尝试调用虚方法 (EditText)

Question

我希望我的应用程序在第一个 AlertDialog 中从 user 中获取用户名并将其发布到第二个 AlertDialog 上，但是当我添加以下代码行时，我的应用程序崩溃了：txt.setText(edt.getText().toString());。 它应该将 textview 更改为在第一个 AlertDialog 上给出的 edittext 值。 以下是错误。

java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable android.widget.EditText.getText()' on a null object reference
    at com.example.myapplication.MainActivity.Alert01(MainActivity.java:73)
    at com.example.myapplication.MainActivity$1.onClick(MainActivity.java:62)
    at android.support.v7.app.AlertController$ButtonHandler.handleMessage(AlertController.java:167)
    at android.os.Handler.dispatchMessage(Handler.java:105)
    at android.os.Looper.loop(Looper.java:164)
    at android.app.ActivityThread.main(ActivityThread.java:6541)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)











EditText edt;
TextView txt;

    @Override
    protected void onCreate(Bundle savedInstanceState) {


edt = (EditText) findViewById(R.id.edit_username);
txt = (TextView) findViewById(R.id.text02);


        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        SectionsPagerAdapter sectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager());
        ViewPager viewPager = findViewById(R.id.view_pager);
        viewPager.setAdapter(sectionsPagerAdapter);


        TabLayout tabs = findViewById(R.id.tabs);
        tabs.setupWithViewPager(viewPager);

Alert();
    } 


public void Alert () {

    final AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this, R.style.MyDialogTheme);

    View v = LayoutInflater.from(MainActivity.this).inflate(R.layout.dialog0, null);

    builder.setMessage("Welcome ! ");
    builder.setView(v);


builder.setPositiveButton("Submit", new DialogInterface.OnClickListener() {
    @Override
    public void onClick(DialogInterface dialog, int which) {

        Alert01();
    }
});  

    AlertDialog alert = builder.create();

    alert.show();

}

public void Alert01 (){

    txt.setText(edt.getText().toString());
        AlertDialog.Builder builder01 = new AlertDialog.Builder(MainActivity.this,R.style.MyDialogTheme);
        View v01 = LayoutInflater.from(MainActivity.this).inflate(R.layout.dialog1,null);
    builder01.setView(v01);


        builder01.setTitle("Congratulations ! ");
        AlertDialog alert = builder01.create();
        alert.show();
    }

}

Answer 1

您需要先对布局进行膨胀，然后在活动中调用 findViewById。

EditText edt是AlertDialog1布局的一部分TextView txt是AlertDialog2布局的一部分，所以你可以访问edt与AlertDialog1的参考和txt通过AlertDialog2的参考。

更新您的代码，如下所示：

    EditText edt;
    TextView txt;

        @Override
        protected void onCreate(Bundle savedInstanceState) {

             //  edt = (EditText) findViewById(R.id.edit_username);<--Remove this line from here 
             // txt = (TextView) findViewById(R.id.text02);<--Remoe this line from here and paste both findViewById below to setContentView(R.layout.activity_main) method.

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

            SectionsPagerAdapter sectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager());
            ViewPager viewPager = findViewById(R.id.view_pager);
            viewPager.setAdapter(sectionsPagerAdapter);


            TabLayout tabs = findViewById(R.id.tabs);
            tabs.setupWithViewPager(viewPager);

    Alert();
        } 


    public void Alert () {

        final AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this, R.style.MyDialogTheme);

        View v = LayoutInflater.from(MainActivity.this).inflate(R.layout.dialog0, null);

        builder.setMessage("Welcome ! ");
        builder.setView(v);
        edt = (EditText)v.findViewById(R.id.edit_username);

    builder.setPositiveButton("Submit", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {

            Alert01();
        }
    });  

        AlertDialog alert = builder.create();

        alert.show();

    }

    public void Alert01 (){
 // txt.setText(edt.getText().toString());<--Remove this line from here and set it below ...

            AlertDialog.Builder builder01 = new AlertDialog.Builder(MainActivity.this,R.style.MyDialogTheme);
            View v01 = LayoutInflater.from(MainActivity.this).inflate(R.layout.dialog1,null);
        builder01.setView(v01);

        txt = (TextView) v01.findViewById(R.id.text02);
        txt.setText(edt.getText().toString());

            builder01.setTitle("Congratulations ! ");
            AlertDialog alert = builder01.create();
            alert.show();
        }

    }

Answer 2

膨胀布局后在 Alert() 方法中移动此行

edt = (EditText) v.findViewById(R.id.edit_username);

同时修改 Alert01() 方法

    AlertDialog.Builder builder01 = new AlertDialog.Builder(MainActivity.this,R.style.MyDialogTheme);
    View v01 = LayoutInflater.from(MainActivity.this).inflate(R.layout.dialog1,null);
    builder01.setView(v01);
    txt = (TextView) v01.findViewById(R.id.text02);
    txt.setText(edt.getText().toString());