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[英]I want to order my data from newest to oldest using Cloud Firestore in Android using Java
[英]In the queue I have implemented below the students are displayed from oldest to newest.I want to display inserted students from the newest to oldest
队列的驱动程序代码
import java.util.Scanner;
public class Driver {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("How many Students are there ? ");
int usercount = sc.nextInt();
Queue obj = new Queue(usercount);
boolean flag = true;
while(flag) {
System.out.println("Queue Operations");
System.out.println("====================");
System.out.println("1.Insert Students Name");
System.out.println("2.Remove First inserted student");
System.out.println("3.Display All Students");
System.out.println("4.Exit");
System.out.println("========================");
System.out.println("Enter Your Choice : ");
String userchice = sc.next();
if(userchice.equalsIgnoreCase("1")) {
for(int i=0; i<usercount; i++) {
System.out.println("Enter Student Name : ");
String sname = sc.next();
obj.Enqueue(sname);
}
}else if(userchice.equalsIgnoreCase("2")) {
obj.Dequeue();
}else if(userchice.equalsIgnoreCase("3")) {
obj.displayall();
}else if(userchice.equalsIgnoreCase("4")) {
System.out.println("End of Program");
flag = false;
}
}
}
}
队列的代码
public class Queue {
int front;
int rear;
String[] username;
int size;
public Queue(int usercount) {
this.size = usercount;
this.front = 0;
this.rear = -1;
this.username = new String[usercount];
}
public Boolean isFull() {
if( this.rear == this.size - 1 ) {
return true;
}else {
return false;
}
}
public Boolean isEmpty() {
if( (this.rear == -1) || (this.rear < this.front) ) {
return true;
}else {
return false;
}
}
public void Enqueue(String item) {
if(isFull()) {
System.out.println("Queue is Full.Cannot Insert");
}else {
this.rear++;
this.username[rear] = item;
System.out.println("Element " + item + " is inserted.");
}
}
public void Dequeue() {
if(isEmpty()) {
System.out.println("Queue is Empty.Cannot Delete");
}else {
String topelem = this.username[front];
this.username[front] = "";
System.out.println("Top Element " + topelem + " is removed.");
for(int i=0; i < rear ; i++) {
this.username[i] = this.username[i+1];
}
this.username[rear] = "";
this.rear--;
}
}
public void displayall() {
for(int i=0; i < this.size ; i++) {
System.out.println("Name = " + username[i]);
}
}
}
**假设我在 1 中的选择中输入 a,b,c 作为学生。当我输入选择为 3(显示所有学生)时,结果是
我想要的是结果
这不是硬件或任务。 这是我自己在练习的东西。我真的不知道如何开始。
**
只需尝试执行反向 for 循环。
for(int i = rear;i>=0;i--) {
System.out.print(this.username[i]);
}
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