java - Intent 问题

Question

我在 Intents 方面遇到了一些困难，但我在这段代码中不断收到错误。我不断收到致命异常..

 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.bac1.problemcodereference/com.example.bac1.problemcodereference.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference 

我的项目代码:

public class MainActivity extends AppCompatActivity implements Serializable    {
    private String cPcode;
    Button help , sub;
    EditText codetext;
    Intent intent = new Intent (this, Main2Activity.class);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        sub = (Button) findViewById(R.id.submit);
        help = (Button) findViewById(R.id.help);



        sub.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String text = codetext.getText().toString();
                MainActivity onew = new MainActivity();
                onew.setcPcode(text);
                intent.putExtra("stuff",onew);
                startActivity(intent);
            }
        });}

Answer 1

在这一行

Intent intent = new Intent (this, Main2Activity.class);

您的 Activity 尚未完全构建，因此请在 oncreate 函数之后或内部使用此调用，因为 this 意味着 context 将为 null 因此出现异常

所以会这样

public class MainActivity extends AppCompatActivity implements Serializable    {
private String cPcode;
Button help , sub;
EditText codetext;
Intent intent;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    intent = new Intent (this, Main2Activity.class);
    sub = (Button) findViewById(R.id.submit);
    help = (Button) findViewById(R.id.help);



    sub.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String text = codetext.getText().toString();
            MainActivity onew = new MainActivity();
            onew.setcPcode(text);
            intent.putExtra("stuff",onew);
            startActivity(intent);
        }
    });}

另一个问题是为什么要传递 mainActivity 的新对象，所以更好的选择是创建一个单独的 POJO 类并使用它来存储数据