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   源码介绍

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package com.example.back;

import android.os.Bundle;
import android.app.Activity;
import android.view.KeyEvent;
import android.view.Menu;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends Activity {
	Button btn;
	long exitTime;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		btn = (Button) findViewById(R.id.button1);
	}

	@Override
	public boolean onKeyDown(int keyCode, KeyEvent event) {
		// TODO Auto-generated method stub
		if (keyCode == KeyEvent.KEYCODE_BACK
				&& event.getAction() == KeyEvent.ACTION_DOWN) {

			if ((System.currentTimeMillis() - exitTime) > 2000) // System.currentTimeMillis()无论何时调用，肯定大于2000
			{
				Toast.makeText(getApplicationContext(), "再按一次退出程序",
						Toast.LENGTH_SHORT).show();
				exitTime = System.currentTimeMillis();
			} else {
				finish();
				System.exit(0);
			}

			return true;
		}
		return super.onKeyDown(keyCode, event);

	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

}