**一下提供伪代码，仅说明自己的理解，思路**/
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AActivity,BActivity,CActivity,DActivity 都extends ActivityGroup.
A1Activity,B1Activity,C1Activity,D1Activity 都extends Activity.
A2Activity,B2Activity,C2Activity,D2Activity 都extends Activity


AActivity.this.startActivity(A1Activity.class)
BActivity.this.startActivity(B1Activity.class)
CActivity.this.startActivity(C1Activity.class)
DActivity.this.startActivity(D1Activity.class)


A1Activity.this.startActivity(A2Activity.class)
B1Activity.this.startActivity(B2Activity.class)
C1Activity.this.startActivity(C2Activity.class)
D1Activity.this.startActivity(D2Activity.class)


A1Activity.this.getParent().startActivity(A2Activity.class)
B1Activity.this.getParent().startActivity(B2Activity.class)
C1Activity.this.getParent().startActivity(C2Activity.class)
D1Activity.this.getParent().startActivity(D2Activity.class)