<span style="color:#333333;"><Button android:text="Button03"  
  android:id="@+id/Button03"  
  android:layout_width="wrap_content"  
  android:layout_height="wrap_content"  
  <span style="color:#FF0000;">android:onClick="</span><span style="color:#ff0000;">Btn3OnClick</span><span style="color:#333333;">"</span>>  
</Button> </span> 

public void Btn3OnClick(View view){  
  Intent intent = new Intent(mainActivity.this, fristActivity.class);  
  intent.putExtra("data", "mainActivity");  
  startActivity(intent);  
}  

protected void onCreate(Bundle savedInstanceState) {  
  super.onCreate(savedInstanceState);  
  setContentView(R.layout.main);  
  findViewById(R.id.Button03).setOnClickListener(new OnClickListener(){  
    @Override  
    public void onClick(View v) {  
      Intent intent = new Intent(mainActivity.this, fristActivity.class);  
      intent.putExtra("data", "mainActivity");  
      startActivity(intent);       
    }      
  });  
}  

protected void onCreate(Bundle savedInstanceState) {  
  super.onCreate(savedInstanceState);  
  setContentView(R.layout.main);  
  findViewById(R.id.Button03).setOnClickListener(onclick);  
}  
OnClickListener onclick = new OnClickListener(){  
  @Override  
  public void onClick(View v) {  
    Intent intent = new Intent(mainActivity.this, fristActivity.class);  
    intent.putExtra("data", "mainActivity");  
    startActivity(intent);   
  }  
}; 

public class mainActivity extends Activity implements OnClickListener{  
  protected void onCreate(Bundle savedInstanceState) {  
    super.onCreate(savedInstanceState);  
    setContentView(R.layout.main);  
    findViewById(R.id.Button02).setOnClickListener(this);  
    findViewById(R.id.Button03).setOnClickListener(this);  
  }  
  public void onClick(View view) {  
    switch (v.getId()) {  
      case R.id.Button03:  
        Intent intent = new Intent(mainActivity.this, fristActivity.class);  
        intent.putExtra("data", "mainActivity");  
        startActivity(intent);   
        break;       
      case R.id.Button02:  
        Intent intent = new Intent(mainActivity.this, loginActivity.class);  
        intent.putExtra("data", "mainActivity");  
        startActivity(intent);   
        break;   
      default:  
        break;  
    }  
  }  
  ……  
}