CS计算机代考程序代写 flex Java COMP 250

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COMP 250
INTRODUCTION TO COMPUTER SCIENCE
Week 3-4: OOD3 Other methods, mutable vs immutable, final variables
Giulia Alberini, Fall 2020

OBJECTS – QUICK REVIEW
Book.java
public class Book { public String title; public String author;
}
TestBook.java
public class TestBook {
public static void main(String[] args) {
Book b = new Book(); b.title = “Matilda”; b.author = “Roald Dahl”; System.out.println(b);
} }

OBJECTS – QUICK REVIEW
public class Book { public String title; public String author;
}
b
title author
null
public class TestBook {
public static void main(String[] args) {
Book b = new Book();
b.title = “Matilda”;
b.author = “Roald Dahl”;
System.out.println(b);
} }
null

OBJECTS – QUICK REVIEW
public class Book { public String title; public String author;
}
“Matilda”
b
title author
public class TestBook {
public static void main(String[] args) {
Book b = new Book();
b.title = “Matilda”;
b.author = “Roald Dahl”;
System.out.println(b);
}
}
null

OBJECTS – QUICK REVIEW
public class Book { public String title; public String author;
}
“Matilda”
b
title author
public class TestBook {
public static void main(String[] args) {
Book b = new Book(); b.title = “Matilda”; b.author = “Roald Dahl”; System.out.println(b);
} }
“Roald Dahl”

OBJECTS – QUICK REVIEW
public class Book { public String title; public String author;
}
public class TestBook {
public static void main(String[] args) {
Book b = new Book();
b.title = “Matilda”;
b.author = “Roald Dahl”;
System.out.println(b);
} }
A
reference

WHAT ARE WE GOING TO DO IN THIS VIDEO?
OOD3
 Other methods
 Mutable vs Immutable final variables

OTHER METHODS

STEP 3
public class ClassName {
// some data declared here ;
}
Data
Method to create an object
public ClassName() { //constructor
}
// declare other methods
File name: ClassName.java
Other methods

OTHER METHODS
 You write all the other methods as before, except:
 If the method is static, it has access to the class variables.
 If the method is not static, it has access to both the class and the instance variables

RECOMMENDED EXERCISES
In the Patient class:
 add a method printName that prints out the name of the patient.
 add a method addTemp that takes as input a double and appends it at the end of the array of temperature of the patient. To do this, you need to create a new array whose length is greater, copy all the values over, append the double, and update the value of the corresponding field.

GET/SET METHODS

GETTERS AND SETTERS
 In general, all fields should be declared as private. We can then declare public method to regulate how they can be accessed.
 Those methods that give access to the value of a field are formally called “accessors”, but commonly referred to as getters.
 Those methods that allows you to modify the value of a field are formally called “mutators”, but commonly referred to as setters.

GET (ACCESSOR) METHOD
Most getters have a very similar format:
public getField() {
return this.field;
}

SET (MUTATOR) METHOD
Most setters have a very similar format:
public void setField( value) { this.field = value;
}

SET, GET, AND REFERENCES
Careful when writing get/set methods:
 If you are returning a reference type, be sure you are actually giving access to the thing you intend.
 If you are setting a value, be sure that you are using the thing you intend.

TRY IT!
 Modify the Patient class to include get and set methods. How should you do it?

ENCAPSULATION
 Process of wrapping data and the code acting on that data in one unit. The idea is to better control the data.
 What to do?
 Make all the fields private
 Provide getters and setters as needed.
 Note: through the methods we can do data validation, while we have little control over the data stored in a public field.

toString()

PRIVATE AND INFORMATION DISPLAY
 When your fields are all private, it might be difficult to display the content of an Object outside of the class.
 If we try to use println on the Object, we just see a reference. References are not very useful when trying to debug a code.

toString() AND println()
 Each time we use println() with objects, it calls the method toString()
on the object and displays the result.
 By default, toString() on an Object returns a String representing the address at which the contents of the Object can be found.
 This is not the case for all the Objects, though. If the method toString() is included inside the class, then when we use println() on variables of such type, not the default toString(), but the one from the class is used.

THE toString() METHOD
You can write a toString() method in any class.
It must have the following header:
public String toString(){
// returns a value of type String
}
If you do that, then when you call print()/println() on an instance of that class, this method is called automatically!

TRY IT!
In the Patient class, add a toString() method that returns a String in the following format:
Name: [name]
Age: [age]
Reported Temperatures: [list of temperatures]
Now, try to use print on an object of type Patient.

MUTABLE vs IMMUTABLE

IMMUTABLE REFERENCE TYPES
 If when you create a class, you make all the fields private, and you do not write any mutator methods, then you have effectively created an Immutable Type!
If the only way to assign values to fields is through the constructor, then the values of the Object cannot be changed after it has been created.

MUTABLE
Mutator methods change the content of the object.
Cat myCat = new Cat(“Sprizt”); myCat.setName(“Small Cat”);
The content of object reference by myCat has changed by changing the name of the cat.

MUTABLE
If we add a second Cat variable referencing to the same cat
Cat myCat = new Cat(“Sprizt”);
Cat aCat = myCat;
myCat.setName(“Small Cat”);
Then, also the content of aCat is changed after the last instruction.

IMMUTABLE
String s = “cats”;
s.charAt(0) = ‘r’; // compile-time error!
There is no method in String that allows us to set the value of a character.

IMMUTABLE
String s = “cats”; String t = s;
t = “dogs”;
The value of s does not change!

MUTABLE VS IMMUTABLE
 Mutable objects: more flexible in what they allow users to do with the object and more efficient because you don’t create a new object each time you want to modify the content. But, they can be error-prone.
 Immutable: easier to keep track of what changes when the contents of the object change. They can save you from long hours of debugging.
Immutable reference types behave like primitive data type.

CONSTRUCTORS AND MUTABLE REFERENCE TYPES
Remember the Patient class:
public class Patient{ private String name; private int age; private double[] temps;
public Patient(String n, int a, double[] t) { this.name = n;
this.age = a;
this.temps = t;
} }
Code like this can cause issues!

EXAMPLE
Patient.java TestPatient.java
public class Patient{ private String name; private int age; private double[] temps;
public Patient(String n, int a, double[] t){
this.name = n;
this.age = a;
this.temps = t; }
}
public class TestPatient{
public static void main(String[] args) {
double[] tempsForP= {37.8, 38.6, 40.0, 37.4, 36.5}; Patient p = new Patient(“John”, 42, tempsForP); tempsForP[0] = 0.0;
} }
The last statement in TestPatient.java will change the value of the field temps even though it is a private field!

GETTERS AND MUTABLE REFERENCE TYPES
Let’s add a get method for the field temps:
public class Patient{
private String name;
private int age;
private double[] temps;
public Patient(String n, int a, double[] t) { …
}
public double[] getTemps() {
return this.temps;
}
}
Also code like this can cause issues!

EXAMPLE
Patient.java TestPatient.java
public class Patient{ private String name; private int age; private double[] temps;
public Patient(String n, int a, double[] t){
…
}
public double[] getTemps() {
return this.temps;
}
}
public class TestPatient{
public static void main(String[] args) {
double[] tempsForP= {37.8, 38.6, 40.0, 37.4, 36.5};
Patient p = new Patient(“John”, 42, tempsForP);
double[] x = p.getTemps();
x[0] = 0.0;
} }
Similarly, the last statement in TestPatient.java will change the values of the field temps even though it is a private field!

GUIDELINE
 Don’t write a constructor initializing a mutable reference type without making a copy!
 Don’t add a get/set method to access/mutate a mutable reference type without making a copy!

Patient.java
TestPatient.java
EXAMPLE
public class TestPatient{
public static void main(String[] args) {
double[] tempsForP= {37.8, 38.6, 40.0, 37.4, 36.5};
Patient p = new Patient(“John”, 42, tempsForP);
double[] x = p.getTemps();
x[0] = 0.0;
} }
public class Patient{
private String name;
private int age;
private double[] temps;
public Patient(String n, int a, double[] t){
…
}
public double[] getTemps() {
// create a copy
int n = this.temps.length; double[] c = new double[n]; for(int i=0; i

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