java - 访问构造函数时出现 NullPointerException

Question

我觉得要么我错过了 OOP 中的一个要点，要么我的构造函数是错误的。创建对象时我不断收到空指针异常。

logcat 说它来自“//默认构造函数”的第一行。

这是我的类(class)的代码:

package com.chriswahlfeldt.homeworkapp;

        import android.app.Activity;
        import android.view.View;
        import android.widget.EditText;

public class MyHomework extends Activity {

    private EditText title, description;
    private View homeworkView, activityView;

    // default constructor
    public MyHomework() {

        homeworkView = getLayoutInflater().inflate(R.layout.add_homework, null);
        activityView = getLayoutInflater().inflate(R.layout.activity_my, null);

        title = (EditText) homeworkView.findViewById(R.id.classTitleET);
        title.setText("");

        description = (EditText) homeworkView.findViewById(R.id.descriptionET);
        title.setText("");

    }

    public View getContentView_activity_my() { return activityView; }

    public View getContentView_add_homework() { return homeworkView; }

    public String getTitleTxt() { return title.getText().toString(); }

    public String getDescriptionTxt() { return description.getText().toString(); }

    public void setTitleTxt(String thatString) { title.setText(thatString); }

    public void setDescriptionTxt(String thatString) { title.setText(thatString); }
}

这里是它的使用地方:

package com.chriswahlfeldt.homeworkapp;

        import android.app.Activity;
        import android.os.Bundle;
        import android.view.View;
        import android.widget.Button;

public class MyActivity extends Activity
{

    private MyHomework hW = new MyHomework();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(hW.getContentView_activity_my());

        final Button addHWBtn = (Button) findViewById(R.id.HWBtn);

        addHWBtn.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        setContentView(hW.getContentView_add_homework());
                    }
                });
    }
}

.xml 文件:activity_my.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context=".MyActivity"
android:id="@+id/mainRelLayout" >

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/HWBtn"
        android:textSize="25sp"
        android:paddingLeft="50dp"
        android:paddingRight="50dp"
        android:paddingTop="10dp"
        android:paddingBottom="10dp"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true"
        android:hint="+Homework"/>

</RelativeLayout> 

.xml 文件:add_homework.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent"
    android:alpha=".8">

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="textPersonName"
        android:ems="10"
        android:id="@+id/classTitleET"
        android:layout_gravity="top"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:capitalize="sentences"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="10dp"
        android:layout_marginBottom="10dp"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true"
        android:layout_marginRight="10dp"
        android:hint="Class Title"
        android:textSize="20sp" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="textMultiLine"
        android:ems="8"
        android:id="@+id/descriptionET"
        android:layout_below="@+id/classTitleET"
        android:layout_alignLeft="@+id/classTitleET"
        android:layout_alignStart="@+id/classTitleET"
        android:layout_alignRight="@+id/classTitleET"
        android:layout_alignEnd="@+id/classTitleET"
        android:hint="Homework Description"
        android:textSize="20sp"
        android:height="120dp"
        android:gravity="top" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/createBtn"
        android:hint="Create"
        android:textSize="20sp"
        android:layout_below="@+id/descriptionET"
        android:layout_alignLeft="@+id/descriptionET"
        android:layout_alignStart="@+id/descriptionET"
        android:layout_alignRight="@+id/descriptionET"
        android:layout_alignEnd="@+id/descriptionET" />

</RelativeLayout>

谢谢!

Answer 1

您不能直接调用 getLayoutInflater()，您需要在 LayoutInflater 类的实例上调用它。 试试这个

LayoutInflater inflater = (LayoutInflater) context.getSystemService( Context.LAYOUT_INFLATER_SERVICE );
homeworkView = inflater.inflate(R.layout.add_homework, null);

如果您在 Activity 中调用此内容，则可以将上下文替换为“this”