Android实现点击两次返回键退出

可以用Handler

public class MainActivity extends Activity{
    //定义一个变量，来标识是否退出
    private static boolean isExit=false;
 
    Handler handler=new Handler(){
        @Override
        public void handleMessage(Message msg){
            super.handleMessage(msg);
            isExit=false;
        }
    };
 
    @Override
    public boolean onKeyDown(int keyCode,KeyEvent event){
        if(keyCode==KeyEvent.KEYCODE_BACK){
            exit();
            return false;
        }
        return super.onKeyDown(keyCode,event);
    }
 
    private void exit(){
        if(!isExit){
            isExit=true;
            Toast.makeText(getApplicationContext(),"再按一
            退出程序",Toast.LENGTH_SHORT).show();
            //利用handler延迟发送更改状态信息
            handler.sendEmptyMessageDelayed(0,2000);
        }
        else{
            finish();
            System.exit(0);
        }
    }

还可以用系统秒数

public class MainActivity extends Activity{
 
    private long exitTime=0;
 
    @Override
    public boolean onKeyDown(int keyCode,KeyEvent event){
        if(keyCode==KeyEvent.KEYCODE_BACK){
            exit();
            return false;
        }
        return super.onKeyDown(keyCode,event);
    }
 
    private void exit(){
        if((System.currentTimeMillis()-exitTime)>2000){
            Toast.makeText(getApplicationContext(),
            "再按一次退出程序",Toast.LENGTH_SHORT).show();
            exitTime=System.currentTimeMillis();
        else{
            finish();
            System.exit(0);
        }
    }