判断当前Activityl里面显示的是哪个Fragment??

首先你建两个Fragment（AFragment和BFragment），定义一个boolean变量(isA=false)，默认显示在rameLayout区域显示AFragment,在
oncreat()方法里调用setDeafultFragment();

 private AFragment a ;
 private BFragment b ;
 private void setDeafultFragment(){
 if(a == null){
 a = new AFragment();
 }
 replaceFragmentWithSelected(a);
 isA = true;
 }

在点击按钮时判断isA是否为true，如果为true则显示BFragment,否则显示AFragment;

 public void onClick(View view){
 switch(view.getId()){
 case R.id.button:
 if(a == null){
 a = new AFragment();
 }
 if(b == null){
 b = new BFragment();
 }
        if(isA){
            replaceFragmentWithSelected(b);
            isA = false;
        }else{
            replaceFragmentWithSelected(a);
        }
        break;
}


}