活动在 setAdapter(ArrayAdapter) 上崩溃；

Question

尝试为 Android 编写我的第一个 ArrayAdapter，目前惨败。它在 setAdapter(adapter); 行崩溃并抛出 NullPointerException。

ContractTestActivity：

public class ContractTestActivity extends Activity {

    private ArrayList<Contract> contracts;
    public final String TAG = "ContractTest";
    //public Contract newContract = new Contract();

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);
        ListView list;
        list = (ListView)findViewById(R.id.list);
        ArrayAdapter<Contract> adapter = new ContractAdapter(this, android.R.layout.simple_list_item_1, contracts);
        list.setAdapter(adapter);
    }
}

ContractAdapter：

public class ContractAdapter extends ArrayAdapter<Contract> {


        private ArrayList<Contract> contracts;

        public ContractAdapter(Context context, int view, ArrayList<Contract> passedContracts) {
                super(context, view, passedContracts);
                contracts = passedContracts;
        }

        @Override
        public int getCount() {
            return contracts.size();
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
                View currentView = convertView;
                LayoutInflater currentViewInflater = (LayoutInflater)getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                currentView = currentViewInflater.inflate(android.R.layout.simple_list_item_1, null);
                Contract currentContract = contracts.get(position);
                TextView text = (TextView) currentView.findViewById(android.R.id.text1);
                text.setText(currentContract.getName());
                return currentView;
        }

}

合同：

public class Contract extends ContractTestActivity {

    private String name;
    private float payRate;
    private int hoursWorked;
    private int holidays;

    public Contract() {

    }

    public String getName() {
        return name;
    }

}

建议？

Answer 1

问题是您没有初始化 ArrayList

private ArrayList<Contract> contracts;

因此您收到错误 NullPointerException。

更新：

ContractTestActivity.java

public class ContractTestActivity extends Activity {
    private List<Contract> contracts = new ArrayList<Contract>();
    public final String TAG = "ContractTest";
    //public Contract newContract = new Contract();

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);
        ListView list;
        list = (ListView)findViewById(R.id.mylist);
        ArrayAdapter<Contract> adapter = new ContractAdapter(this, android.R.layout.simple_list_item_1, myContracts());
        list.setAdapter(adapter);
    }

    private List<Contract> myContracts(){

        List<Contract> list = new ArrayList<Contract>();

        list.add(new Contract("Friend1"));
        list.add(new Contract("Friend2"));
        list.add(new Contract("Friend3"));
        list.add(new Contract("Friend4"));
        return list;
    }
}

Contract.java

public class Contract {

    private String name;

    public Contract(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
}

ContractAdapter.java

public class ContractAdapter extends ArrayAdapter<Contract> {


    private List<Contract> contracts;

    public ContractAdapter(Context context, int view, List<Contract> passedContracts) {
            super(context, view, passedContracts);
            contracts = passedContracts;
    }

    @Override
    public int getCount() {
        return contracts.size();
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
            View currentView = convertView;
            LayoutInflater currentViewInflater = (LayoutInflater)getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            currentView = currentViewInflater.inflate(android.R.layout.simple_list_item_1, null);
            Contract currentContract = contracts.get(position);
            TextView text = (TextView) currentView.findViewById(android.R.id.text1);
            text.setText(currentContract.getName());
            return currentView;
    }
}

Answer 2

问题是您没有向 ArrayList 合约添加任何值。

 ArrayAdapter<Contract> adapter = new ContractAdapter(this, android.R.layout.simple_list_item_1, contracts);

在此行之前，您应该向 contracts 添加一些值，

因此向 List 添加值，如下所示。

 contracts = new ArrayList<Contract>();
 contracts.add(new Contract("name"));
 ArrayAdapter<Contract> adapter = new ContractAdapter(this,android.R.layout.simple_list_item_1, contracts);

现在，合约大小将返回 ArrayList 的长度。

Answer 3

ArrayAdapter<Contract> adapter = new ContractAdapter(this, android.R.layout.simple_list_item_1, contracts); 

此时合同还没有确定吗？

Answer 4

私有 ArrayList 合约 = ???

应该为它分配一些值...或者简单地分配一个 null，如下所示：

contracts = new ArrayList ;

Also try to String[] names = new String[] { "Linux", "Windows7", "Eclipse", "Suse",
                "Ubuntu", "Solaris", "Android", "iPhone"};
        // Create an ArrayAdapter, that will actually make the Strings above
        // appear in the ListView
        list.setAdapter(new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_1, names));

如果异常仍然存在...然后检查 xml 中列表视图的 Id。

这或许只是因为这个原因。

Answer 5

在main.xml中创建一个列表视图
以及 listview.xml 中的文本视图

public void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    t=(EditText)findViewById(R.id.editText1);
    lv=(ListView)findViewById(R.id.listView1);
    listItems = new ArrayList<String>();
    dataAdapter = new ArrayAdapter<String>(this, R.layout.listview,R.id.row);
    //row is id of textview
     lv.setAdapter(dataAdapter);

}

在按钮单击事件中

dataAdapter.add(t.getText().toString());

dataAdapter.notifyDataSetChanged();

，以便单击按钮时 edittext 的内容将添加到列表视图