Chapter 3 - In person Class

Question 1

What does scan.nextline(); do? (differently than scan.nextInt).

Answer 1

It gobbles up the enter key you used for the inputted numbers.

Question 2

How do you make sure your scan can take a string as an integer.

Answer 2

val1 = Integer.parseInt(scan.nextLine());

Question 3

What is a null pointer?

Answer 3

It’s when the String title; has not been initialized yet. It points to nothing and we call it “null pointer”.

Question 4

How do you create an object?

Answer 4

title = new String (“Java Software Solutions”);

Question 5

What is the constructor?

Answer 5

The constructor is a special method that sets up the object (the name must match).

It is a special method that has the same name as the class.

Question 6

How do you invoke a method?

Answer 6

count = title.length();

Question 7

How do you access the methods of the scanner object scan?

Answer 7

You type a period after the name i.e.
scan.

String s; -> declares a variable s.
s = new String (“Hello World”); - -> assigns the string value.

We can do both by combining step 1 and 2.

String s = new String (“Hello World”);

s. (—–methods…
- —–method 2
- —–method 3
- —–method 4…)

Question 8

What happens to the memory space and object that doesn’t have an ARROW pointing to it?

Answer 8

That object becomes an ORPHAN, and it needs to be taken out like GARBAGE.

Have controls/documentation in place to stop this from happening. Garbage will not modify your code.

Question 9

String is the only object that allows you to create WITHOUT using NEW. (primitive types are not objects).

Answer 9

title = “Star Wars”;

Question 10

What does it mean when we say a string is immutable?

Answer 10

A string is immutable because you cannot change it once it’s created. BUT you can create new string from it.

Question 11

How do we replace the object with a new string?

Answer 11

String s = new String("Hello World");
s = s.replace( 'l' , 'p' );

Question 12

What do we assume about the following initialization?

String phrase = “Change is Inevitable”;

Answer 12

String phrase = new String(“Change is Inevitable”);

For Strings Java allows us to exclude new String();

Question 13

Where is the method in this code?

String phrase = “Change is Inevitable”;
String mutation 1, mutation2, mutation3, mutation4;
mutation1 = phrase.concat (“,except from vending machines.”);

Answer 13

Right after the phase, where we say .concat();

.Concat is one of the many methods of the String Class.

Question 14

Do the methods belong to the String Class or String Object?

Answer 14

The methods come from the String Object.

Question 15

How can we compare two strings to see if they are equal?

Can we use if ( a1 == a2); ?

Answer 15

we use if (a1.equal(a2));

The == sign can only be used to compare two PRIMITIVE types. Everything else needs a method.

Question 16

Is it better to use

import java.util.*; or the specific package class name?
import.java.util.Scanner;

Answer 16

It depends on if you’re going to use more than 3 classes. If it’s 3 or less then you should name each for documentation purposes. This will be helpful in case java makes an update to the util package and you need to know if you need to change any classes.

For GUI packages we often use * because we are often using more than 3 classes.

Question 17

What does it mean when the Math class has static methods?

Answer 17

Static methods can be invoked through the class name without using an object.

Question 18

There is no way to exponentiate in java, then how do we calculate 3^2?

Answer 18

We use the Math class that is a static class. So we would say Math.pow(3,2);

.pow stands for power.

Question 19

How can you create your own type?

Answer 19

You can create your own type by using ENUM.

enum Season (winter, spring, summer, fall);

Each Value has an ordinal value in addition to the actual value. There are methods for going back and forth between them.

time = Season.fall;

Question 20

Enumarated variables are a list of data types created outside the main method.

Answer 20

enum Flavor {vanilla. chocolate}

Flavor cone1, cone2, cone3;

cone1 = Flavor.rockyRoad
cone2 = Flavor.butterPecan

Flavor cone1, cone2, cone3;

Question 21

What are the methods of enum class?

Answer 21

It is cone1.ordinal() and cone2.name();

We are creating an enumarated data type called Flavor.

Question 22

What does the DecimalFormat df = new DecimalFormat (“0.0#”) do? Why can’t we have 0s after #s?

Answer 22

the second zero requires a zero to be displayed even if there is not number. The # sign requires a display only if the is a non-zero number.

Question 23

What are wrapper classes?

Answer 23

A wrapper class is not primitive data BUT AN OBJECT. It wraps an object around a primitive data so it’s STORED using a REFERENCE.

Its so you can treat everything can b treated in the same way IF THEY ARE ALL OBJECTS (consistency and continuinity).

z = x;
wrapper and int.
z = new Integer(x);

Question 24

HW help

Answer 24

public static void main(String[]args)

Integer.parseInt(null); => extracts an integer value from a string.

Question 25

Where is the integer methods for chapter 3.

Answer 25

It may be used in quiz.

Question 26

What is a conditional operator?

Answer 26

It’s an operator and not a statement. It has three operands.

? :
(t/f test ? true value : false value);
(pounds>1 ? “pounds” : “pound”);