[JAVA] I wrote a design pattern in kotlin, Iterator edition
To learn the concept of Interface and the reusability of objects, which are important in object orientation ["Introduction to design patterns learned in Java language"](https://www.amazon.co.jp/%E5%A2%97% E8% A3% 9C% E6% 94% B9% E8% A8% 82% E7% 89% 88Java% E8% A8% 80% E8% AA% 9E% E3% 81% A7% E5% AD% A6% E3% 81% B6% E3% 83% 87% E3% 82% B6% E3% 82% A4% E3% 83% B3% E3% 83% 91% E3% 82% BF% E3% 83% BC% E3% 83% B3% E5% 85% A5% E9% 96% 80-% E7% B5% 90% E5% 9F% 8E-% E6% B5% A9 / dp / 4797327030 / ref = sr_1_1? __mk_ja_JP =% E3% 82% AB % E3% 82% BF% E3% 82% AB% E3% 83% 8A & keywords = java% E8% A8% 80% E8% AA% 9E% E3% 81% A7% E5% AD% A6% E3% 81% B6 % E3% 83% 87% E3% 82% B6% E3% 82% A4% E3% 83% B3% E3% 83% 91% E3% 82% BF% E3% 83% BC% E3% 83% B3% E5 I learned about% 85% A5% E9% 96% 80 & qid = 1559563427 & s = gateway & sr = 8-1) and decided to write in Java and then in kotlin. First, I will summarize Iterator.
- Also, thank you to @sdkei for reviewing this time in the comment section. I have reflected the revised content based on the review, so I will write it based on that point as well.
What is Iterator
It is a pattern that scans the elements of a set such as List in order from the beginning, which means "iterator" in Japanese.
Aggregate interface
The Agreagate interface has the role of an aggregate that can be counted. Interfaces are not inherited, they have a role to force them to implement methods.
Aggregate.java
interface Aggregate {
public abstract Iterator iterator();
}
Aggregate.kt
interface Aggregate {
fun iterator(): Iterator
}
Iterator interface
An interface that has methods for manipulating elements because it plays a role in counting elements. Here, the movement of the next method is important, and it is necessary to acquire the element and advance the index so that the next element can be acquired. Also, after receiving a review of this Iterator interface, it was said that the Object type in Java is equivalent to Any? In Kotlin. The difference between Any and Any? Is whether or not it allows nulls, and the API has been modified to allow nulls for various implementations.
Iterator.java
interface Iterator {
public abstract boolean hasNext();
public abstract Object next();
}
Iterator.kt
interface Iterator {
fun hasNext(): Boolean
fun next(): Any?
}
Member class
Create a class that will be retrieved as an element. With kotlin, you can get it with member.name without implementing getter / setter.
Member.java
class Member {
private String name;
public Member(String name) {
this.name = name;
}
public String getName() {
return name;
}
}
Member.kt
class Member(val name: String)
Staff class
Create a Staff class as an implementation of Aggregate, and make the Staff class a collection of Members. Implement the Iterator method defined in the interface.
Staff.java
class Staff implements Aggregate{
private Member[] members;
private int last;
public Staff(int maxsize) {
this.members = new Member[maxsize];
}
public Member getMemberAt(int index) {
return members[index];
}
public void appendMember(Member member) {
this.members[last] = member;
last++;
}
public int getLength() {
return last;
}
public Iterator iterator() {
return new StaffIterator(this);
}
}
Staff.kt
class Staff internal constructor(private val maxSize: Int): Aggregate {
private val members: Array<Member?> = arrayOfNulls(maxSize)
private var last: Int = 0
fun getMemberAt(index: Int): Member? = members[index]
fun appendMember(member: Member) {
members[this.last] = member
this.last++
}
fun getLength():Int = this.last
override fun iterator(): Iterator = StaffIterator(this)
}
StaffIterator class
Create the StaffIterator class as an implementation of Iteraltor and implement the methods so that Staff can be scanned. In the next method, the element is acquired and the index is advanced to the next, and it has the role of i ++ in the For syntax. Also, I allowed Null using the next method C Any? Of the Iterator interface, but I did not intend to return the member object as null, so I set Any, which is Non-Null, as the return value.
StaffIterator.java
class StaffIterator implements Iterator{
private Staff staff;
private int index;
public StaffIterator(Staff staff) {
this.staff = staff;
this.index = 0;
}
public boolean hasNext() {
if(index < staff.getLength()) {
return true;
}else {
return false;
}
}
public Object next() {
Member member = staff.getMemberAt(index);
index++;
return member;
}
}
StaffIterator.kt
class StaffIterator(private val staff: Staff): Iterator {
private var index: Int = 0
override fun hasNext(): Boolean = index < staff.getLength()
override fun next(): Any {
val member: Member = staff.getMemberAt(index) as Member
index++
return member
}
}
Main class
This is the Main class that actually operates the above class. The point here is that the implementation of Staff does not interfere with the while statement.
IteratorSample.java
public class IteratorSample {
public static void main(String[] args) {
Staff staff = new Staff(3);
staff.appendMember(new Member("sato"));
staff.appendMember(new Member("suzuki"));
staff.appendMember(new Member("saito"));
Iterator it = staff.iterator();
while(it.hasNext()) {
Member member = (Member)it.next();
System.out.println(member.getName());
}
}
}
IteratorSample.kt
fun main(args: Array<String>){
val staff = Staff(3).apply {
appendMember(Member("sato"))
appendMember(Member("suzuki"))
appendMember(Member("saito"))
}
val it: Iterator = staff.iterator()
while (it.hasNext()) {
val member: Member = it.next() as Member
println(member.name)
}
}
Execution result
sato
suzuki
saito
Class diagram
Impressions
- Since the processing of the while statement does not depend on the implementation of the subclass, I learned that even if there is a change in the implemented method, it does not affect the processing of the while statement.
- With kotlin, the amount of code is considerably reduced and it is easy to see
- For the first time, I learned that a subclass that inherits Iterator can be filled with objects in an instance of Interface.
- With kotlin, it seems that null and nullable are not handled yet, so it would be helpful if you could comment or point out.
Problem 1-1
Changed the array of Staff class to Vector. As a result of actually implementing it, I was able to confirm that the range of influence of the change was within the Staff class, and learned the importance of low dependency.
package practice.design_pattern_java;
import java.util.Vector;
interface Aggregate {
public abstract Iterator iterator();
}
interface Iterator {
public abstract boolean hasNext();
public abstract Object next();
}
class Member {
private String name;
public Member(String name) {
this.name = name;
}
public String getName() {
return name;
}
}
class StaffIterator implements Iterator{
private Staff staff;
private int index;
public StaffIterator(Staff staff) {
this.staff = staff;
this.index = 0;
}
public boolean hasNext() {
if(index < staff.getLength()) {
return true;
}else {
return false;
}
}
public Object next() {
Member member = staff.getMemberAt(index);
index++;
return member;
}
}
class Staff implements Aggregate{
private Vector<Member> members = new Vector<Member>();
private int last;
public Member getMemberAt(int index) {
return members.get(index);
}
public void appendMember(Member member) {
this.members.addElement(member);;
last++;
}
public int getLength() {
return last;
}
public Iterator iterator() {
return new StaffIterator(this);
}
}
public class IteratorVectorSample {
public static void main(String[] args) {
Staff staff = new Staff();
staff.appendMember(new Member("sato"));
staff.appendMember(new Member("suzuki"));
staff.appendMember(new Member("saito"));
Iterator it = staff.iterator();
while(it.hasNext()) {
Member member = (Member)it.next();
System.out.println(member.getName());
}
}
}
reference
It was very easy to read and understand by referring to the following.
Introduction to Kotlin for Java Programmers
For Kotlin's Any, I referred to the following. Where Java programmers tend to trip over Kotlin
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