What is different between be going to and will...??

As I know be going to and will has same meaning that is used to express future time. But, in the fact we can compare Be going to and will become 3 comparation to make us believe how Be going to and Will is used.

*To express a PREDICTION, When the speaker is making a prediction ( a statement about she/he thinks will be true or will occur in future) either Be going to and Will is possible.

{ jadi intinya jika kita memprediksikan sesuatu yang akan terjadi atau kegiatan yang kita ingin lakukan di masa yang akan datang maka kita bisa menggunakan Be going to atau Will karena keduanya mempunyai pengertian yang sama}
Ex:
1. According to the weather report, it will be cloudy tomorrow.
2. According to the weather reaport, it is going to cloudy tomorrow.
# There is no difference in meaning between 1 and 2

*To express a PRIOR PLAN, When the speaker is expressing a prior plan (Something the speaker intends to do in the future because in past she/he has made a plan or decision to do it), only Be going to is used.

{jika kita sudah mempunyai rencana yang diprioritaskan atau telah menentukan apa yang ingin kita lalukan dari masa lalu yang akan dikerjakan dimasa yang akan datang maka sebaiknya kita menggunakan Be going to}
Ex:
1. I talked to Bob yesterday. He is tried of taking the bus to work. He is going to buy a car.
# The speaker knows Bob's intention to buy a car. Bob made the decition in the past and he intends to act on this desition in the future. So, Will is not appropriate.

*To express a WILLINGNESS, When the speaker is expressing a willingness only will is possible.

Ex:
A: The phone's ringing

B: I will get it.

# Speker B is saying :"I am happy to get phone." He isn't making a prediction, He has made no prior to answer the phone. he is, insted,volunteering to answe the phone and uses will to show his willingness.