in place of the 3 and <identifier> expected A variable name is expected after a[ ] .

19.	What is the prototype of the default constructor?
Answer» Option A and B are wrong because they use the default access modifier and the access modifier for the class is public (remember, the default constructor has the same access modifier as the class). Option D is wrong. The void makes the compiler think that this is a method specification - in fact if it were a method specification the compiler would spit it out.