Are the following incremental code threads safe in Java?

Java code:

public class IncreaseTest {
    public static int value = 0;

    public synchronized int increment() {
        return value++;
    }
}

Is method increment () thread safe? Do I have to add the modifier keyword volatile, as follows:

public static volatile int value = 0;

Solution

This code is not thread safe Instance methods will synchronize on instances. If you have multiple instances, they will not use the same monitor, so updates can be interleaved

You need to remove static from the value field or add static to the increment () method

In addition, since you have exposed the value, there is another problem, that is, you can change or read the value outside this method without using synchronization that may cause the old value to be read

Therefore, changing the code to the following makes it thread safe:

public class IncreaseTest {
    private int value = 0;

    public synchronized int increment() {
        return value++;
    }
}