Java Abstract Methods and Arrays

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

Which of the following identifiers is a constant according to Java naming conventions?

maxValue

Test

ReadInt

COUNT (correct)

What will be the result of adding an int, a byte, a long, and a double?

long

int

byte

double (correct)

What is the outcome of the expression (double)5/2?

3.0

2.0

2.5 (correct)

What is the reason for the compile error in the provided switch statement?

The switch value cannot be a floating-point number. Signup and view all the answers

What will be printed when the following code runs? int x = 0; if (x < 4) { x = x + 1; } System.out.println("x is " + x);

1 Signup and view all the answers

What is a valid output from the statement System.out.println((int)(Math.random() * 4))?

0 Signup and view all the answers

In the code 'int x; double d = 1.5; switch (d)', what type is the variable used for the switch?

double Signup and view all the answers

What result do you expect if you combine the statements 'int x = 0;' and 'if (x < 4) { x = x + 2; }'?

x is 2 Signup and view all the answers

What will be the final value of the variable 'balance' after the provided code executes?

1 Signup and view all the answers

What will the program output when executing the method xmethod with an integer parameter?

The program displays 'int' followed by 5. Signup and view all the answers

Which expression correctly generates a random integer value of 0 or 1?

(int)(Math.random() + 0.5) Signup and view all the answers

What is the term for a variable defined within a method in Java?

a local variable Signup and view all the answers

In method definitions, where do arguments always appear?

within parentheses Signup and view all the answers

What will the output of System.out.println(java.util.Arrays.toString(scores)) be given the array int[] scores = {1, 20, 30, 40, 50}?

{1, 20, 30, 40, 50} Signup and view all the answers

Which statement accurately describes a compiler error when trying to invoke the xmethod?

The compiler cannot distinguish which version of xmethod to invoke. Signup and view all the answers

Which of the following options correctly describes the function of a break statement within a loop?

It exits the loop immediately, skipping any remaining iterations. Signup and view all the answers

What modifier allows class members to be inaccessible to other classes in different packages but accessible to any subclasses?

protected Signup and view all the answers

Which statement about constructors is incorrect?

A constructor may be static. Signup and view all the answers

What will be the output of the following code? `java.util.ArrayList list = new java.util.ArrayList(); list.add("New York"); java.util.ArrayList list1 = (java.util.ArrayList)(list.clone()); list.add("Atlanta"); list1.add("Dallas"); System.out.println(list1);`

[New York, Dallas] Signup and view all the answers

If A is an interface and B is a concrete class implementing A, which of the following is a valid way to instantiate A?

A a = new B(); Signup and view all the answers

In Java, what does a constructor do?

It creates an instance of the class. Signup and view all the answers

Which option correctly describes a valid constructor capability?

A constructor can invoke a static method like any other method. Signup and view all the answers

What happens if a constructor does not invoke an overloaded or superclass constructor?

It defaults to invoking the superclass's no-arg constructor. Signup and view all the answers

Which of the following statements about class members is true?

Protected members can be accessed by subclasses outside their package. Signup and view all the answers

Which statement about abstract classes is true?

A class with abstract methods must itself be declared abstract. Signup and view all the answers

What is true about the dimensions of the array defined as double[][] x = new double[4][5];?

x.length is 4 and x[0].length is 5. Signup and view all the answers

What will happen when the following code is executed? public class Test { public static void main(String[] args) { int[][] values = {{3, 4, 5, 1}, {33, 6, 1, 2}}; for (int row = 0; row < values.length; row++) { java.util.Arrays.sort(values[row]); for (int column = 0; column < values[row].length; column++) System.out.print(values[row][column] + " "); System.out.println(); } }}

The program prints two sorted rows: 1 3 4 5 and 1 2 6 33. Signup and view all the answers

Given the array declaration int[][] a = new int[3][2];, what is the index of the first element?

a[0][0] Signup and view all the answers

Which of the following statements regarding abstract methods is correct?

An abstract method cannot exist in a non-abstract class. Signup and view all the answers

What is the potential output of the program if the following statement is modified? System.out.println(values[row][column] + " ");

The program will output the original array without any sorting. Signup and view all the answers

If the array double[][] b = new double[5][4];, what are the lengths of b and b[0]?

5 and 4 Signup and view all the answers

Which statement is false about abstract classes?

Abstract classes can be instantiated. Signup and view all the answers

Which syntax correctly creates an array of five empty Strings?

String[] a = {"", "", "", "", ""}; Signup and view all the answers

Do Program I and Program II produce the same output?

Yes, both reverse the array in the same manner. Signup and view all the answers

Which of the following statements about array initialization is true?

char[] c = new char[]{'a', 'b', 'c', 'd'}; Signup and view all the answers

What is the output of the following code snippet? 'System.out.println("n is " + n);'

The code prints n is 2. Signup and view all the answers

Identify the correct way to create an array of integers with the values 3, 4, 3, and 2.

int[] i = {3, 4, 3, 2}; Signup and view all the answers

What will happen if 'char[] c = new char;' is executed?

It throws a compilation error. Signup and view all the answers

How many elements are in the array initialized as 'int[] list = {1, 2, 3, 4, 5};'?

5 elements. Signup and view all the answers

What is the primary difference between Program I and Program II regarding their array handling?

They have different variable names for the arrays. Signup and view all the answers

Study Notes

Abstract Methods

A class that contains abstract methods must be abstract
An abstract class can have no abstract methods
An abstract method cannot be contained in a non-abstract class
Abstract classes can have constructors
Data fields cannot be declared abstract, only methods and classes can be declared abstract

Array Declarations

double[][] x = new double[4][5] uses two dimensions, x.length returns 4 and x[0].length returns 5

Array Sorting

java.util.Arrays.sort(values[row]) sorts the elements of each row in the array values

Array Indexing

The index variable for the element at the first row and first column in array a is a[0][0]

Doubles

(double)5/2 will result in a value of 2.5, the cast occurs before the division

Constants

Constants should be named following the Java naming convention using uppercase letters, separated by underscores
COUNT and MAX_VALUE are valid constant names based on the convention

Random Number Generation

Math.random() generates a random double between 0 and 1 (exclusive 1)
The code below will produce a random int between 0 and 3 (inclusive):
- (int)(Math.random() * 4)

Variables

Local variables are defined within a method
Arguments to methods are always defined within parentheses

Arrays with `Arrays.toString`

System.out.println(java.util.Arrays.toString(scores)) prints the contents of an array of the form: [1, 2, 3, 4, 5]

Array Creation

Arrays can be created using the curly brace notation: example: String[] a = {"", "", "", "", ""};

Method Overloading and Compile Errors

If multiple methods in a class share the same name but have different parameters (number or data types), the methods are overloaded
During compilation, Java can determine which method to call based on the arguments provided
If a method call does not match any overloaded method, there will be a compile-time error

Modifiers Access Levels

protected modifier for members of a class is accessible to subclasses in any package, but not to other classes in a different package

Constructors

Constructors cannot be defined as static
Constructors can be private, which means the class itself can create its own instances, but other classes cannot
Constructors can invoke static methods

ArrayList & Cloning

The clone() method creates a shallow copy of an ArrayList, meaning changes made to the original ArrayList will NOT be reflected in the copy

Inheriting from Interfaces

An interface is a blueprint that defines methods without implementations
A class that implements an interface must provide implementations of the interface methods
When creating instances of concrete classes that implement interfaces: A a = new B(), where A is the interface and B is the concrete class. This is valid because an instance of the concrete class is also an instance of the interface.

Example - Method Overloading and Compile Errors

If there are two overloaded xMethod implementations:
- public static int xMethod(n, t)
- public static long xMethod(n)
The code below would not compile because the compiler cannot determine which xMethod to call:
- System.out.println(xmethod(5)); since 5 could be a long or an int.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz focuses on key concepts in Java programming, including abstract methods, array declarations, sorting, and constants. Test your understanding of these foundational elements of Java programming and how they interact. Perfect for students learning Java in a structured course.