public class RestService {

public static Retrofit retrofit = null;

public static Retrofit getClient(String url) {
    if (retrofit == null) {
        retrofit = new Retrofit.Builder()
                .baseUrl(url)
                .addConverterFactory(GsonConverterFactory.create())
                .build();
    }
    return retrofit;
}

public class ApiUtils {

public static final String NService = "http://localhost:5108/NSureServices.svc/";

public static Service getLoginDetails(){
    return RestService.getClient(NService).create(Service.class);
}

public interface Service {

@FormUrlEncoded
@POST("GetLoginDetails")
Call<User> login(@Field("USERID") String UserId, @Field("PASSWORD") String Password, @Field("apiKey") String ApiKey);

public class LoginActivity extends AppCompatActivity {

EditText email, password;
Button login;
TextView register, forgotPassword;
Service service;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);

    email = (EditText)findViewById(R.id.emailIdEt);
    password = (EditText)findViewById(R.id.passwordEt);
    register = (TextView)findViewById(R.id.signUpTv);
    forgotPassword = (TextView)findViewById(R.id.forgotPasswordTv);
    login = (Button)findViewById(R.id.loginBtn);
    service = ApiUtils.getLoginDetails();

    login.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            User user = new User();
            user.setUSERID(email.getText().toString());
            user.setPASSWORD(password.getText().toString());


            if(validateLogin(user.getUSERID(),user.getPASSWORD())){
                doLogin(user.getUSERID(),user.getPASSWORD(),user.getApiKey());
            }

        }
    });

}

public boolean validateLogin(String UserId, String Password){
    if(UserId == null || UserId.trim().length() == 0){
        Toast.makeText(this, "Username is required", Toast.LENGTH_LONG).show();
        return false;
    }
    if(Password == null || Password.trim().length() == 0){
        Toast.makeText(this, "Password is required", Toast.LENGTH_LONG).show();
        return false;
    }
    return true;
}



public void doLogin(String uid, String pwd, String apikey){
    try {
        Call<User> call =service.login(uid, pwd, apikey);
        call.enqueue(new Callback<User>() {
            @Override
            public void onResponse(Call<User> call, Response<User> response) {
                if(response.isSuccessful()){
                    Toast.makeText(LoginActivity.this, "Successful", Toast.LENGTH_LONG).show();
                }
                else{
                    Toast.makeText(LoginActivity.this, "Failure", Toast.LENGTH_LONG).show();

                }
            }

            @Override
            public void onFailure(Call<User> call, Throwable t) {
                Toast.makeText(LoginActivity.this, "Error", Toast.LENGTH_LONG).show();

            }
        });

    } catch (Exception ex){
        Toast.makeText(LoginActivity.this, ex.getMessage(), Toast.LENGTH_LONG).show();

    }

}

dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
    exclude group: 'com.android.support', module: 'support-annotations'
})
compile 'com.android.support:appcompat-v7:26.+'
compile 'com.android.support.constraint:constraint-layout:1.0.2'
compile 'com.squareup.retrofit2:retrofit:2.3.0'
compile 'com.google.code.gson:gson:2.6.1'
compile 'com.squareup.retrofit2:converter-gson:2.3.0'
compile 'com.squareup.okhttp:okhttp:2.4.0'
provided 'org.glassfish:javax.annotation:10.0-b28'

compile 'com.android.support:support-v4:26.+'
testCompile 'junit:junit:4.12'