Object Oriented Programming (SWE211) Quiz

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Questions and Answers

Suppose the xMethod() is invoked from a main method in a class as follows:

public static void main(String[] args) {
    xMethod();
}

xMethod() is ______ in the class.

a static method (correct)

an instance method

None of them

a static method or an instance method

If a variable is declared `static` in a particular class, what does that mean?

It can only be changed by the instances of the class in which it is declared

Each instance of the class has its own copy of the variable

There is only one copy of it that all instances of the class can access or share (correct)

It cannot be changed

Declare and construct an `ArrayList` with an initial capacity of 20 references to `Object`.

`ArrayList list[20] = new ArrayList();`

`ArrayList<Object> list = new ArrayList<Object>(20);` (correct)

`Object list(20) = new ArrayList();`

`ArrayList[Object] list = new ArrayList(20);`

Which of these cannot be declared `static`?

<code>Object</code> (D) Signup and view all the answers

Which of the following statements are true?

i. A subclass is a subset of a superclass. ii. A subclass is usually extended to contain more functions and more detailed information than its superclass. iii. "`class A extends B`" means A is a subclass of B. iv. "`class A extends B`" means B is a subclass of A.

i, ii and iii only (D) Signup and view all the answers

Given a class named `Student`, which of the following is a valid constructor declaration for the class?

<code>Student (Student s) { }</code> (C) Signup and view all the answers

Which option can we use in the try block to stop the execution of the "finally" block?

try {
    // option
} finally {
    // code
}

<code>No correct answer</code> (A) Signup and view all the answers

Suppose the xMethod() is invoked from a main method in a class as follows: `public static void main(String[] args) { xMethod(); }` xMethod() is ____ in the class.

a static method (C) Signup and view all the answers

Which option can we use in the try block to stop the execution of the "finally" block?

No correct answer (A) Signup and view all the answers

What is printed as a result of executing the following code segment? `ArrayList p = new ArrayList(); p.add(7); p.add(2); p.add(2); p.remove(2); p.add(1); p.add(2); System.out.println(p);`

[7, 2, 1] (C) Signup and view all the answers

What is printed as a result of executing the following code segment? `ArrayList aList = new ArrayList<Integer>(); alist.add(1); alist.add(2); alist.remove(1); alist.add(1, 3); alist.set(1, 4); alist.add(5); System.out.println(aList);`

[1, 4, 3, 5] (D) Signup and view all the answers

Examine the following code: `ArrayList<String> list = new ArrayList<String>(); list.add("Andy" ); list.add("Bart" ); list.add("Carl" ); list.add("Doug"); list.add("Elmo" );` Which of the following will change the list so that it looks like: `Andy Bart Carl Doug`

list.remove(list.size()-1); (B) Signup and view all the answers

What is the output of the following java code?

`import java.util.ArrayList; class TestList { public static void main(String[] args) { ArrayList<String> al = new ArrayList<String>(); al.add("A"); al.add("B"); al.add("A"); al.add(null); System.out.println(al); } }`

[A,B,A,null] (A) Signup and view all the answers

Examine the following code: `public class Main { public static void main(String[] args) { List<Integer> mylist = new ArrayList(); System.out.println(mylist.add(5)); } }` What is the output of this java code?

5 (A) Signup and view all the answers

Analyze the following code. `public class Test { public static void main(String args[]) { NClass nc = new NClass(); nc.t = nc.t++; } } class NClass{ int t; private NClass() { } }`

The program has a compile error because the NClass class has a private constructor. (B) Signup and view all the answers

Analyze the following code. `double[] array = {1, 2, 3}; ArrayList<Double> list = new ArrayList<>(Arrays.asList(array)); System.out.println(list);`

The code is correct and displays [1.0, 2.0, 3.0]. (B) Signup and view all the answers

Analyze the following code.

`public class Test { public static void main(String[] args) { B b = new B(); b.m(5); System.out.println("i is " + b.i); } } class A { int i; public void m(int i) { this.i = i; } } class B extends A { public void m(String s) { } }`

The method m is not overridden in B. B inherits the method m from A and defines an overloaded method m in B. (D) Signup and view all the answers

Every JavaFX main class _____.

implements javafx.application.Application (B) Signup and view all the answers

Which of the following statements is true?

A Node can be placed in a Pane. (A) Signup and view all the answers

To place a node in the left of a BorderPane p, use _____.

p.setLeft(node); (A) Signup and view all the answers

Which of the following methods is NOT defined in the Animation class?

resume() (A) Signup and view all the answers

To remove a node from the pane, use _____.

pane.getChildren().remove(node); (B) Signup and view all the answers

Which of the following statements is correct to create a DataOutputStream to write to a file named out.dat?

DataOutputStream outfile = new DataOutputStream(new FileOutputStream("out.dat")); (B) Signup and view all the answers

After the following program is finished, how many bytes are written to the file t.dat? `import java.io.*; public class Test { public static void main(String[] args) throws IOException { DataOutputStream output = new DataOutputStream(new FileOutputStream("t.dat")); output.writeShort(1234); output.writeShort(5678); output.close(); } }`

8 bytes. (B) Signup and view all the answers

Interfaces can be instantiated.

False (B) Signup and view all the answers

The modifiers public and static can be written in either order “public static” or “static public”.

True (A) Signup and view all the answers

Abstraction is supported by method overriding in java.

False (B) Signup and view all the answers

You can override a static method defined in a superclass.

False (B) Signup and view all the answers

You can write a constructor for an abstract class.

True (A) Signup and view all the answers

If AbsClass is an abstract class, the expression x instanceof AbsClass is always false.

False (B) Signup and view all the answers

Classes, methods, and instance variables can all be declared final.

True (A) Signup and view all the answers

What does the `instanceof` operator evaluate to, if the left-hand-side object is a child of the right-hand-side class or is the same type as the class provided?

true Signup and view all the answers

Does the `instanceof` operator work with interfaces?

yes Signup and view all the answers

What error does the program give if `instanceof` operation is applied with a type that is always false?

compile time error Signup and view all the answers

A null reference is an instance of any class.

False (B) Signup and view all the answers

What is the common use case of `instanceof` operation?

to check the actual type of an object at runtime and perform different actions based on its type Signup and view all the answers

How do you count the number of lines in a text file?

Read the file line by line using a <code>Scanner</code> and increment a counter for each line. Signup and view all the answers

How do you get the file name as user input?

Prompt the user to enter the file name using <code>System.out.print</code> and store the input using a <code>Scanner</code>. Signup and view all the answers

Write a java method that takes an array of different elements' type and returns an array of only the ints from the array.

<pre><code class="language-java">import java.util.ArrayList; import java.util.Arrays; public class JavaTest { public static int[] integers(Object[] in) { ArrayList<Integer> ints=new ArrayList<>(); for(Object o: in) if(o instanceof Integer) ints.add((Integer)o); int[] out = new int[ints.size()]; for(int i=0; i<ints.size(); i++) out[i] = ints.get(i); return out; } public static void main(String[] args) { Object[] a = {89,"hello",12,0.23,"java"}; int[] ints = integers(a); System.out.println(Arrays.toString(ints)); } } </code></pre> Signup and view all the answers

Write a Java program that simulates a stock account with deposit and withdrawal operations. Use two threads: one for performing deposits and another for withdrawals. Ensure thread safety without using notify() or wait() methods.

<pre><code class="language-java">class Stock { private int balance = 0; public synchronized void deposit(int amount) { balance += amount; System.out.println(Thread.currentThread().getName() + " deposited: " + amount + ", Balance: " + balance); } public synchronized void withdraw(int amount) { if (balance >= amount) { balance -= amount; System.out.println(Thread.currentThread().getName() + " withdrew: " + amount + ", Balance: " + balance); } else { System.out.println(Thread.currentThread().getName() + " insufficient balance to withdraw: " + amount); } } } class DepositThread extends Thread { private Stock stock; public DepositThread(Stock stock) { this.stock = stock; } @Override public void run() { for (int i = 1; i <= 5; i++) { stock.deposit(i * 10); // Deposit multiples of 10 try { Thread.sleep(100); } catch (InterruptedException e) { Thread.currentThread().interrupt(); System.out.println("Thread interrupted: " + e.getMessage()); } } } } class WithdrawThread extends Thread { private Stock stock; public WithdrawThread(Stock stock) { this.stock = stock; } @Override public void run() { for (int i = 1; i <= 5; i++) { stock.withdraw(i * 15); // Withdraw multiples of 15 try { Thread.sleep(150); } catch (InterruptedException e) { Thread.currentThread().interrupt(); System.out.println("Thread interrupted: " + e.getMessage()); } } } } public class StockExample { public static void main(String[] args) { Stock stock = new Stock(); DepositThread depositThread = new DepositThread(stock); WithdrawThread withdrawThread = new WithdrawThread(stock); depositThread.setName("DepositThread"); withdrawThread.setName("WithdrawThread"); depositThread.start(); withdrawThread.start(); } } </code></pre> Signup and view all the answers

Flashcards

What is a static method?

A method that can be called without creating an instance of the class. It belongs to the class itself.

What is an instance method?

A method that needs an object of the class to be called. It operates on the object's data.

What does it mean for a variable to be declared static?

A static variable is shared by all instances of a class. Changes to it affect every object.

How do you declare and construct an ArrayList with an initial capacity?

An ArrayList is a dynamic list that can grow or shrink as needed. You can specify an initial capacity to reduce resizing overhead.