private boolean isExit;
    private Handler handler;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        handler = new Handler(){
            @Override
            public void handleMessage(Message msg) {
                super.handleMessage(msg);
                isExit = false;
            }
        };
    }
　　public boolean onKeyDown(int keyCode, KeyEvent event) {
        if(keyCode==KeyEvent.KEYCODE_BACK){
            if(!isExit){
                isExit = true;
                handler.sendEmptyMessageDelayed(0, 1500);
                Toast.makeText(this, "再按一次推出程序", 1000).show();
                return false;
            }else{
                finish();
            }
        }
        return super.onKeyDown(keyCode, event);
    }

private long exitTimeMillis = System.currentTimeMillis();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if(keyCode==KeyEvent.KEYCODE_BACK){
            long currentTime = System.currentTimeMillis();
            if(currentTime-exitTimeMillis==0||currentTime-exitTimeMillis>1500){
                exitTimeMillis = System.currentTimeMillis();
                Toast.makeText(this, "再按一次推出程序", 1000).show();
                return false;
            }else{
                finish();
            }
        }
        return super.onKeyDown(keyCode, event);
    }