将意图发送到其他活动有什么错？

Question

我知道如何使用ArrayList发送捆绑包，但我的代码会导致NullPointerException。但我不知道为什么。

主要活动

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);
        }
    });
}

维护活动2（此处接收）

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();
}

这里，哪一部分错了Arraylist被添加到捆绑包中，但每次抛出NullPointerException时。

Answer 1

你在传递一个键，然后试图用另一个键获取值，这是错误的。为了清楚你的理解，让我给你举个例子→

凯斯：你店里有很多T恤衫。你订购了一些T恤衫出售，只得到了红色和绿色T恤衫。现在，当一个顾客来问他黄色T恤时，你说它不在那里。这在编码术语中称为null。你这边也是这样。让我们检查一下解决方案。

当你用一个键发送一个值时，用同一个键接收它。试试这个代码。

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();

不要试图看到toast中的值。我在这里看到：

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

尝试调试或在日志中显示它。