Java CopyOnWriteArrayList addIfAbsent()用法及代碼示例

如果列表中不存在該參數，則CopyOnWriteArrayList的addIfAbsent(E e)方法會將在參數中傳遞的元素附加到列表的末尾。向列表中添加新元素時，該函數返回true。

用法:

public boolean addIfAbsent(E e)

參數：該函數接受單個強製性參數element ，該元素指定要添加到列表的元素(如果不存在)。

返回值：如果將其添加到列表，則該函數返回true。

以下示例程序旨在說明上述函數：

示例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"); 
    } 
}

輸出：

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

示例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"); 
    } 
}

輸出：

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

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

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