Java CopyOnWriteArrayList addAllAbsent()用法及代码示例

CopyOnWriteArrayList的addAllAbsent(E e)方法将指定集合中尚未包含在此列表中的所有元素追加到此列表的末尾，并按指定集合的迭代器返回的顺序进行添加。

用法:

public int addAllAbsent(Collection<E> collection)

参数：该函数接受单个强制性参数集合，该集合指定了不存在的附加到列表的集合。

返回值：该函数返回一个整数值，该整数值是在列表中添加元素时的数字。

异常：如果指定的集合为null，则此方法将引发NullPointerException。

以下示例程序旨在说明addAllAbsent()函数：

示例1:

// Java Program to illustrate the CopyOnWriteArrayList 
// addAllAbsent() method in Java 
  
import java.util.concurrent.CopyOnWriteArrayList; 
import java.util.*; 
  
public class GFG { 
    public static void main(String[] args) 
    { 
  
        // create object of CopyOnWriteArrayList 
        CopyOnWriteArrayList<Integer> ArrLis 
            = new CopyOnWriteArrayList<Integer>(); 
  
        // Add elements 
        ArrLis.add(2); 
        ArrLis.add(3); 
        ArrLis.add(4); 
        ArrLis.add(7); 
  
        // print CopyOnWriteArrayList 
        System.out.println("CopyOnWriteArrayList: "
                           + ArrLis); 
  
        // creating an Empty Integer ArrayList 
        ArrayList<Integer> arr = new ArrayList<Integer>(4); 
  
        // using add() to initialize values 
        // [1, 2, 3, 4] 
        arr.add(1); 
        arr.add(2); 
        arr.add(3); 
        arr.add(4); 
  
        // list initially 
        System.out.println("The collection to be appended: "
                           + arr); 
  
        // Append the list arr in the list ArrLis 
        // using addAllAbsent() method 
        System.out.println("\nNumber of elements appended"
                           + " using addAllAbsent() method: "
                           + ArrLis.addAllAbsent(arr)); 
  
        // print CopyOnWriteArrayList 
        System.out.println("Modified CopyOnWriteArrayList: "
                           + ArrLis); 
    } 
}

输出：

CopyOnWriteArrayList: [2, 3, 4, 7]
The collection to be appended: [1, 2, 3, 4]

Number of elements appended using addAllAbsent() method: 1
Modified CopyOnWriteArrayList: [2, 3, 4, 7, 1]

示例2:演示NullPointerException

// Java Program to illustrate the CopyOnWriteArrayList 
// addAllAbsent() method in Java 
  
import java.util.concurrent.CopyOnWriteArrayList; 
import java.util.*; 
  
public class GFG { 
    public static void main(String[] args) 
    { 
  
        // create object of CopyOnWriteArrayList 
        CopyOnWriteArrayList<Integer> ArrLis 
            = new CopyOnWriteArrayList<Integer>(); 
  
        // Add elements 
        ArrLis.add(2); 
        ArrLis.add(3); 
        ArrLis.add(4); 
        ArrLis.add(7); 
  
        // print CopyOnWriteArrayList 
        System.out.println("CopyOnWriteArrayList: "
                           + ArrLis); 
  
        // creating a null Integer ArrayList 
        ArrayList<Integer> arr = null; 
  
        // list initially 
        System.out.println("The collection to be appended: "
                           + arr); 
  
        try { 
            // Append the list arr in the list ArrLis 
            // using addAllAbsent() method 
            System.out.println("\nTrying to append null collection\n"); 
            ArrLis.addAllAbsent(arr); 
        } 
        catch (Exception e) { 
            System.out.println(e); 
        } 
    } 
}

输出：

CopyOnWriteArrayList: [2, 3, 4, 7]
The collection to be appended: null

Trying to append null collection

java.lang.NullPointerException

参考： https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/CopyOnWriteArrayList.html#addAllAbsent-java.util.Collection-

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