Singleton Pattern - Thread Safety

public class SingleTest {
    private SingleTest(){
        
    }

    //Static inner class implementation, this method is the best, it can ensure thread safety under concurrency better than double-layer verification
    static class SingleInner{
        private static final SingleTest stest = new SingleTest();
    }
    public SingleTest getInstance(){
        return SingleTest.SingleInner.stest;
    }
}