Java language basics

Question 1

Data types specify….

Answer 1

specify the kind of thing a value represents

Question 2

Variables are….

Answer 2

used to store values for later use. must have a specified data type

Question 3

Arrays are….

Answer 3

used to store multiple values in a single variable which all must be the same data type

Question 4

Conditional Statements allow…..

Answer 4

Allow for branching and looping in Java

Question 5

entry point for the program to begin

Answer 5

main method - every java program must have one

Question 6

main method syntax

Answer 6

public static void main (String[] args) {
//code goes here
}

Question 7

Class names should be _________ case?

Answer 7

PascalCase

Question 8

One of the Class names much match the?

Answer 8

filename.java

Question 9

Sequence Diagrams show…..

Answer 9

show how application components or separate applications interact with each other and in the order of interactions.

Question 10

Sequence Diagrams are structured…..

Answer 10

are structured to represent a timeline beginning at the top and descending to depict the sequence of interactions.

Question 11

Class diagrams are used…..

Answer 11

are used to describe classes and their relationships

Question 12

Class diagrams show….

Answer 12

the names and attributes of the classes, relationships between the classes, and sometimes also the methods of the classes.

Question 13

What do the plus and minus signs indicate in this class diagram

Answer 13

+ before the attribute name means the attribute is public
- means the attribute is private so it is not visible to outside sources

Question 14

What do the words following the colons represent

Answer 14

In a class diagram this item when following attributes, it’s the data type for the attribute. When following methods, it’s the return type for the methods.

Question 15

What is the class name?

Answer 15

Person

Question 16

What data type does setName return?

Answer 16

None the void means it doesn’t return anything

Question 17

What are the items below the second line?

Answer 17

This row of the diagram is a section containing the Constructors and Methods. Constructors are listed first and then methods.

Question 18

What are the items at the top of this sequence diagram

Answer 18

the customer object who is outside our diagram and the theater and server objects inside our diagram

Question 19

What is the vertical dashed line?

Answer 19

A lifeline that represents the different objects or parts that interact over time.

Question 20

What are the solid horizontal lines?

Answer 20

Messages being sent between objects

Question 21

What are the dashed horizontal lines?

Answer 21

Direct response messages that are the return for a message that was sent.

Question 22

What is the vertical rectangles/activation boxes in some sequence diagrams?

Answer 22

The Activation box is present under the object next to the code for each process which the object is involved.

Question 23

what is casting?

Answer 23

The conversion of one data type to another

Question 24

what is the memory size order from smallest to biggest of these data types:
int
float
byte
double
long
short

Answer 24

byte < short < int < long < float < double

Question 25

what is the syntax for casting a larger data type (float) into a smaller(int) (Narrowing Casting)

Answer 25

float floatVar = 45.67
int intVar = int(floatVar)

Question 26

What is the size and range of values of a byte?

Answer 26

byte | 8-bit | -128 to 127

Question 27

What is the size and range of values of a short?

Answer 27

short | 16-bit | -32,768 to 32,767

Question 28

What is the size and range of values of a int?

Answer 28

int | 16-bit | -2 billion to 2 billion

Question 29

What is the size and range of values of a long?

Answer 29

long | 32-bit| -9 quintillion to 9 quintillion

Question 30

What is the size and range of values of a float?

Answer 30

float | 32-bit | 6 decimal digits

Question 31

What is the size and range of values of a double?

Answer 31

double | 64-bit | 15 decimal digits

Question 32

What is the size and range of values of a char?

Answer 32

char | 16-bit |single character

Question 33

What is the size and range of values of a boolean?

Answer 33

boolean | 1-bit | true or false

Question 34

What is a wrapper class?

Answer 34

A corresponding class to the primitive data types that allow us to define the primitive data type using that class instead.

Question 35

what is the Java.lang package?

Answer 35

A package that loads automatically with Java.

Question 36

What are the hundreds of included classes in the java language called?

Answer 36

Packages

Question 37

What separator do java package paths use?

Answer 37

Unlike folders, Java package paths use dots (.) as separators instead of slashes (\ or /).

Question 38

What java package is imported by default and contains built-in classes like String?

Answer 38

Java.lang (lang is short for language)

Question 39

When you want to use a class that’s part of a package what the common way?

Answer 39

The common way is to add an import statement to your class file where the other class will be used. This allows you to only use the class name in your code versus the full package name.

Question 40

What will this line of code do? import java.util.*;

Answer 40

It will import all of the classes in the java.util package for use.

Question 41

Why is code line a) preferred over b) when we only need the Scanner class?
a) import java.util.Scanner;
b) import java.util.*;

Answer 41

Importing only what you need will lead to improved maintainability, the readability of your code, and you won’t have problems with ambiguous class names (2 classes with the same name in 2 different packages).

Question 42

Where do the lines importing packages/classes go in your code?

Answer 42

At the very top.

Question 43

What is the difference between ++i and i++?

Answer 43

++i increases the value of i by 1 before executing the code on the line.
i++ increases the value of i by 1 after executing the code on the line.

Question 44

What do the dots represent when referring to packages?

Answer 44

The dots represent subdirectories.

Question 45

Why should we list the specific class to import and not use the star import (*)?

Answer 45

Better readability
Reduced errors

Question 46

What is a jar file?

Answer 46

A jar file is like a zip file that contains complied code. Java can search inside jar files and use the classes included, without expanding the archive on the disk.

Question 47

What is reference equality?

Answer 47

When two references point to the same object.

Question 48

What is the difference between equality and equivalence when comparing objects?

Answer 48

Equality check is the objects point to the same memory location. Equivalence checks to see if the object have the same variables and data.

Question 49

what is the syntax to compare stringOne and stringTwo?

Answer 49

stringOne.equals(stringTwo);

Question 50

What is the syntax to check if one object is equal to another?

Answer 50

objectOne == objectTwo
This only returns true if both of the objects are assigned to the same object.

Question 51

What is a simple explanation of a hashCode?

Answer 51

It is an integer used to quickly search for objects on Java.

Question 52

When you override the equals method you also need to override the……..

Answer 52

When you override the equals method, you also need to override the hashCode method.

Question 53

What does overriding the “equals” method do?

Answer 53

I allows us to establish equivalence between two objects. That way the “equals” method will return true even if == returns false, meaning you are dealing with two separate, but equivalent, objects.

Question 54

What are the six steps to overriding .equals() for the class City?

Answer 54

Check for self-reference: Check if the object being compared is the same instance as this object. If it is, return true, since an object is always equal to itself.

Check for null: Begin by checking if the object being compared is null. If it is, return false, since a null object cannot be equal to any other object.

Check for class compatibility: Check if the object being compared is an instance of the same class as this object. If it is not, return false, since objects of different classes cannot be equal.

Cast the object: If the object being compared is an instance of the same class, cast it to the appropriate type.

Compare object fields: Compare the fields of the object being compared to the fields of this object. Use the appropriate comparison method for each field type (for example, == for primitive types, and equals() for objects).

Return the result: Return true if all fields are equal, or false otherwise.

Question 55

Use this card to memorize the syntax for overriding the equals method in a class

Answer 55

@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null) return false;
if (getClass() != o.getClass()) return false;
Person person = (Person) o;
return age == person.age && name.equals(person.name);

Question 56

Create the equals method that does the following:
1) Take in an Object class argument and call it spare.
2) Check if the current object and the spare object are the same object by returning true if the current object (this) is equal to (using ==) as other.
3) Check if the spare object is null by returning false if other is null.
4) Check if the spare object is of the same class as the current object by returning false if getClass() is not equal to (using !=) to spare.getClass();.
5) Check if the spare object’s fullName and trueAge attributes are the same as the current objects by:
First casting the spare object into a Person class.
Comparing if the current objects fullName and trueAge are equal to the cast object’s fullName and trueAge.

Answer 56

@Override
public boolean equals(Object spare) {
if (this == spare) return true;
if (spare == null) return false;
if (getClass() != spare.getClass()) return false;
Person sparePerson = (Person) spare ;
return fullName.equals(sparePerson.fullName) && trueAge == sparePerson.trueAge);

OR
@Override
public boolean equals(Object spare) {
if (this == spare) return true;
if ((spare == null) || (getClass() != spare.getClass()))
return false;
Person sparePerson = (Person) spare ;
return fullName.equals(sparePerson.fullName) && trueAge == sparePerson.trueAge);

Question 57

a and b are two objects, and a.equals(b) returns true. Also, a.hashCode() returns 1234, what will be returned by b.hashCode()? Assume that both implement equals and hashCode methods correctly.

Answer 57

1234 – Equal objects have equal hash codes. Since a and b are equal, and the hash code of a is 1234, the hash code of b will also be 1234.

Question 58

What will happen when you compile and run the following code?
String s1 = “Manish”;
String s2 = s1;
s1 = null;
System.out.println(s2.equals(s1));

Answer 58

false – Whenever null is passed as an argument to the equals method which is invoked on a non-null reference; it always returns false. In this code, s2 refers to the String “Manish”, and s1 becomes null. Hence the call s2.equals(s1) returns false.

Question 59

What will be printed when the following code is compiled is run? Type only the output that will printed in correct case without any quotes or spaces.
Object i = new Integer(7);
Object f = new Float(7);
Object d = new Double(7);
System.out.println(i.equals(f) == d.equals(f));

Answer 59

true – All the wrapper classes return false if the argument of the equals method is of different type. i.e. In the case of all wrapper classes, a.equals(b) will return false if a and b are objects of different classes, irrespective of the value they represent. Here, the calls i.equals(f) and d.equals(f) both return false, and comparing two false with an equality operator (==) prints true.

Question 60

What is the code for importing Java’s built in hashCode method?

Answer 60

import java.util.Objects;

Question 61

What is the code to use the variables name and age to override the hashCode using Objects.hash?

Answer 61

public int hashCode() {
return Objects.hash(name, age);
}

Question 62

What’s the first if statement code inside the method: public boolean equals(Object o) {
}

Answer 62

if (o == this) {
return true;
}

Question 63

What’s the 2nd if statement code inside the method: public boolean equals(Object o) {
}

Answer 63

if (o == null) {
return false;
}

Question 64

What’s the 3rd if statement code inside the method: public boolean equals(Object o) {
} (note the class is Person)

Answer 64

if (!(o instanceof Person)) {
return false;
}

Question 65

What’s the last statement code inside the method: public boolean equals(Object o) {
} (note the class is Person with the variables name and age)

Answer 65

Person other = (Person) o;
return this.name.equals(other.name) && this.age == other.age;
}

Question 66

What does the .equals() method compare by default?

Answer 66

By default, the equals method in Java compares the memory addresses of two objects to determine if they are equal. If two objects are of different classes, they will have different memory addresses and therefore equals will return false.

Question 67

What’s wrong with this initialization: float myfloat = 3.14;

Answer 67

An f needs to be added: float myfloat = 3.14f;
When you initialize a float variable with a floating-point literal, the compiler needs to know whether the value should be interpreted as a float or a double. If not the line with cause an error.

Question 68

What’s the default value of a boolean?

Answer 68

false

Question 69

What’s the default value of a char?

Answer 69

‘\u0000’ (the unicode representation of the ASCII code for null)

Question 70

What’s the default value of a String?

Answer 70

null

Question 71

What are the naming restrictions on variable names?

Answer 71

Cannot contain whitespace
Cannot start with a digit
Cannot be a Java reserved word (Ex. data type names)
Needs to be unique within its scope (ex: only one declared with this name in a class or method)

Question 72

With Arrays if you don’t initialize it at the same time as the declaration what must the declaration statement contain?

Answer 72

new :
int[] numbers;
numbers = new int[5];
or
int[] numbers;
numbers = new int[]{1, 5, 6, 7};

Question 73

What is a use case?

Answer 73

A use case in programming refers to a description of how a user or an external system interacts with a software system to achieve a particular goal or task. It is a technique used in software development to identify, analyze, and document the requirements of a software system from a user’s perspective.

Question 74

What is composition?

Answer 74

Composition is a fundamental concept in object-oriented programming (OOP) where a class is made up of one or more objects of other classes. It allows creating complex types by combining simpler types, and it is used to build more complex objects by combining simpler objects.

In composition, the objects that are part of a class are called its “components” or “parts”, and they are typically stored as private member variables of the containing class. The containing class can then use the methods of its components to provide its own functionality. This allows for code reuse, as the same components can be used in different classes to provide similar functionality.

Question 75

does the java compiler require each public class to be it’s own file or in a shared file?

Answer 75

In Java, each public class must be defined in its own file with the name of the file matching the name of the class. However, you can have multiple non-public classes defined in the same file as the public class.

Question 76

The Java compiler requires that if a class is in a package, and if it uses any import statements, they must be in the order of:

Answer 76

Package declaration (if the class belongs to a package)
Import statements
Class declaration

Question 77

will .* import all the packages and child packages?

Answer 77

No, .* does not include the child packages

Question 78

What’s the method signature used to override the .equals method?

Answer 78

public boolean equals(Object o) {
}