1. Java Flashcards
What is the main purpose of Java’s garbage collector?
To automatically manage memory by reclaiming memory used by objects that are no longer referenced.
True or False: Java supports multiple inheritance through classes.
False
What keyword is used to inherit a class in Java?
extends
Fill in the blank: In Java, the _______ keyword is used to declare a constant.
final
Which of the following is a valid way to declare an array in Java? (A) int arr[]; (B) int[] arr; (C) Both A and B
C
What is the default value of a boolean variable in Java?
false
What does JVM stand for?
Java Virtual Machine
True or False: An interface can contain method implementations in Java.
False
What is the purpose of the ‘synchronized’ keyword in Java?
To control access to a method or block by multiple threads.
What is the output of the following code: System.out.println(1 + 1 + ‘1’);? (A) 21 (B) 11 (C) 2
21
Which collection class allows duplicate elements in Java?
ArrayList
What is the primary function of the ‘try-catch’ block?
To handle exceptions that may occur during program execution.
Fill in the blank: A _______ is a blueprint for creating objects in Java.
class
What is method overloading?
Defining multiple methods with the same name but different parameters in the same class.
True or False: Java provides a mechanism called ‘autoboxing’ for converting primitive types to their corresponding wrapper classes.
True
What is the purpose of the ‘this’ keyword in Java?
To refer to the current object instance.
Which of the following is NOT a primitive data type in Java? (A) int (B) String (C) boolean
B
What does the ‘transient’ keyword indicate in Java?
That a field should not be serialized.
What is a ‘static’ method?
A method that belongs to the class rather than any specific instance.
Fill in the blank: Java uses _______ to achieve platform independence.
bytecode
What is the difference between ‘== ‘ and ‘.equals()’ in Java?
’==’ checks reference equality, while ‘.equals()’ checks for value equality.
True or False: A Java interface can extend multiple other interfaces.
True
What is the purpose of the ‘finalize()’ method in Java?
To perform cleanup before an object is garbage collected.
What is the output of the following code: System.out.println(5 / 2);?
2
Which exception is thrown when a program attempts to access an array with an illegal index?
ArrayIndexOutOfBoundsException
What is a ‘constructor’ in Java?
A special method used to initialize objects.
Fill in the blank: The _______ block is used to execute code regardless of whether an exception is thrown or not.
finally
What does the ‘volatile’ keyword do in Java?
It indicates that a variable may be changed by different threads.
True or False: Java supports operator overloading.
False
What is the difference between ‘List’ and ‘Set’ in Java Collections Framework?
‘List’ allows duplicates and maintains order, while ‘Set’ does not allow duplicates and does not guarantee order.
What is the purpose of the ‘instanceof’ operator?
To test whether an object is an instance of a specific class or interface.
What is the default access modifier for class members in Java?
Package-private
Which keyword is used to create an anonymous inner class in Java?
new
Fill in the blank: The Java Development Kit (JDK) includes a _______ for compiling Java source code.
compiler
What is a ‘Java Bean’?
A reusable software component that follows specific conventions for property management.
True or False: Java supports multiple inheritance through interfaces.
True
What is the purpose of the ‘super’ keyword?
To refer to the superclass of the current object.
Which of the following is a feature of Java? (A) Platform independence (B) Automatic memory management (C) Both A and B
C
What does ‘JAR’ stand for in Java?
Java Archive
What is the significance of the ‘main’ method in Java?
It serves as the entry point for any Java application.
Fill in the blank: The _______ package contains the Java Collections Framework.
java.util
What is the purpose of ‘Java Reflection’?
To inspect classes, interfaces, fields, and methods at runtime without knowing the names of the classes.
True or False: The ‘String’ class in Java is mutable.
False
What is the role of a ‘Comparator’ in Java?
To define a custom sorting order for objects.
What is the purpose of the ‘assert’ statement in Java?
To provide a means for debugging by testing assumptions in the code.
What is a ‘static block’ in Java?
A block of code that runs when the class is loaded into memory.
Fill in the blank: Java uses _______ to manage access control for classes and members.
access modifiers
What is the primary use of ‘Java Annotations’?
To provide metadata about a program element.
What is a ‘Checked Exception’?
An exception that must be either caught or declared in the method signature.
True or False: The ‘StringBuilder’ class is synchronized.
False
What is the purpose of the ‘volatile’ keyword?
To ensure visibility of changes to variables across threads.
Which Java keyword is used to create a new thread?
Thread
Fill in the blank: A _______ is used to handle a group of related exceptions in Java.
catch block
What is the difference between ‘HashMap’ and ‘TreeMap’?
‘HashMap’ is unordered and allows null keys, while ‘TreeMap’ is ordered and does not allow null keys.
What does ‘JVM’ do when it encounters a ‘ClassNotFoundException’?
It indicates that the Java Virtual Machine cannot find a class that is being referenced.
What is the output of the expression ‘10 % 3’ in Java?
1
Fill in the blank: The _______ interface is used to implement a callback mechanism in Java.
Runnable
What is the purpose of the ‘default’ keyword in interfaces?
To provide a default implementation for a method in an interface.
True or False: The ‘main’ method must always be declared as public.
True
What is the function of the ‘Thread.sleep()’ method?
To pause the execution of the current thread for a specified time.
What does the ‘final’ keyword indicate when used with a class?
That the class cannot be subclassed.
Fill in the blank: An _______ is a template for creating objects and contains methods and variables.
object
What is the role of ‘JavaFX’?
To provide a platform for creating rich internet applications.
What is the purpose of the ‘volatile’ keyword in concurrent programming?
To ensure that updates to a variable are visible to other threads.
What happens when you try to access an index of an array that is out of bounds?
ArrayIndexOutOfBoundsException is thrown.
Fill in the blank: Java uses _______ to manage the lifecycle of an application.
threads
What is the difference between ‘abstract class’ and ‘interface’?
‘Abstract class’ can have both abstract and concrete methods, while ‘interface’ can only have abstract methods (until Java 8).
What is ‘Java Streams’?
A feature that allows functional-style operations on collections of objects.
True or False: A constructor can have a return type.
False
What is the role of the ‘java.lang’ package?
It contains fundamental classes that are essential for Java programming.
What is the output of the following code: System.out.println(3 + 4 + ‘7’);?
74
Fill in the blank: The _______ method is called when an exception is not caught.
printStackTrace
What is the main purpose of the ‘synchronized’ keyword?
To prevent thread interference by allowing only one thread to access a block of code at a time.
What does the term ‘boxing’ refer to in Java?
The conversion of a primitive type to its corresponding wrapper class.
True or False: A synchronized block is less flexible than synchronized methods.
True
Why were Generics Introduced?
Generics were introduced in J2SE 5.0 to deal with type-safe objects, provide a way to define classes, interfaces, and methods with placeholders for types.
This capability allows for the creation of flexible and reusable code components that work with different data types while maintaining type safety at compile time. It makes the code stable by detecting the bugs at compile time.
Why Generics in Java?
Type Safety: Type safety is a critical aspect of Java programming. Before generics, collections in Java could hold any type of objects, which meant that type errors could only be caught at runtime, not compile time. Lack of type safety could lead to ClassCastException errors when an object was cast to the wrong type.
Code Reusability: Generics allow for the creation of more general and reusable code. By parameterizing types, we can write methods and classes that can operate on any type, reducing code duplication and increasing flexibility.
Elimination of Casting: Generics reduce the need for explicit type casting, making the code cleaner and less error-prone. Without generics, developers often have to cast objects to the desired type, leading to cluttered code and potential runtime errors.
Enabling Generic Algorithms: Generics enable the creation of generic algorithms that can work with any type, enhancing the versatility of code. It is particularly useful in collections and other data structures.
Enhancing the Java Collections Framework: The introduction of generics significantly enhanced the Java Collections Framework, making it more powerful and type-safe. Collections can now be parameterized with specific types, ensuring type safety and reducing the risk of runtime errors.
Supporting More Readable and Maintainable Code: Generics make the code more readable and maintainable by explicitly stating what types are being used. This clarity helps other developers understand the intended use of collections and methods, reducing the likelihood of errors.
Facilitating Type Inference: Java’s type inference mechanisms work well with generics, allowing the compiler to deduce the type parameters in many cases, which simplifies the code for the developer.
Backward Compatibility: Generics in Java were designed with type erasure to ensure backward compatibility with older versions of Java. Type erasure means that generic type information is only available at compile time and is removed at runtime. It allows generic code to interoperate with legacy code that does not use generics.
What does Type Safety mean and why do we need it?
We can hold only a single type of objects in generics. It does not allow to store other objects.
Without Generics, we can store any type of objects.
Is type casting required with Generics?
**No, it’s not! **
]There is no need to typecast the object.
Before Generics, we need to type cast.
What is Generics effect on Compile-Time Checking?
ClassOrInterface<Type></Type>
ArrayList<String></String>
It is checked at compile time so problem will not occur at runtime. The good programming strategy says it is far better to handle the problem at compile time than runtime.
Give a full example of Generics
We are using the ArrayList class
Here, we are using the ArrayList class, but we can use any collection class such as ArrayList, LinkedList, HashSet, TreeSet, HashMap, Comparator etc.
element is: jai
rahul
jai
Show an example using Map
Now we are going to use map elements using generics. Here, we need to pass key and value.
1 vijay
2 ankit
4 umesh
What is a Generic class?
A class that can refer to any type is known as a generic class. Here, we are using the T type parameter to create the generic class of specific type.
Let’s see a simple example to create and use the generic class.
What are the Type parameters for Generics?
The type parameters naming conventions are important to learn generics thoroughly. The common type parameters are as follows:
T - Type
E - Element
K - Key
N - Number
V - Value
What are Generic methods?
Like the generic class, we can create a generic method that can accept any type of arguments. Here, the scope of arguments is limited to the method where it is declared. It allows static as well as non-static methods.
Let’s see a simple example of Java generic method to print array elements. We are using here E to denote the element.
What are wild cards in Java Generics?
The ? (question mark) symbol represents the wildcard element. It means any type. If we write <? extends Number>, it means any child class of Number. For example, Integer, Float, and double. Now we can call the method of Number class through any child class object.
We can use a wildcard as a type of a parameter, field, return type, or local variable. However, it is not allowed to use a wildcard as a type argument for a generic method invocation, a generic class instance creation, or a supertype.
What are upper bounded wildcards?
The purpose of upper bounded wildcards is to decrease the restrictions on a variable. It restricts the unknown type to be a specific type or a subtype of that type. It is used by declaring wildcard character (“?”) followed by the extends (in case of, class) or implements (in case of, interface) keyword, followed by its upper bound.
Syntax
*List<? extends Number> *
Here,
? is a wildcard character.
extends, is a keyword.
Number, is a class present in java.lang package
Suppose, we want to write the method for the list of Number and its subtypes (like Integer, Double). Using List<? extends Number> is suitable for a list of type Number or any of its subclasses whereas **List<Number> **works with the list of type Number only. So, **List<? extends Number>** is less restrictive than ***List<Number>***.</Number></Number>
What are unbounded wildcards?
The unbounded wildcard type represents the list of an unknown type such as List<?>. This approach can be useful in the following scenarios: -
When the given method is implemented by using the functionality provided in the Object class.
When the generic class contains the methods that don’t depend on the type parameter.
Disadvantages of Java Generics
Type Erasure: One of the fundamental limitations of Java Generics is type erasure. This design choice ensures backward compatibility with older versions of Java but introduces several issues.
No Runtime Type Information: Because type information is erased at runtime, generic types cannot be used to obtain type-specific information. This limitation means you cannot directly check the type parameters of a generic instance at runtime using reflection.
Type Casting and Type Safety: Type erasure sometimes requires type casting, which can introduce ClassCastException at runtime if the type is not handled correctly.
What is the Complexity and Learning Curve?
Generics add a layer of complexity to the Java language, which can be challenging for developers to learn and use correctly:
Syntax Complexity: The syntax for generics can be complex and lengthy, especially when dealing with bounded type parameters, wildcards, and nested generic types. This complexity can make the code harder to read and understand, particularly for new developers.
Debugging Difficulty: Debugging code that uses generics can be more challenging. Since type information is erased at runtime, the error messages related to type issues can be less informative and harder to trace back to the source.
Advanced Features: Features like bounded wildcards (<? extends T> and <? super T>), generic methods and generic constructors can be difficult to master and correctly apply in practical scenarios.
What Restrictions and Limitations?
Generics come with several restrictions that can limit their flexibility and usability:
Cannot Use Primitive Types: Java Generics do not support primitive types directly. You have to use wrapper classes like Integer and Double instead of int and double, which can lead to additional boxing and unboxing overhead.
