一 单例模式

public class Singleton  {
 
    private static Singleton INSTANCE = null;
 
    // other instance variables can be here
     
    private Singleton() {};
 
    public static Singleton getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new Singleton();
        }
        return(INSTANCE);
    }
     
    // other instance methods can follow 
}

二 MVC（Model View Controller) mvc_pattern_uml_diagram.jpg

三 MVP(Model View Presenter)

public interface LoginPresenter 
{
/*when user click on login button from Activity*/
void handleLogin(String username, String password);
}

public interface LoginView 
{
    void showValidationErrorMsg();
    void loginSuccessFully();
    void loginFail();
}

public class LoginPresenterImpl implements LoginPresenter
{
    private LoginView loginView;
    public LoginPresenterImpl(LoginView loginView) 
    {
        this.loginView = loginView;
    }
    @Override
    public void handleLogin(String username, String password) 
    {
        if ((TextUtils.isEmpty(username) || TextUtils.isEmpty(password))
        {
            loginView.showValidationErrorMsg();
        } 
        else
        {
            if (username.equals("Standerd") && password.equals("Standerd"))
            {
                loginView.loginSuccessFully();
            }
            else
            {
                loginView.loginFail();
            }
        }
    }
}

public class MainActivity extends AppCompatActivity implements LoginView
{
    private LoginPresenter presenter;
    private TextView textViewUserName;
    private TextView textViewPassword;
    private Button buttonLogin;
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initializeView();
        presenter = new LoginPresenterImpl(this);
        buttonLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                presenter.login(textViewUserName.getText().toString(), textViewPassword.getText().toString());
              }
        });
    }
    private void initializeView()
    {
        textViewUserName = findViewById(R.id.textViewUserName);
        textViewPassword = findViewById(R.id.textViewPassword);
        buttonLogin = findViewById(R.id.buttonLogin);
    }
    @Override
    public void showValidationErrorMsg()
    {
        Toast.makeText(this, "Username or Password is incorrect", Toast.LENGTH_SHORT).show();
    }
    @Override
    public void loginSuccessFully()
    {
        Toast.makeText(this, "Login SuccessFully", Toast.LENGTH_SHORT).show();
    }
    @Override
    public void loginFail()
    {
        Toast.makeText(this, "Something went wrong", Toast.LENGTH_SHORT).show();
    }
}

MVC_MVP.png

四 适配器模式（adapter)

public class CustomList extends ArrayAdapter<String>{
    private final Activity context;
    private final String[] web;
    private final Integer[] imageId;
    public CustomList(Activity context,
                      String[] web, Integer[] imageId) {
        super(context, R.layout.list_single, web);
        this.context = context;
        this.web = web;
        this.imageId = imageId;
    }
    @Override
    public View getView(int position, View view, ViewGroup parent) {
        LayoutInflater inflater = context.getLayoutInflater();
        View rowView= inflater.inflate(R.layout.list_single, null, true);
        TextView txtTitle = (TextView) rowView.findViewById(R.id.txt);
        ImageView imageView = (ImageView) rowView.findViewById(R.id.img);
        txtTitle.setText(web[position]);
        imageView.setImageResource(imageId[position]);
        return rowView;
    }
}

public class MainActivity extends Activity {
    ListView list;
    String[] web = {
            "Java",
            "C++",
            "C#",
            "HTML",
            "CSS"
    } ;
    Integer[] imageId = {
            R.drawable.java,
            R.drawable.cplus,
            R.drawable.csharp,
            R.drawable.html,
            R.drawable.download
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        CustomList listAdapter = new
                CustomList(MainActivity.this, web, imageId);
        list=(ListView)findViewById(R.id.list);
        list.setAdapter(listAdapter);
        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view,
                                    int position, long id) {
                Toast.makeText(MainActivity.this, "You Clicked at " +web[+ position], Toast.LENGTH_SHORT).show();
            }
        });
    }
}

五 观察者模式

observer.jpg