Change between Activities

Main Activity 

package itesm.mx.activity3_aboutmyself;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;


public class MainActivity extends AppCompatActivity {

    private EditText input;
    private Button b1;
    private static final int PERRITO_CODE = 0;

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

        input = findViewById(R.id.editText);
        b1 = findViewById(R.id.button);


    }

    public void buttonTest(View v) {
        Intent intent = new Intent(this, MenuActivity.class);
        //startActivity(intent);        startActivityForResult(intent, PERRITO_CODE);
    }

}

Menu Activity / Perrito Activity

package mx.tec.jan21;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

public class PerritoActivity extends AppCompatActivity {

    private TextView textito;
    @Override    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_perrito);
        textito = findViewById(R.id.textView);

        Intent intent = getIntent();
        String message = intent.getStringExtra("userName") + " age: " + intent.getIntExtra("age", 0);
        textito.setText(message);

    }

    public void finishActivity(View v){

        // go back sending info        // use an intent!        Intent intent = new Intent();

        intent.putExtra("returnValue", "going back!");
        setResult(Activity.RESULT_OK, intent);
        finish();
    }
}

Tips

On AndroidManifest.xml:

<activity android:name=".PerritoActivity"></activity>