Question

我有这段代码。当我启动调试时，我收到错误

java.lang.RuntimeException：无法实例化活动 ComponentInfo {nicola.pokerstarsleaguecalculator / nicola.pokerstarsleaguecalculator.MainActivity}： java.lang.NullPointerException：尝试调用虚方法 'android.view.Window $ Callback android.view.Window.getCallback（）'上 null对象引用。

我想错误就在这里：

if (JTIscritti.getText().length()!=0)

我该如何纠正？感谢

public class MainActivity extends AppCompatActivity {
    public EditText JTIscritti = (EditText)findViewById(R.id.JTIscritti);
    public TextView JLPremiati = (TextView) findViewById(R.id.JLPremiati);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        JTIscritti.addTextChangedListener(new TextWatcher() {
        @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
            }

            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void afterTextChanged(Editable s) {
            premiati();
            }
            public void premiati() {
                if (JTIscritti.getText().length()!=0) {
                    PuntiLeague aPunti = new PuntiLeague();
                    String strIscritti = String.valueOf(JTIscritti.getText());
                    int iscritti = Integer.parseInt((strIscritti));
                    int premiati = aPunti.aPremio(iscritti);
                    String strPremiati = Integer.toString(premiati);
                    JLPremiati.setText(strPremiati);
                }
                else JLPremiati.setText("0");
            }
        });
    }
}

更新：我解决了谢谢@ΦXocę웃Пepeúpaツ和@VishnuSP移动一些声明+作业

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final EditText JTIscritti = (EditText)findViewById(R.id.JTIscritti);
    final TextView JLPremiati = (TextView) findViewById(R.id.JLPremiati);

Answer 1

你必须做这个初始化

public EditText JTIscritti = (EditText)findViewById(R.id.JTIscritti);
public TextView JLPremiati = (TextView) findViewById(R.id.JLPremiati);

在onCreate方法中，请记住在回调之前布局没有膨胀/初始化，因此 JTIscritti 和 JLPremiati 将被初始化为空引用，所以在空引用上执行此JTIscritti.getText()将抛出NPE！

Answer 2

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        JTIscritti = (EditText)findViewById(R.id.JTIscritti);
        JLPremiati = (TextView) findViewById(R.id.JLPremiati);

       //----your code/logic----
}

只有在

之后才会创建视图

super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

执行。之前`findViewById'将返回null。

空指针异常启动调试（无法实例化活动）。坏IF条件？

2 个答案: