test2 Flashcards
What is the Java Virtual Machine (JVM) and what role does it play?
The JVM is an abstract machine that executes Java bytecode. It provides platform independence, memory management (via garbage collection), security, and manages runtime processes.
What is the Java Runtime Environment (JRE) and how does it differ from the JVM and JDK?
The JRE contains the JVM and the core libraries required to run Java applications, whereas the JDK (Java Development Kit) includes the JRE along with development tools like the compiler and debugger.
What is the entry point of a Java program?
The entry point is the main method, defined as: public static void main(String[] args) (or with varargs: public static void main(String… args)).
What is a class in Java?
A class is a blueprint that defines the structure (fields) and behavior (methods) for objects. It encapsulates data and functions to operate on that data.
What is an object in Java?
An object is an instance of a class that occupies memory and has a state (via instance variables) and behavior (via methods).
What are the main components of a Java class?
The components include fields (instance and static variables), methods, constructors, nested classes/interfaces, and static initialization blocks.
What types of variables exist in Java?
There are instance variables (fields), local variables (declared within methods or blocks), and class (static) variables, which are shared among all instances.
What are the default values for uninitialized instance variables?
Numeric types default to 0 (or 0.0), boolean defaults to false, char defaults to \u0000 (null character), and object references default to null.
What are the eight primitive data types in Java?
byte, short, int, long, float, double, char, and boolean.
What are wrapper classes and why are they important?
Wrapper classes (e.g., Integer, Double) provide object representations for primitives. They allow primitives to be used in collections and provide utility methods for conversion and manipulation.
What are autoboxing and unboxing?
Autoboxing is the automatic conversion of a primitive to its corresponding wrapper object, while unboxing converts a wrapper object back to its primitive type.
What kinds of operators exist in Java?
Java operators include arithmetic (+, -, *, /, %), relational (==, !=, >, <, >=, <=), logical (&&,
How does integer division behave in Java?
Integer division truncates any fractional part. For example, 7 / 2 yields 3.
What is the difference between == and equals() when comparing objects?
== checks for reference equality (same memory location), while equals() typically checks for logical equality (same content), as defined by the class’s implementation.
How do you perform type casting in Java and what happens when you cast a double to an int?
Implicit casting occurs automatically when converting a smaller to a larger type; explicit casting is needed for incompatible types. Casting a double to an int truncates the decimal portion.
What are the primary control flow statements in Java?
Control flow statements include conditionals (if, if-else, switch), loops (for, while, do-while), and branch statements (break, continue, return).
How do if/else statements work in Java?
They evaluate a boolean condition and execute a block if true; an optional else block executes if false. Multiple conditions can be chained with else if.
How does a switch statement operate, and what types can be used?
A switch statement evaluates an expression and executes the matching case block. Since Java 7, switch statements can work with strings, as well as integral types and enums.
When is a for loop preferred and what is its syntax?
A for loop is preferred when the number of iterations is known. Syntax: for (initialization; condition; update) { /* code */ }
What is the difference between a for loop and a while loop?
A while loop checks its condition before executing the loop body, making it ideal when the iteration count is unknown.
What is unique about the do-while loop?
The do-while loop executes the loop body at least once before evaluating the condition.
How can an infinite loop occur and why might it be used?
An infinite loop (e.g., while(true)) never terminates. It can be used in server applications or event loops where the program must continuously run until explicitly stopped.
What are the purposes of the break and continue statements in loops?
break exits the loop immediately, while continue skips the remainder of the current iteration and proceeds with the next iteration.
What is a method in Java and why are methods important?
A method is a block of code that performs a specific task. Methods promote modularity, code reuse, and readability.
What constitutes a method signature?
The signature includes the method name and its parameter list (number, type, and order of parameters). The return type is not part of the signature.
What is method overloading and why is it used?
Method overloading allows multiple methods with the same name but different parameters within the same class. It increases flexibility and readability.
Can methods be overloaded solely by return type?
No. Overloaded methods must differ by parameter list; a difference in return type alone is not enough.
What is method overriding and how does it support polymorphism?
Overriding occurs when a subclass provides a new implementation for a method inherited from its superclass with the same signature. This enables runtime polymorphism, where the method called depends on the actual object type.
What is a constructor and what is its role in a Java class?
A constructor initializes a new instance of a class. It has the same name as the class and no return type.
What happens if no constructor is defined in a class?
The compiler automatically provides a default no-argument constructor that initializes the object with default values.
How can you overload constructors and why would you do it?
You can define multiple constructors with different parameter lists to allow various ways to initialize an object.
How is the this keyword used in constructors and methods?
this refers to the current object’s instance, and it is used to differentiate between instance variables and parameters or to call another constructor in the same class using this().
Is a private constructor valid, and when would you use one?
Yes, a private constructor is valid. It’s often used in the Singleton pattern or in utility classes to prevent external instantiation.
What is encapsulation and why is it important?
Encapsulation is the bundling of data (fields) and methods that operate on the data, while restricting direct access to some of the object’s components. It enhances security, maintainability, and modularity.
What is a package in Java and how does it benefit code organization?
A package groups related classes and interfaces, helping to avoid naming conflicts and making the code easier to manage and maintain.
What is the purpose of the import statement?
The import statement allows you to refer to classes from other packages without using their fully qualified names, simplifying the code.
What types of comments are available in Java and what is the purpose of Javadoc comments?
Java supports single-line (//), multi-line (/* … */), and documentation (/** … */) comments. Javadoc comments are used to generate API documentation and include tags such as @param, @return, and @throws.
How do you compile and run a Java program from the command line?
Compile with javac FileName.java to produce bytecode, then run with java FileName (without the .java extension).
What does the static keyword signify in a class?
static indicates that a field or method belongs to the class rather than any instance, meaning it is shared among all instances and can be accessed without creating an object.
How can you call a static method from a non-static context?
You call a static method using the class name (e.g., ClassName.methodName()), even within non-static methods.
What is garbage collection in Java and can it be forced?
Garbage collection is the automatic reclamation of memory occupied by objects that are no longer referenced. You can request garbage collection with System.gc(), but it is not guaranteed to run immediately.
What is inheritance and how does it work in Java?
Inheritance allows a class (subclass) to inherit fields and methods from another class (superclass), promoting code reuse and establishing a hierarchical relationship.
How does polymorphism work in Java?
Polymorphism allows a superclass reference to refer to a subclass object, enabling dynamic method invocation. This is primarily achieved through method overriding.
What is an interface and what members can it contain?
An interface defines a contract with abstract methods (and, since Java 8, default and static methods) and constant fields (implicitly public, static, and final).
What is an abstract class and when would you use one?
An abstract class cannot be instantiated and may contain abstract methods (without implementation) alongside concrete methods. It’s used when classes share common behavior but should not be instantiated on their own.
What is a static initialization block and when is it executed?
A static block runs once when the class is loaded, and is used to initialize static variables or execute startup code that needs to run before any instances are created.
What does declaring a field or method as final imply?
A final field can be assigned only once, ensuring its value remains constant after initialization. A final method cannot be overridden by subclasses.
What are the key characteristics of an immutable class and how can you design one?
An immutable class is declared final, all its fields are private and final, and it does not provide any setters. Defensive copies of mutable objects are returned if necessary, ensuring the state cannot change after construction.
Can method parameters be modified within a method and how does that affect the caller?
Primitive parameters are passed by value, so modifications do not affect the original variable. Object references are also passed by value; however, modifying the object’s internal state (via its methods) will affect the same object outside the method.
What is the difference between checked and unchecked exceptions?
Checked exceptions are enforced at compile time and must be either caught or declared in the method’s throws clause. Unchecked exceptions (subclasses of RuntimeException) are not required to be caught or declared.
What happens if an exception is not caught in a Java program?
If an exception is not caught, it propagates up the call stack. If it reaches the main method without being handled, the program terminates and prints a stack trace.
Can static methods be overridden? How do they behave differently from instance methods?
Static methods are not overridden; they are hidden if redefined in a subclass. The method invoked is determined by the reference type, not the object type, unlike instance methods which exhibit polymorphic behavior.
How does the ternary operator work and what is its syntax?
The ternary operator is a shorthand for an if-else statement. Syntax: condition ? valueIfTrue : valueIfFalse. It returns one of two values based on the evaluation of the condition.
What are bitwise operators used for and name a few.
Bitwise operators (e.g., &,
What is the entry point of a Java program?
The entry point is the main method with the signature public static void main(String[] args).
What is a class in Java?
A class is a blueprint for creating objects; it encapsulates data (fields) and behavior (methods) and defines how objects should be structured and interact.
What is the difference between instance variables and local variables?
Instance variables are declared in a class but outside any method and belong to an object, while local variables are declared inside methods or blocks and only exist during the execution of that block.
What are the default initialization values for instance variables in Java?
For object references, the default is null; for numeric types (e.g., int, double), it is 0 or 0.0; for boolean, it is false; and for char, it is the null character (\u0000).
What are the differences between the access modifiers: private, protected, public, and default (package-private)?
Private restricts access to the class itself; protected allows access within the same package and subclasses; public allows access from any class; if no modifier is specified (default), access is restricted to classes in the same package.
What is the purpose of the final keyword in Java?
final can be used to declare variables whose values cannot change, methods that cannot be overridden, or classes that cannot be subclassed.
What constitutes a method signature in Java?
A method signature consists of the method name and its parameter list (including types, order, and count). The return type and exceptions are not part of the signature.
How are arguments passed in Java—by value or by reference?
Java always passes arguments by value. For objects, the reference is passed by value, meaning the reference copy points to the same object.
What is the difference between static and instance methods/variables?
Static members belong to the class itself and can be accessed without creating an instance, while instance members belong to individual objects of the class.
What is the difference between a while loop and a do-while loop?
A while loop checks its condition before executing the loop body, whereas a do-while loop executes the body first and then checks the condition, guaranteeing at least one execution.
What are the purposes of the break and continue statements in loops?
break terminates the loop immediately, while continue skips the current iteration and proceeds with the next iteration of the loop.
How do nested loops work and what is their impact on time complexity?
Nested loops place one loop inside another. If the outer loop runs n times and the inner loop runs m times per iteration, the total iterations are n × m, often resulting in O(n·m) time complexity.
What are common pitfalls when using nested loops?
Pitfalls include increased time complexity leading to performance issues, incorrect loop bounds, and off-by-one errors when iterating through arrays or matrices.
Provide an example of using nested loops to print a multiplication table.
Example: java<br></br>for (int i = 1; i <= 10; i++) {<br></br> for (int j = 1; j <= 10; j++) {<br></br> System.out.printf(“%4d”, i * j);<br></br> }<br></br> System.out.println();<br></br>}<br></br>
What does Math.random() return?
It returns a double value in the range [0.0, 1.0), meaning it can be 0.0 but will always be less than 1.0.
How do you use Math.pow(x, y) and what does it do?
Math.pow(x, y) calculates x raised to the power of y, returning the result as a double.
What are the purposes of Math.floor() and Math.ceil()?
Math.floor(x) returns the largest integer less than or equal to x, while Math.ceil(x) returns the smallest integer greater than or equal to x.
How does the substring() method work in the String class?
substring(int beginIndex) returns a new string starting from the specified index, while substring(int beginIndex, int endIndex) returns a string from the start index up to, but not including, the end index.
What is the difference between using equals() and the == operator when comparing strings?
equals() compares the actual content (sequence of characters) of two strings, whereas == checks if the two references point to the same object in memory.
How do you generate a random integer between 0 (inclusive) and n (exclusive) using the Random class?
Instantiate a Random object and call nextInt(n), which returns a random integer in the range 0 (inclusive) to n (exclusive).
How can you seed a Random instance to produce predictable results?
By passing a seed value (long) to the constructor, e.g., new Random(12345L).
What is the purpose of StringBuffer and how does it differ from StringBuilder?
Both provide mutable sequences of characters; however, StringBuffer is synchronized (thread-safe) while StringBuilder is not, making the latter faster in single-threaded environments.
How do you convert a StringBuffer to a String?
Use the toString() method, for example: String s = myStringBuffer.toString();.
What is the difference between String, StringBuffer, and StringBuilder?
String objects are immutable (unchangeable), whereas both StringBuffer and StringBuilder are mutable. StringBuffer is thread-safe due to synchronization; StringBuilder is not, so it is generally faster in non-threaded scenarios.
How do you declare and initialize a one-dimensional array with values in Java?
You can declare and initialize it in one line using: int[] arr = {1, 2, 3, 4, 5}; or separately declare and then allocate memory with int[] arr = new int[5];.
How do you determine the length of an array and what is the significance of its length property?
Use the array.length property to obtain the number of elements. This property is final and cannot be modified once the array is created.
Are arrays considered objects in Java?
Yes, arrays are objects in Java and are created on the heap.
How do you declare a two-dimensional array and what are the common ways to initialize it?
You can declare a 2D array with int[][] matrix = new int[rows][columns]; or initialize it using initializer lists, e.g., int[][] matrix = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} };.
What is a jagged array and how is it different from a rectangular two-dimensional array?
A jagged array is an array of arrays where each inner array can have different lengths, unlike a rectangular array where every row has the same number of columns.
How do you iterate safely over a jagged array?
Use a nested loop where the outer loop iterates over the rows and the inner loop uses matrix[i].length to determine the number of columns for that specific row.
What exception is thrown when trying to access an array index out-of-bounds?
An ArrayIndexOutOfBoundsException is thrown if you try to access an invalid index (less than 0 or greater than or equal to the array’s length).
What is encapsulation and how is it implemented in Java classes?
Encapsulation is the bundling of data and methods that operate on that data within a class, and restricting direct access to some of the object’s components using private fields and public getters/setters.
What is method overloading and how does it differ from method overriding?
Overloading is defining multiple methods with the same name in the same class but with different parameter lists. Overriding occurs when a subclass provides its own implementation of a method already defined in its superclass, using the same signature.
What is the purpose of a constructor in a Java class?
A constructor initializes a new object instance of the class, setting up initial values and states. It can be overloaded, and its access level controls how instances of the class are created.
How does the this keyword function in a constructor?
The this keyword is used to refer to the current object’s fields or methods, commonly to distinguish between instance variables and parameters with the same name in a constructor or method.
Can a class have multiple constructors? What is this concept called?
Yes, a class can have multiple constructors with different parameter lists. This is known as constructor overloading.
When and why would you use a private constructor?
A private constructor is used to restrict object creation from outside the class. It is often used in Singleton patterns or in utility classes where instantiation is not required.
Can a class be declared as final and what does that imply?
Yes, declaring a class as final means it cannot be subclassed. This is used for security and to maintain immutability in certain designs.
What is the purpose of a static block in a class?
A static block is used to initialize static variables or perform setup operations that need to occur once when the class is loaded, before any instances are created.
How can you implement the Singleton pattern in Java?
Implement a Singleton by using a private constructor, a private static instance variable, and a public static method (or static block) that returns the single instance. This ensures that only one instance of the class is ever created.
How do you override the toString() method and why is it useful?
Override toString() to provide a meaningful string representation of an object, which is useful for debugging and logging. For example, inside your class, define: java<br></br>@Override<br></br>public String toString() {<br></br> return “MyClass details…”;<br></br>}<br></br>
What is the significance of getter and setter methods in a class?
Getters and setters provide controlled access to private fields, allowing validation or additional processing when getting or setting a value. They are a key aspect of encapsulation.
