根据子类从基类指定抽象方法的返回类型

public abstract class BaseClass<T> {
    public abstract T getResult();
}

public class SubClass extends BaseClass<String> {
    @Override
    public String getResult() {
        return "Hello, world!";
    }
}

public class Main {
    public static void main(String[] args) {
        SubClass subClass = new SubClass();
        String result = subClass.getResult();
        System.out.println(result);
    }
}