別クラスからViewの変更

package com.example.temp;

import android.app.Activity;
import android.os.Bundle;

public class MainActivity extends Activity {

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  
  new vewChange(this);
 }

}

package com.example.temp;

import android.content.Context;
import android.widget.TextView;

public class vewChange {

 public vewChange(Context context) {
  // TODO Auto-generated constructor stub
  
  TextView tv = (TextView) ((com.example.temp.MainActivity) context).findViewById(R.id.textView1);
  tv.setText("change");
 }

}

別パターン

package com.example.temp;

import android.widget.TextView;

public class vewChange {

 public vewChange(MainActivity mainActivity) {
  // TODO Auto-generated constructor stub
  TextView tv = (TextView) mainActivity.findViewById(R.id.textView1);
  tv.setText("change");
 }
}