Cách sử dụng size() trong PriorityBlockingQueue trên Android

Giới thiệu về size() trong PriorityBlockingQueue

size() trả về số phần tử hiện có trong queue. Khác với ConcurrentLinkedQueue, PriorityBlockingQueue.size() là O(1) và thread-safe (atomic) nhờ sử dụng ReentrantLock.

Ví dụ minh họa

Bước 1: Tạo project

Android Studio → File → New Project.

Bước 2: Layout

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="https://schemas.android.com/apk/res/android"
    xmlns:app="https://schemas.android.com/apk/res-auto"
    xmlns:tools="https://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:gravity="center"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:orientation="vertical">
    <TextView
        android:id="@+id/actionEvent"
        android:textSize="40sp"
        android:layout_marginTop="30dp"
        android:layout_width="wrap_content"
        android:layout_height="match_parent" />
</LinearLayout>

Bước 3: Code (src/MainActivity.java)

package com.example.myapplication;

import android.os.Build;
import android.os.Bundle;
import android.support.annotation.RequiresApi;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;
import java.util.concurrent.PriorityBlockingQueue;

public class MainActivity extends AppCompatActivity {
    PriorityBlockingQueue<String> priorityBlockingQueue;

    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        priorityBlockingQueue = new PriorityBlockingQueue<>();
        final TextView actionEvent = findViewById(R.id.actionEvent);

        priorityBlockingQueue.add("sai");
        priorityBlockingQueue.add("ram");
        priorityBlockingQueue.add("krishna");
        priorityBlockingQueue.add("prasad");
        priorityBlockingQueue.add("ram");

        actionEvent.setText("Queue: " + priorityBlockingQueue + "\nSize: " + priorityBlockingQueue.size());

        actionEvent.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int size = priorityBlockingQueue.size();
                actionEvent.setText("Size: " + size + "\nQueue: " + priorityBlockingQueue);
            }
        });
    }
}

Kết quả

Ban đầu: Size: 5\nQueue: [prasad, ram, ram, sai, krishna] (sorted alphabet)

Click: Hiển thị size = 5

Đặc điểm PriorityBlockingQueue

Đặc điểm	Mô tả
Kiểu	Unbounded priority queue
Thread-safe	ReentrantLock (single lock)
Sắp xếp	Natural ordering hoặc Comparator
size()	O(1) - atomic
Cho phép null	Không (NPE)
Blocking operations	take() block khi rỗng, put() không bao giờ block

So sánh size() các BlockingQueue

Queue	size()	Blocking	Sắp xếp
PriorityBlockingQueue	O(1) atomic	take() block	Priority
ArrayBlockingQueue	O(1) atomic	put/take block	FIFO
LinkedBlockingQueue	O(1) atomic	put/take block	FIFO
DelayQueue	O(1) atomic	take() block	Time-based
ConcurrentLinkedQueue	O(n) không atomic	Không	FIFO

Ứng dụng thực tế - Monitoring Queue

PriorityBlockingQueue<Task> taskQueue = new PriorityBlockingQueue<>(
    Comparator.comparingInt(Task::getPriority).reversed() // High priority first
);

// Producer
taskQueue.put(new Task("Critical", 10));
taskQueue.put(new Task("Normal", 5));

// Monitoring thread
new Thread(() -> {
    while (true) {
        int size = taskQueue.size(); // O(1) - nhanh, an toàn
        int remaining = taskQueue.remainingCapacity(); // Luôn MAX_VALUE
        log("Queue size: " + size);
        Thread.sleep(5000);
    }
}).start();

// Consumer
while (true) {
    Task task = taskQueue.take(); // Block cho đến khi có task
    execute(task);
}

Lưu ý

size() O(1) nhờ biến count được bảo vệ bởi lock
remainingCapacity() luôn trả về Integer.MAX_VALUE (unbounded)
Iterator weakly consistent
Không cho phép null
Phù hợp cho task scheduling theo priority