Buttonクリックイベントを設定する4つの方法

1つ目(匿名の内部クラス):

    private Button btn1;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_day5);
        btn1 = findViewById(R.id.btn1);
 
        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.i("flag"," ");
            }
        });
    }

2つ目(内部実装インタフェース):

    private Button btn1;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_day5);
        btn1 = findViewById(R.id.btn1);
 
        btn1.setOnClickListener(new MyLinstenter());
    }
   class MyLinstenter implements View.OnClickListener {
       @Override
       public void onClick(View v) {
           btn1.setOnClickListener(new View.OnClickListener() {
               @Override
               public void onClick(View v) {
                   Log.i("flag", " !");
               }
 
           });
       }
   }
 

3つ目(外部クラス実装インタフェース):

    private Button btn1;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_day5);
        btn1 = findViewById(R.id.btn1);
 
        btn1.setOnClickListener(new MyLinstenter(btn1));
    }
 
}
class MyLinstenter implements View.OnClickListener {
    private Button btn1;
 
    public MyLinstenter(Button btn1) {
        this.btn1 = btn1;
    }
 
    @Override
    public void onClick(View v) {
        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.i("flag", " !");
            }
 
        });
    }

4つ目(このクラスのオブジェクトを使用してインタフェースを実装):

public class day5 extends AppCompatActivity implements View.OnClickListener{
    private Button btn1;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_day5);
        btn1 = findViewById(R.id.btn1);
        btn1.setOnClickListener(this);
    }
    @Override
    public void onClick(View v) {
            Log.i("flag", " !");
        }
 
}