What is wrong with using sending intent to another Activity?

Question

I what to send a bundle with an ArrayList, but my code causes a NullPointerException. But I don't know why.

Main Activity

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.act1);
    context = getApplicationContext();
    txtName = findViewById(R.id.name);
    txtDepartment = findViewById(R.id.department);
    txtUndergred = findViewById(R.id.undergred);
    txtUrl = findViewById(R.id.url);
    txtPhone = findViewById(R.id.phone);
    btnphone = findViewById(R.id.btnphone);
    btnlogin = findViewById(R.id.btnlogin);
    btnweb = findViewById(R.id.btnweb);

    btnlogin.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String name = txtName.getText().toString();
            String department = txtDepartment.getText().toString();
            String undergrad = txtUndergred.getText().toString();
            Bundle Sender = new Bundle();
            ArrayList list = new ArrayList<>();
            list.add(name);
            list.add(department);
            list.add(undergred);
            Sender.putStringArrayList("list", list);
            ArrayList shit = Sender.getStringArrayList("list");
            Intent intent = new Intent(getApplicationContext(),MainActivity2.class);
            intent.putExtra("Sender", Sender);
            startActivity(intent);
        }
    });
}

MainActivity2(receive here)

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.act2);
        txtUrl = findViewById(R.id.url);
        txtPhoneNum = findViewById(R.id.phoneNum);
        button0 = findViewById(R.id.goback);
        Intent passedIntent = getIntent();
        if (passedIntent != null) {
            Bundle mybundle = passedIntent.getBundleExtra("Sender");
            ArrayList list = mybundle.getStringArrayList("Sender");
            Toast.makeText(getApplicationContext(), "Student info: "
                    +list.get(0)+list.get(1)+list.get(2),duration).show();
        }
        String Url = txtUrl.getText().toString();
        String PhoneNum = txtPhoneNum.getText().toString();
}

Here, which part is wrong? Arraylist is added to the bundle but every time a NullPointerException is thrown.

Answer 1

The issue

---

You are passing a key and then trying to get the value with another key. This is wrong. To clear your understanding, let me give you an example →

Case: You have many tee-shirts in your shop. You ordered some tee-shirts to sale and got red and green tee-shirts only. Now, when a custom comes and he asks yellow tee-shirts, then you say that it is not there. That is called null in coding terms. This is the same case with you over here. Let's check the solution.

The solution

---

When you send a value with a key, receive it with same key. Just try this code.

MainActivity 2

super.onCreate(savedInstanceState);
        setContentView(R.layout.act2);
        txtUrl=findViewById(R.id.url);
        txtPhoneNum=findViewById(R.id.phoneNum);
        button0=findViewById(R.id.goback);
        Intent passedIntent= getIntent();
        if(passedIntent != null){
            Bundle mybundle= passedIntent.getBundleExtra("Sender");
            ArrayList list= mybundle.getStringArrayList("list");
            Toast.makeText(getApplicationContext(), "Student info: "
                    +list.get(0)+list.get(1)+list.get(2),duration).show();
        }
        String Url=txtUrl.getText().toString();
        String PhoneNum=txtPhoneNum.getText().toString();

The not related but helpful thing

---

Do not try to see the values in toast. I see that here:

Toast.makeText(getApplicationContext(), "Student info: "
                    +list.get(0)+list.get(1)+list.get(2),duration).show();

Try to debug or display it in log instead.