Android - Button with if condition : 

I am newbie in Android, and i am trying to make a one button open 2 activities but is not working for me.
for ex:
on `Mainacitivity`, there is `btn_mathematics` and `btn_physics` open the same activity (`Main2acitivity`) and find `btn_semester1` and `btn_semester2`, each button will open 2 other activities for the semester modules.

If the user on Mainacitivity clicked on:
`btn_mathematics` ---> `btn_semester1`---> will have `ModulesMAT`
and if clicked on the btn_semester1 same button: 
`btn_physics` ---> `btn_semester1` ---> will have `ModulesPHY` .


**MainActivity XML:**

    <Button
        android:id="@+id/btn_mathematics"
        android:onClick="btn_mathematics"
        android:text="@string/btn_mathematics/>

    <Button
        android:id="@+id/btn_physics"
        android:onClick="btn_physics"
        android:text="@string/btn_physics"/>


**Main2Activity XML:**

    <Button
        android:id="@+id/btn_semester1"
        android:onClick="btn_semester1"
        android:text="@string/btn_semester1"/>

    <Button
        android:id="@+id/btn_semester2"
        android:onClick="btn_s2"
        android:text="@string/btn_semester2"/>

I guess that no need to add xml for ModulesMAT and ModulesPHY, its pretty similar to the the others.


and now the java code:

**MainActivity.java:**

    @Override
    protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
        }

        public void btn_mathematics (View v)  {
            Intent intent = new Intent(MainActivity.this, Main2Activity.class);
            startActivity(intent);
        }

        `public void btn_physics (View v) {
            Intent intent = new Intent(MainActivity.this, Main2Activity.class);
            startActivity(intent);
        }


    }


**Main2Activity.java:**

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
    }



    public void btn_semester1 (View v)
    {
        Intent i = getIntent();
        String id = i.getStringExtra("id");
        if(id == "btn_mathematics")
        {
             i = new Intent(this, ModulesMAT.class);
            startActivity(i);
        }
        else if (id == "btn_physics")
        {
             i = new Intent(this, ModulesPHY.class);
            startActivity(i);
        }

    }


    public void btn_semester2 (View v)
    {
        Intent i = getIntent();
        String id = i.getStringExtra("id");
        if(id == "btn_mathematics")
        {
            i = new Intent(this, ModulesMAT2.class);
            startActivity(i);
        }
        else if (id == "btn_physics")
        {
            i = new Intent(this, ModulesPHY2.class);
            startActivity(i);
        }

    }
