Button(this) android why this code example

Example 1: android studio button usage

//this for xml file 
 

//this for java file 
Button btn;
  @Override  
    protected void onCreate(Bundle savedInstanceState) {  
        super.onCreate(savedInstanceState);  
        setContentView(R.layout.activity_main);  
  btn = (Button) findViewById(R.id.button);
        btn.setOnClickListener(new View.OnClickListener() {  
            @Override  
            public void onClick(View view) {  
               //do something
            }  
        }); 
    }

Example 2: button android stdui

Button b = findById();