Java AtomicReference compareAndExchange()用法及代碼示例

如果AtomicReference對象的當前值(稱為見證值)等於期望值，則使用AtomicReference類的compareAndExchange()方法以原子方式將值newValue設置為AtomicReference對象。此方法將返回見證值。將與期望值相同。此方法使用讀取的內存語義來處理操作，就像將該變量聲明為volatile一樣。

用法:

public final V compareAndExchange(V expectedValue,
                                  V newValue)

參數：該方法接受ExpectedValue(期望值)和newValue(新值)來設置新值。

返回值：此方法返回見證值，如果成功，它將與期望值相同。

以下示例程序旨在說明compareAndExchange()方法：
程序1:

// Java program to demonstrate 
// AtomicReference.compareAndExchange() method 
  
import java.util.concurrent.atomic.AtomicReference; 
  
public class GFG { 
    public static void main(String[] args) 
    { 
  
        // create an atomic reference object 
        // which stores Integer. 
        AtomicReference<Integer> ref 
            = new AtomicReference<Integer>(); 
  
        // set some value 
        ref.set(999); 
  
        // apply compareAndExchange() 
        Integer oldValue 
            = ref.compareAndExchange( 
                999, 
                999999); 
  
        // print value 
        System.out.println("Witness Value= "
                           + oldValue); 
    } 
}

輸出：

程序2:

// Java program to demonstrate 
// AtomicReference.compareAndExchange() method 
  
import java.util.concurrent.atomic.AtomicReference; 
  
public class GFG { 
    public static void main(String[] args) 
    { 
  
        // create an atomic reference object. 
        AtomicReference<String> ref 
            // = new AtomicReference<String>(); 
  
            // set some value 
            ref.set("GFG"); 
  
        // apply compareAndExchange() 
        String oldValue 
            = ref.compareAndExchange( 
                "GFG", 
                "Geeks for Geeks"); 
  
        // print value 
        System.out.println("Witness Value= "
                           + oldValue); 
    } 
}

輸出：

參考文獻： https://docs.oracle.com/javase/10/docs/api/java/util/concurrent/atomic/AtomicReference.html#compareAndExchange(V, V)

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