Java PriorityBlockingQueue toArray()方法

toArray()

PriorityBlockingQueen的 toArray 方法用于创建一个数组，该数组的元素与该PriorityBlockingQueen的元素相同，且顺序适当。实际上，这个方法是把PriorityBlockingQueen中的所有元素复制到一个新的数组中。这个方法在数组和PriorityBlockingQueue之间起到了桥梁作用。

语法

public Object[] toArray()

返回值： 该方法返回一个包含PriorityBlockingQueue元素的数组。

下面的程序说明了PriorityBlockingQueue类的toArray()方法。

例1： 演示PriorityBlockingQueue的toArray()方法的程序，从一个包含数字的PriorityBlockingQueue创建一个数字数组。

// Java Program Demonstrate toArray()
// method of PriorityBlockingQueue
 
import java.util.concurrent.PriorityBlockingQueue;
public class GFG {
 
    public static void main(String[] args)
    {
        // define capacity of PriorityBlockingQueue
        int capacityOfQueue = 5;
 
        // create object of PriorityBlockingQueue
        PriorityBlockingQueue<Integer> PrioQueue
            = new PriorityBlockingQueue<Integer>(capacityOfQueue);
 
        // Add numbers to PriorityBlockingQueue
        PrioQueue.offer(35658786);
        PrioQueue.offer(5278367);
        PrioQueue.offer(74381793);
        PrioQueue.offer(87625142);
 
        // create an array of elements of PriorityBlockingQueue
        Object[] array = PrioQueue.toArray();
 
        // print elements of array
        System.out.println("Array Contains:");
        for (Object i : array) {
            System.out.print(i + " ");
        }
    }
}

输出

Array Contains:
5278367 35658786 74381793 87625142

例2： 演示PriorityBlockingQueue的toArray()方法的程序，从一个包含字符串值的PriorityBlockingQueue中创建一个字符串数组。

// Java Program Demonstrate toArray()
// method of PriorityBlockingQueue
 
import java.util.concurrent.PriorityBlockingQueue;
import java.util.*;
 
public class GFG {
    public static void main(String[] args)
    {
 
        // define capacity of PriorityBlockingQueue
        int capacityOfQueue = 5;
 
        // create object of PriorityBlockingQueue which contains
        // name of students
        PriorityBlockingQueue<String> names
            = new PriorityBlockingQueue<String>(capacityOfQueue);
 
        // Add names of students of girls college
        names.offer("Joyita");
        names.offer("Bristi");
        names.offer("Riya");
 
        // Create a array by calling toArray() method
        Object[] array = names.toArray();
 
        // Print List of names
        System.out.println("Queue is " + names);
 
        // Print elements of array
        System.out.println("The array created by toArray() is:");
        for (Object i : array) {
            System.out.println(i + " ");
        }
    }
}

输出

Queue is [Bristi, Joyita, Riya]
The array created by toArray() is:
Bristi 
Joyita 
Riya

toArray(T[] a)

PriorityBlockingQueen 的toArray(T[] a)方法用于返回一个包含与该PriorityBlockingQueen相同元素的数组，并以适当的顺序排列。这个方法与toArray()的不同之处只有一个条件。如果PriorityBlockingQueue的大小小于或等于传递的数组，返回的数组类型与参数中传递的数组相同。否则，将分配一个新的数组，其类型与指定的数组相同，并且数组的大小等于该队列的大小。这个方法作为数组和集合之间的桥梁。

语法

public <T> T[] toArray(T[] a)

参数： 该方法接受一个数组作为参数，如果它足够大的话，队列中的所有元素将被复制到该数组中。否则，一个新的相同运行时类型的数组将被分配到此。

返回值： 该方法返回一个包含该队列中所有元素的数组。

异常该方法抛出以下异常。

ArrayStoreException： 当传递的数组与PriorityBlockingQueue的元素类型不同。
NullPointerException： 如果传递的数组是空的。

下面的程序说明了PriorityBlockingQueue类的toArray(T[] a)方法。

例1： 演示PriorityBlockingQueue的toArray(T[] a)方法的程序。

// Java Program Demonstrate toArray(T[] a)
// method of PriorityBlockingQueue
 
import java.util.concurrent.PriorityBlockingQueue;
import java.util.concurrent.TimeUnit;
 
public class GFG {
 
    public static void main(String[] args)
        throws InterruptedException
    {
        // define capacity of PriorityBlockingQueue
        int capacityOfQueue = 3;
 
        // create object of PriorityBlockingQueue
        PriorityBlockingQueue<String> PrioQueue
            = new PriorityBlockingQueue<String>(capacityOfQueue);
 
        // Add names of students of girls college
        PrioQueue.offer("Joyi");
        PrioQueue.offer("Rani");
        PrioQueue.offer("Riya");
 
        // Creating the array
        String[] array = new String[capacityOfQueue];
 
        // Calling toArray(T[] a) method
        Object[] ReturnArray = PrioQueue.toArray(array);
 
        // Print queue
        System.out.println("Queue is " + PrioQueue);
 
        // Print elements of array passed as parameter
        System.out.println();
        System.out.println("The array passed to toArray() is:");
        for (Object i : array) {
            System.out.println(i);
        }
 
        // Print elements of array returned by method toArray()
        System.out.println();
        System.out.println("The array returned by toArray() is:");
        for (Object i : ReturnArray) {
            System.out.println(i);
        }
    }
}

输出

Queue is [Joyi, Rani, Riya]

The array passed to toArray() is:
Joyi
Rani
Riya

The array returned by toArray() is:
Joyi
Rani
Riya

例2： 演示PriorityBlockingQueue的toArray(T[] a)方法所产生的异常的程序。

// Java Program Demonstrate toArray(T[] a)
// method of PriorityBlockingQueue
 
import java.util.concurrent.PriorityBlockingQueue;
import java.util.concurrent.TimeUnit;
 
public class GFG {
 
    public static void main(String[] args)
        throws InterruptedException
    {
        // define capacity of PriorityBlockingQueue
        int capacityOfQueue = 3;
 
        // create object of PriorityBlockingQueue
        PriorityBlockingQueue<String> PrioQueue
            = new PriorityBlockingQueue<String>(capacityOfQueue);
 
        // Add names of students of girls college
        PrioQueue.offer("Joyi");
        PrioQueue.offer("Rani");
        PrioQueue.offer("Riya");
 
        // Creating the array with null value
        String[] array = null;
 
        try {
            // Calling toArray(T[] a) method
            Object[] ReturnArray = PrioQueue.toArray(array);
        }
        catch (Exception e) {
            System.out.println("Exception thrown:" + e);
        }
    }
}

输出

Exception thrown:java.lang.NullPointerException

参考资料

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/PriorityBlockingQueue.html#toArray-
https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/PriorityBlockingQueue.html#toArray-T:A-