当我调用FragmentTransactionadd(Fragmentfragment,Stringtag)时android在哪里添加fragment我写了这段代码，但看不到fragment的布局。它显示一个空白屏幕。@OverrideprotectedvoidonCreate(BundlesavedInstanceState){super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);Fragmentfragment=newTestFragment();FragmentTransactionft