在封装过程中获取“空对象引用”

Question

这可能是愚蠢的简单，但我似乎找不出是什么问题。我正在尝试创建一个展示封装的演示。

Encapsulation.class：

public class Encapsulation {
// 'numberOfEggs' is an instance variable
// it was made 'private', because we do NOT want it to be directly accessed by other classes
// being 'private' also means that only code within the class can read or write its value
private int numberOfEggs = 10;

// getter
public int getNumberOfEggs() {
    return numberOfEggs;
}

// setter
public void setNumberOfEggs(int numberOfEggs) {
    if (numberOfEggs > 0) {
        // we used the 'this' keyword to differentiate between the method parameter 'currentNumberOfEggs' and
        //  the instance variable 'currentNumberOfEggs'
        this.numberOfEggs = numberOfEggs;
    }
}
}

MainActivity.class：

public class MainActivity extends AppCompatActivity {

TextView defaultNumberOfEggs;
Encapsulation encapsulation;

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

    defaultNumberOfEggs = (TextView) findViewById(R.id.default_number_of_eggs);
    defaultNumberOfEggs.setText(encapsulation.getNumberOfEggs());
}
}

activity_main.xml：

<RelativeLayout android:id="@+id/activity_main"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.vladp.studyandroid1.MainActivity">

<TextView
    android:id="@+id/default_number_of_eggs"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />
</RelativeLayout>

当启动应用程序并通过defaultNumberOfEggs.setText(encapsulation.getNumberOfEggs());设置文本时，我会得到一个“空对象引用”错误，它应该显示"10“(没有引号)。

Answer 1

在onCreate方法中，将第一行添加为

encapsulation = new Encapsulation();

Answer 2

您忘记初始化您的encapsulation对象。默认情况下，它指向null。

所以，要么在类级别上定义它：

Encapsulation encapsulation = new Encapsulation();

或在构造函数中：

MainActivity() {
    encapsulation = new Encapsulation();
}

或者在调用超级后的onCreate方法中：

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    encapsulation = new Encapsulation();
    setContentView(R.layout.activity_main);

    ....
}