Java ConcurrentLinkedDeque removeFirstOccurrence()方法

java.util.concurrent.ConcurrentLinkedDeque.removeFirstOccurrence() 方法是Java中的一个内置方法，它接受一个参数并删除该元素在deque中的第一次出现。因此，如果该元素在deque中不存在，它将保持不变。

语法

public boolean removeFirstOccurrence(Object o)

参数： 该函数接受一个对象 elem 作为参数，表示要从deque中删除其第一次出现的对象。
返回值： 如果 elem 在deque中存在，该函数返回true，否则返回false。

异常： 如果作为参数传递给该函数的指定元素为null，则该函数抛出NullPointerException 。下面的程序说明了 removeFirstOccurrence() 方法的使用：

程序1 :

/* Java program to demonstrate
   removeFirstOccurrence() method
   of ConcurrentLinkedDeque   */
 
import java.util.concurrent.*;
 
class ConcurrentLinkedDequeDemo {
    public static void main(String[] args)
    {
        ConcurrentLinkedDeque<String> cld = new ConcurrentLinkedDeque<String>();
 
        cld.addFirst("GFG");
        cld.addFirst("Geeks");
        cld.addFirst("Gfg");
        cld.addFirst("gfg");
        cld.addFirst("Geeks");
 
        // Displaying the existing LinkedDeque
        System.out.println("Elements in "
                           + "the LinkedDeque: " + cld);
 
        // Remove first occurrence of  element
        cld.removeFirstOccurrence("Geeks");
 
        // Displaying the modified LinkedDeque
        System.out.println("Elements in "
                           + "the LinkedDeque: " + cld);
    }
}

输出

Elements in the LinkedDeque: [Geeks, gfg, Gfg, Geeks, GFG]
Elements in the LinkedDeque: [gfg, Gfg, Geeks, GFG]

代码2

/* Java program to demonstrate
   removeFirstOccurrence() method
   of ConcurrentLinkedDeque   */
 
import java.util.concurrent.*;
 
class ConcurrentLinkedDequeDemo {
    public static void main(String[] args)
    {
        ConcurrentLinkedDeque<Integer> cld = new ConcurrentLinkedDeque<Integer>();
 
        cld.addFirst(12);
        cld.addFirst(280);
        cld.addFirst(1008);
        cld.addFirst(1050);
        cld.addFirst(379);
 
        // Displaying the existing LinkedDeque
        System.out.println("Elements in "
                           + "the LinkedDeque: " + cld);
        try {
            // Remove first occurrence of  element
            cld.removeFirstOccurrence(null);
        }
        catch (Exception e) {
            System.out.println(e);
        }
    }
}

输出

Elements in the LinkedDeque: [379, 1050, 1008, 280, 12]
java.lang.NullPointerException

参考资料: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentLinkedDeque.html#removeFirstOccurrence()