Java中的 CopyOnWriteArrayList addIfAbsent() 方法

如果元素不存在于列表中， CopyOnWriteArrayList的addIfAbsent(E e)方法会将参数中传递的元素附加到列表的末尾。该函数在将新元素添加到列表时返回 true。

句法：

public boolean addIfAbsent(E e)

参数：该函数接受单个强制参数元素，该元素指定要附加到列表中的元素（如果不存在）。

返回值：如果添加到列表中，该函数返回true 。

下面的程序说明了上述函数：

方案一：

// Java Program to illustrate the CopyOnWriteArrayList
// addIfAbsent(element) 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 ArrLis
            = new CopyOnWriteArrayList();
  
        // Add elements
        ArrLis.addIfAbsent(2);
        ArrLis.addIfAbsent(3);
        ArrLis.addIfAbsent(4);
        ArrLis.addIfAbsent(7);
  
        // print CopyOnWriteArrayList
        System.out.println("CopyOnWriteArrayList: " 
                                           + ArrLis);
  
        System.out.println("On adding 4 it returns " 
          + ArrLis.addIfAbsent(4) + " as it is present");
    }
}

输出：

CopyOnWriteArrayList: [2, 3, 4, 7]
On adding 4 it returns false as it is present

方案二：

// Java Program to illustrate the CopyOnWriteArrayList
// addIfAbsent(element) 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 ArrLis
            = new CopyOnWriteArrayList();
  
        // Add elements
        ArrLis.addIfAbsent("gopal");
        ArrLis.addIfAbsent("gfg");
        ArrLis.addIfAbsent("jgec");
        ArrLis.addIfAbsent("sudo");
  
        // print CopyOnWriteArrayList
        System.out.println("CopyOnWriteArrayList: " + ArrLis);
  
        System.out.println("On adding 'sudo' it returns " 
            + ArrLis.addIfAbsent("sudo") + " as it is present");
    }
}

输出：

CopyOnWriteArrayList: [gopal, gfg, jgec, sudo]
On adding 'sudo' it returns false as it is present

参考： https: Java/util/concurrent/CopyOnWriteArrayList.html#addIfAbsent-E-