Java CopyOnWriteArrayList addIfAbsent()方法

CopyOnWriteArrayList 的 addIfAbsent(E e) 方法在列表中不存在的情况下，将参数中的元素追加到列表的末尾。当新元素被添加到列表中时，该函数返回true。

语法

public boolean addIfAbsent(E e)

参数： 该函数接受一个强制参数 元素 ，如果该元素不存在，则指定将其追加到列表中。

返回值： 如果它被添加到列表中，该函数返回true。

下面的程序说明了上述函数。

程序1 :

// 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<Integer> ArrLis
            = new CopyOnWriteArrayList<Integer>();
  
        // 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

程序2

// 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<String> ArrLis
            = new CopyOnWriteArrayList<String>();
  
        // 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

参考资料: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CopyOnWriteArrayList.html#addIfAbsent-E-