[OCPJP]AtomicInteger的用法

Given the code fragments:

class MyThread implements Runnable {

    private static AtomicInteger count = new AtomicInteger(0);

    public void run() {
        int x = count.incrementAndGet();
        System.out.print(x + " ");
    }
}

class MyThread implements Runnable {

private static AtomicInteger count = new AtomicInteger(0);

public void run() {

int x = count.incrementAndGet();

System.out.print(x + " ");

}

and

Thread thread1 = new Thread(new MyThread());
Thread thread2 = new Thread(new MyThread());
Thread thread3 = new Thread(new MyThread());
Thread[] ta = {thread1, thread2, thread3};
for (int x = 0; x < 3; x++) {
    ta[x].start();
}

Thread thread1 = new Thread(new MyThread());

Thread thread2 = new Thread(new MyThread());

Thread thread3 = new Thread(new MyThread());

Thread[] ta = {thread1, thread2, thread3};

for (int x = 0; x < 3; x++) {

ta[x].start();

}

Which statement is true?

A. The program prints 1 2 3 and the order is unpredictable.
B. The program prints 1 2 3.
C. The program prints 1 1 1.
D. A compilation error occurs.

題解

AtomicInteger的incrementAndGet方法，可以使AtomicInteger所表示的整數值加一後，同時回傳整數結果，在操作AtomicInteger物件的數值時，同一時間永遠只有一個執行緒可以存取，所以可以達成thread-safe。

程式第8行的x區域變數，在每個執行緒中的數值都不一樣，排序後為1、2、3。但是執行緒執行第9行的順序不同，因此輸出結果會是1、2、3的任意排列組合。

MagicLen

[OCPJP]AtomicInteger的用法

題解

關於作者

Magic Len

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