java - Java 初始化的奇怪行为

Question

/**
 * Example Activity to demonstrate the lifecycle callback methods.
 */
public class ActivityA extends Activity {

    private String mActivityName;
    private TextView mStatusView;
    private TextView mStatusAllView;
    private StatusTracker mStatusTracker = StatusTracker.getInstance();
    private AlertDialog alertBox;
    private static int number = 0;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_a);

        init();

        String output = getString(R.string.on_create) + number++;
        showAlertDialog(alertBox, output);
        mStatusTracker.setStatus(mActivityName, output);
        Utils.printStatus(mStatusView, mStatusAllView);
    }

    @Override
    protected void onStart() {
        super.onStart();
        mStatusTracker.setStatus(mActivityName, getString(R.string.on_start));
        Utils.printStatus(mStatusView, mStatusAllView);
    }

    @Override
    protected void onRestart() {
        super.onRestart();
        mStatusTracker.setStatus(mActivityName, getString(R.string.on_restart));
        Utils.printStatus(mStatusView, mStatusAllView);
    }

    @Override
    protected void onResume() {
        super.onResume();
        mStatusTracker.setStatus(mActivityName, getString(R.string.on_resume));
        Utils.printStatus(mStatusView, mStatusAllView);
    }

    @Override
    protected void onPause() {
        super.onPause();
        mStatusTracker.setStatus(mActivityName, getString(R.string.on_pause));
        Utils.printStatus(mStatusView, mStatusAllView);
    }

    @Override
    protected void onStop() {
        super.onStop();
        mStatusTracker.setStatus(mActivityName, getString(R.string.on_stop));
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        mStatusTracker.setStatus(mActivityName, getString(R.string.on_destroy));
        mStatusTracker.clear();
    }

    public void startDialog(View v) {
        Intent intent = new Intent(ActivityA.this, DialogActivity.class);
        startActivity(intent);
    }

    public void startActivityB(View v) {
        Intent intent = new Intent(ActivityA.this, ActivityB.class);
        startActivity(intent);
    }

    public void startActivityC(View v) {
        Intent intent = new Intent(ActivityA.this, ActivityC.class);
        startActivity(intent);
    }

    public void finishActivityA(View v) {
        ActivityA.this.finish();
    }

    private void showAlertDialog(AlertDialog ad, String msg) {
        ad.setTitle("Logging Msg");
        ad.setMessage(msg);
        ad.show();
    }

    private void init() {
        System.out.println("Value = before &&&&&&&&& " + alertBox);
        if (alertBox == null) { // Why always null?
            alertBox = new AlertDialog.Builder(this).create();
            mActivityName = getString(R.string.activity_a);
            mStatusView = (TextView) findViewById(R.id.status_view_a);
            mStatusAllView = (TextView) findViewById(R.id.status_view_all_a);
            Log.e("ActivityA", "Init done **************************************");
            Log.e("ActivityA", "Init done **************************************");
            System.out.println("Value = after &&&&&&&&&&& " + alertBox);
        }
    }
}

仅在第一次调用 init 时，我设置了 alertBox 并且我认为 if block 不应在任何其他时间执行。但我错了，每次调用 init 时它都会运行。为什么？

我是Javascript出身，如果我把上面的代码翻译成JS(类似)，if block 只会求值一次。

Answer 1

alertBox 是类的成员变量，即类的每个实例都有自己的副本。因此，如果您有多个 ActivityA 实例，每个实例都有自己的 alertBox 最初都设置为 null。

如果你想分享alertBox你需要声明它为static:

private static AlertDialog alertBox;

尽管如此，这可能会导致多线程应用程序出现(额外的)问题，因此请谨慎使用!