Android onItemSelected - NullPointer

Question

以下代码抛出 NullPointerException ：

public class test extends Activity implements OnItemSelectedListener {
    private TextView explanation;

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

        explanation = (TextView) findViewById(R.id.picker_explanation_text);

        Spinner spinner = (Spinner) findViewById(R.id.spinner);
        ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(
                this, R.array.picker_array,
                android.R.layout.simple_spinner_item);
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinner.setOnItemSelectedListener(new test());
        spinner.setAdapter(adapter);
    }

    @Override
    public void onItemSelected(AdapterView<?> parent, View view, int pos,
            long id) {
        explanation = (TextView) findViewById(R.id.picker_explanation_text);
    }
}

由于explanation = (TextView) findViewById(R.id.picker_explanation_text);方法中的onItemSelected(...)引发了它，但我不明白为什么。毕竟，它是在onCreate(...)方法中工作的。

Answer 1

而不是

    spinner.setOnItemSelectedListener(new test());

使用

    spinner.setOnItemSelectedListener(this);

您希望将您的真实活动用作目标;你正在创建一个永远不会被上下文初始化的新对象，因此当它被调用时会崩溃。