工具类DoubleClickExitHelper

public class DoubleClickExitHelper {  
    private final Activity mActivity;  
    private boolean isOnKeyBacking;  
    private Handler mHandler;  
    private Toast mBackToast;  
      
    public DoubleClickExitHelper(Activity activity) {  
        mActivity = activity;  
        mHandler = new Handler(Looper.getMainLooper());  
    }  
      
    /** 
     * Activity onKeyDown事件 
     * */  
    public boolean onKeyDown(int keyCode, KeyEvent event) {  
        if(keyCode != KeyEvent.KEYCODE_BACK) {  
            return false;  
        }  
        if(isOnKeyBacking) {  
            mHandler.removeCallbacks(onBackTimeRunnable);  
            if(mBackToast != null){  
                mBackToast.cancel();  
            }  
            mActivity.finish();  
            return true;  
        } else {  
            isOnKeyBacking = true;  
            if(mBackToast == null) {  
                mBackToast = Toast.makeText(mActivity, "再按一次返回", 2000);  
            }  
            mBackToast.show();  
            //延迟两秒的时间，把Runable发出去  
            mHandler.postDelayed(onBackTimeRunnable, 2000);  
            return true;  
        }  
    }  
      
    private Runnable onBackTimeRunnable = new Runnable() {  
          
        @Override  
        public void run() {  
            isOnKeyBacking = false;  
            if(mBackToast != null){  
                mBackToast.cancel();  
            }  
        }  
    };  
}  

接下来我们只需要在回调onKeyDown中加入该工具方法即可，当然要初始化一下

DoubleClickExitHelper doubleClick = new DoubleClickExitHelper(this);  

最后重写重写onKeyDown:

 @Override  
    public boolean onKeyDown(int keyCode, KeyEvent event) {  
  
        if (event.getKeyCode() == KeyEvent.KEYCODE_BACK) {  
            return doubleClick.onKeyDown(keyCode, event);  
        }  
        return super.onKeyDown(keyCode, event);  
    }  

转载至：http://javaeedyc.iteye.com/blog/2165098

最近又发现一个更简单的方法

private long mPreTime = 0;

   @Override
    public void onBackPressed() {
        if (System.currentTimeMillis() - mPreTime > 2000) {
            Toast.makeText(this, "再按一次退出应用", Toast.LENGTH_SHORT).show();
            mPreTime = System.currentTimeMillis();
            return;
        } else {
            finish();
            super.onBackPressed();
        }
    }