webview.goback()

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event){
        if(event.getAction()==KeyEvent.ACTION_DOWN){
            switch (keyCode){
                case KeyEvent.KEYCODE_BACK:
                    if(webView.canGoBack()){
                        webView.goBack();
                    }else{
                        finish();
                    }
                    return true;
            }
        }
        return super.onKeyDown(keyCode, event);
    }