重写返回键

private long tempTime = 0;

/**
 * 双击退出
 */
@Override
public void onBackPressed() {
    long firstClick = System.currentTimeMillis();
    if (firstClick - tempTime < 500) {
        ToastUtil.cancel();
        super.onBackPressed();
    } else {
        ToastUtil.show(this, "再按一次退出");
    }
    tempTime = firstClick;
}

Toast工具类

package com.bitmain.btccom.utils;

import android.content.Context;
import android.widget.Toast;

/**
 * Created by kongqw on 2015/11/21.
 */
public final class ToastUtil {

    private static Toast mToast;

    // 工具类私有化
    private ToastUtil() {
    }

    // 单例模式 显示Toast
    public static void show(Context context, String text) {
        if (null == mToast) {
            mToast = Toast.makeText(context, "", Toast.LENGTH_SHORT);
        }
        mToast.setText(text);
        mToast.show();

    }

    // 关闭Toast
    public static void cancel() {
        if (null != mToast)
            mToast.cancel();
    }
}