1.启动activity
Intent intent = new Intent(MainActivity.this,Other.class);
intent.putExtra("message", a+" + "+b+" = ");
startActivityForResult(intent, 1000);
2.重写onActivityResult 比较请求码和返回码
if(requestCode==1000 && resultCode==1001){}
3.返回结果
Intent intent = new Intent();
intent.putExtra("result", value);
setResult(1001, intent);
finish();