Creating SOAP Web Service in Apex

Create the apex class and generate its wsdl. Download Partner wsdl as well.

global class MathOperations
{
    webservice static Integer getSum (Integer a, Integer b)
    {
        return a + b;
    }
}

Convert the wsdl files that you have downloaded to jar files using the command below:

java -classpath force-wsc-40.1.1.jar;rhino-1.7.7.1.jar;ST-4.0.8.jar;C:\Java\jdk1.8.0_121\lib\tools.jar com.sforce.ws.tools.wsdlc partner.wsdl partner_stub.jar

Using SoapUI, get the sessionId from partner wsdl:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:urn="urn:partner.soap.sforce.com">
   <soapenv:Header>
   </soapenv:Header>
   <soapenv:Body>
      <urn:login>
         <urn:username>username</urn:username>
         <urn:password>password</urn:password>
      </urn:login>
   </soapenv:Body>
</soapenv:Envelope>

Enter the sessionId value in custom apex class wsdl. Check the below request and response:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:mat="http://soap.sforce.com/schemas/class/MathOperations">
   <soapenv:Header>
      <mat:SessionHeader>
         <mat:sessionId>00D6A000001Vait!ARwAQPfREKhyBFwVu3F.vFEXYDvLIiR3w8K5hlZvemfv_vFPGECj.QLj3HXsOVFQzVb7OQAG.T9QFWqUzh5P0PP0rPhASOdo</mat:sessionId>
      </mat:SessionHeader>
   </soapenv:Header>
   <soapenv:Body>
      <mat:getSum>
         <mat:a>5</mat:a>
         <mat:b>8</mat:b>
      </mat:getSum>
   </soapenv:Body>
</soapenv:Envelope>




<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns="http://soap.sforce.com/schemas/class/MathOperations">
   <soapenv:Body>
      <getSumResponse>
         <result>13</result>
      </getSumResponse>
   </soapenv:Body>
</soapenv:Envelope>

You can call this web service using Java like below:

package wsc;

import com.sforce.soap.MathOperations.Connector;
import com.sforce.soap.MathOperations.SoapConnection;
import com.sforce.soap.partner.PartnerConnection;
import com.sforce.ws.ConnectionException;
import com.sforce.ws.ConnectorConfig;

public class SalesforceSoapStarter {

    private static final String AUTH_ENDPOINT =  "https://login.salesforce.com/services/Soap/u/40.0";
    private static final String USERNAME = "username";
    private static final String PASSWORD = "password";

    public static void main(String[] args) throws ConnectionException {

     ConnectorConfig config = new ConnectorConfig();
        config.setUsername(USERNAME);
        config.setPassword(PASSWORD);
        config.setAuthEndpoint(AUTH_ENDPOINT);
        
        PartnerConnection connection = new PartnerConnection(config);

        String sessionId = config.getSessionId();
        
        SoapConnection soap = Connector.newConnection("","");
        soap.setSessionHeader(sessionId);
        System.out.println(soap.getSum(4, 9));
    }

}

Refer this link to set up Java developer environment:
https://developer.salesforce.com/docs/atlas.en-us.salesforce_developer_environment_tipsheet.meta/salesforce_developer_environment_tipsheet/salesforce_developer_environment_overview.htm