Question

Is it possible to have this behavior:

One thread (T1) calls some method, for instance, compute(10);

While that function is still executing, another thread calls it again (with other parameters), say, compute(20);

If the method

public int compute(int i) {
    return i+20;
}

will each thread get the correct result?

I expect that T1 gets 20, and T2 gets 40.

Answer 1

Since you use local variables only (method parameters are local), yes it is safe.

public class Computer {
    public int compute(int i) {
        return i+20;
    }
}

If you would use instance variables or static variables, then you should have synchronized every read and write to the variables.

public class Computer {
    private int increment;

    public synchronized int compute(int i) {
        return i+increment;  // <-- reads increment
    }

    private synchronized void setIncrement(int increment) {
         this.increment = increment;  // <-- writes increment
    }
}