Synthetic Class in Java

Question

What is a synthetic class in Java? Why should it be used? How can I use it?

Answer 1

A synthetic class does not appear in your code: it is made up by compiler. E.g. A bridge method made up by compiler in java is typically synthetic.

public class Pair {
    private T first;
    private T second;
    public void setSecond(T newValue) {
        second = newValue;
    }
}


public class DateInterval extends Pair {
    public void setSecond(String second) {
        System.out.println("OK sub");
    }

    public static void main(String[] args) throws NoSuchFieldException, SecurityException {
        DateInterval interval = new DateInterval();
        Pair pair = interval;
        pair.setSecond("string1");
    }
}

Using the command javap -verbose DateInterval, you can see a bridge method:

public void setSecond(java.lang.Object);
flags: ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC

This was made up by compiler; it does not appear in your code.