如何在对象上检查null,然后如果它为NULL则继续？

Question

在Java中,检查对象是否有价值或正在返回的最佳方法是null什么？我发现的大多数例子都不是很好.基本上我有这个代码:

mDBApi.getSession().setAccessTokenPair(reAuthTokens);
System.out.println(reAuthTokens);
if(reAuthTokens.equals(null)) {
    mDBApi.getSession().startAuthentication(Main.this);
    Log.e(TAG, "Keys not set -- I'm starting authentication");
}

我正在尝试reAuthTokens检查其价值,如果没有,请继续进行身份验证.但是,我只是NullPointerException在if语句行中获得了一个.有什么我可以做得更好吗？

__ _ __ _ __ _ __ _ OnCreate for rcook _ __ _ __ _ __ _ __ _ __ _ __ _ __ _ __

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    getWindow().requestFeature(Window.FEATURE_NO_TITLE);
    setContentView(R.layout.main);


    //AccessTokenPair tokens=null;      
    //AccessTokenPair tokens = getStoredKeys();   
    //System.out.println(access + "here I am"); 
    //clearKeys();
    //Log.e(TAG, "keys cleared");


    AppKeyPair appKeys = new AppKeyPair(APP_KEY, APP_SECRET);
    AndroidAuthSession session = new AndroidAuthSession(appKeys, ACCESS_TYPE);
    mDBApi = new DropboxAPI<AndroidAuthSession>(session);
    AccessTokenPair reAuthTokens = new AccessTokenPair(APP_KEY, APP_SECRET);
    mDBApi.getSession().setAccessTokenPair(reAuthTokens);

        System.out.println(reAuthTokens);

    if(reAuthTokens == null) {       
    mDBApi.getSession().startAuthentication(Main.this);
    Log.e(TAG, "Keys not set -- I'm starting authentication");



    }


//



    /*read settings*/
    mSettings = getSharedPreferences(PREFS_NAME, 0);
    boolean hide = mSettings.getBoolean(PREFS_HIDDEN, false);
    boolean thumb = mSettings.getBoolean(PREFS_THUMBNAIL, true);
    int space = mSettings.getInt(PREFS_STORAGE, View.VISIBLE);
    int color = mSettings.getInt(PREFS_COLOR, -1);
    int sort = mSettings.getInt(PREFS_SORT, 3);



    mFileMag = new FileManager();
    mFileMag.setShowHiddenFiles(true);
    mFileMag.setSortType(sort);

    if (savedInstanceState != null)
        mHandler = new EventHandler(Main.this, mFileMag, savedInstanceState.getString("location"));
    else
        mHandler = new EventHandler(Main.this, mFileMag);

    mHandler.setTextColor(color);
    mHandler.setShowThumbnails(thumb);
    mTable = mHandler.new TableRow();

    /*sets the ListAdapter for our ListActivity and
     *gives our EventHandler class the same adapter
     */
    mHandler.setListAdapter(mTable);
    setListAdapter(mTable);

    /* register context menu for our list view */
    registerForContextMenu(getListView());

    mStorageLabel = (TextView)findViewById(R.id.storage_label);
    mDetailLabel = (TextView)findViewById(R.id.detail_label);
    mPathLabel = (TextView)findViewById(R.id.path_label);
    mPathLabel.setText("path: /sdcard");

    updateStorageLabel();
    mStorageLabel.setVisibility(space);

    mHandler.setUpdateLabels(mPathLabel, mDetailLabel);

    /* setup buttons */
    int[] img_button_id = {R.id.help_button, R.id.home_button, 
                           R.id.back_button, R.id.info_button, 
                           R.id.manage_button, R.id.multiselect_button,
                           R.id.dropbox_button
                           };

    int[] button_id = {R.id.hidden_copy, R.id.hidden_attach,
                       R.id.hidden_delete, R.id.hidden_move};

    ImageButton[] bimg = new ImageButton[img_button_id.length];

    Button[] bt = new Button[button_id.length];

    for(int i = 0; i < img_button_id.length; i++) {
        bimg[i] = (ImageButton)findViewById(img_button_id[i]);
        bimg[i].setOnClickListener(mHandler);

        if(i < 4) {
            bt[i] = (Button)findViewById(button_id[i]);
            bt[i].setOnClickListener(mHandler);
        }
    }

    Intent intent = getIntent();

    if(intent.getAction().equals(Intent.ACTION_GET_CONTENT)) {
        bimg[5].setVisibility(View.GONE);
        mReturnIntent = true;

    } else if (intent.getAction().equals(ACTION_WIDGET)) {
        Log.e("MAIN", "Widget action, string = " + intent.getExtras().getString("folder"));
        mHandler.updateDirectory(mFileMag.getNextDir(intent.getExtras().getString("folder"), true));

    }
}

Answer 1

请if (reAuthTokens == null))改用.你不是要比较对象的内容; 你试图比较参考文献."是reAuthTokens指向同一地址null吗？"

编辑跟随OP的更新:reAuthTokens是类型的AccessTokenPair(我打赌许多读者最初认为这是List...我知道我做了).它在这一行中被实例化:

AccessTokenPair reAuthTokens = new AccessTokenPair(APP_KEY, APP_SECRET);

这就是为什么以下情况总是错误的:reAuthTokens == null.这就是为什么在编码时if (reAuthTokens == null)会出现"死代码"警告:编译器知道这种情况永远不会成立,因为你实例化reAuthTokens了几行.

因此,您所追求的比较类型不是参考,而是内容.你想检查是否reAuthTokens"空".但是从你引用的代码中没有意义.你怎么实例化对象,然后想检查它是否"空"？

我认为你的逻辑不对.您应该首先从您期望的位置获取访问令牌对(会话？),并将结果与​​之比较null.像这样的东西:

AccessTokenPair reAuthTokens = mDBApi.getSession().getAccessTokenPair();

if (reAuthTokens == null) {
    reAuthTokens = new AccessTokenPair(...);
    mDBApi.getSession().setAccessTokenPair(reAuthTokens);
}