AtomicInteger.incrementAndGet() vs. AtomicInteger.getAndIncrement()

问题

When return value is not of interest, is there any (even irrelevant in practice) difference between AtomicInteger.getAndIncrement() and AtomicInteger.incrementAndGet() methods, when return value is ignored?

I'm thinking of differences like which would be more idiomatic, as well as which would put less load in CPU caches getting synchronized, or anything else really, anything to help decide which one to use more rationally than tossing a coin.

回答1:

The code is essentially the same so it does not matter:

public final int getAndIncrement() {
    for (;;) {
        int current = get();
        int next = current + 1;
        if (compareAndSet(current, next))
            return current;
    }
}

public final int incrementAndGet() {
    for (;;) {
        int current = get();
        int next = current + 1;
        if (compareAndSet(current, next))
            return next;
    }
}

回答2:

Since no answer to the actual question has been given, here's my personal opinion based on the other answers (thanks, upvoted) and Java convention:

incrementAndGet()

is better, because method names should start with the verb describing the action, and intended action here is to increment only.

Starting with verb is the common Java convention, also described by official docs:

"Methods should be verbs, in mixed case with the first letter lowercase, with the first letter of each internal word capitalized."

回答3:

No, there's no difference (if you don't care about the return value).

The code of those methods (in the OpenJDK) differs only in that one uses return next and the other uses return current.

Both use compareAndSet under the hood with the exact same algorithm. Both need to know both the old and the new value.

回答4:

Just want to add to existing answers: there could be very small non-noticeable difference.

If you look at this implementation:

public final int getAndIncrement() {
    return unsafe.getAndAddInt(this, valueOffset, 1);
}

public final int incrementAndGet() {
    return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
}

Note - both function call exactly the same function getAndAddInt, except +1 part, which means that in this implementation getAndIncrement is faster.

But, here is older implementation:

public final int getAndIncrement() {
    for (;;) {
        int current = get();
        int next = current + 1;
        if (compareAndSet(current, next))
            return current;
    }
}

public final int incrementAndGet() {
    for (;;) {
        int current = get();
        int next = current + 1;
        if (compareAndSet(current, next))
            return next;
    }
}

The only difference is return variable, so both functions perform exactly the same.

回答5:

Here I am giving an example. Hope it will clear your doubt.

Suppose I have a variable i as

AtomicInteger i = new AtomicInteger();

In this case:

i.getAndIncrement() <==> i++;

And

i.incrementAndGet() <==> ++i;

Please have a look of the below programs

public class Test1
{   
    public static void main(String[] args) 
    {               
        AtomicInteger i = new AtomicInteger();
        System.out.println(i.incrementAndGet());  
        System.out.println(i);  

    }

}

**output

1 1 ======================================**

public class Test2
{   
    public static void main(String[] args) 
    {               
        AtomicInteger i = new AtomicInteger();
        System.out.println(i.getAndIncrement());  
        System.out.println(i); 

    }

}

**output

0 1 -------------**

Comment: 1) In the class Test1, incrementAndGet() will first increment the i value and then print.

2) In the class Test2, getAndIncrement() will first print the i value and then increment.

That's all.

来源：https://stackoverflow.com/questions/15137308/atomicinteger-incrementandget-vs-atomicinteger-getandincrement

标签

java

atomic

java.util.concurrent