Error adding to JAVA [duplicate]

I would like to know how to add hours in Java. For example: 10:30 + 00:30 = 11:00.

Does anyone know how?

Code sample:

package teste1;

import java.io.UnsupportedEncodingException;
import java.io.ObjectInputStream.GetField;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.time.Duration;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.util.Calendar;
import java.util.Date;

import javax.annotation.Generated;
import javax.swing.plaf.synth.SynthSeparatorUI;
import javax.xml.bind.DatatypeConverter;

public class converter {

    public static void main(String[] args)  {

        double Disponibilidade;
        double atividade;
        double tempotrabalhado = Disponibilidade - atividade;

        System.out.println(tempotrabalhado);

    }

}