关于Activity数据保存的问题

package com.ocular.htcvive.activitylifecycletest;
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.view.Window;
import android.widget.Button;
public class MainActivity extends Activity {
    public static final String TAG="MainActivity";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d(TAG,"onCreate");
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.activity_main);
        Button startNormalActivity=(Button)findViewById(R.id.start_normal_activity);
        Button startDialogActivity=(Button)findViewById(R.id.start_dialog_activity);
        startNormalActivity.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent=new Intent(MainActivity.this,NormalActivity.class);
                startActivity(intent);
            }
        });
        startDialogActivity.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent=new Intent(MainActivity.this,DialogActivity.class);
                startActivity(intent);
            }
        });
    }
    @Override
    protected void onStart(){
        super.onStart();
        Log.d(TAG,"onStart");
    }
    @Override
    protected void onResume(){
        super.onResume();
        Log.d(TAG,"onResume");
    }
    @Override
    protected void onPause(){
        super.onPause();
        Log.d(TAG,"onPause");
    }
    @Override
    protected void onStop(){
        super.onStop();
        Log.d(TAG,"onStop");
    }
    @Override
    protected void onRestart(){
        super.onRestart();
        Log.d(TAG,"onRestart");
    }
    @Override
    protected void onDestroy(){
        super.onDestroy();
        Log.d(TAG,"onDestroy");
    }
}

如上面MainActivity代码所示，这是本人买的一本安卓开发教程上的一个体会活动生命周期的示例代码，然后讲到活动数据保存的问题时，书本叙述如下所示
之后上面百度了相关信息
本人的问题是：1.教材上面说的添加onSaveInstanceState()方法和修改onCreate()方法可以保存数据，那么onSaveInstanceState()方法的参数是怎么传进onCreate（）方法内的
2.百度到的相关信息上说系统内存不足时会回收，activity的实例会被回收，本人想问问具体是怎么的情况才会回收activity
谢谢各位！