java - 尝试封装时获取 "Null Object Reference"

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 6 年前。

这可能非常简单，但我似乎无法弄清楚出了什么问题。我正在尝试创建一个演示来展示封装。

封装类:

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>

启动应用程序并通过 defaultNumberOfEggs.setText(encapsulation.getNumberOfEggs()); 设置文本时，我收到“空对象引用”错误，应显示“10”(不带引号)。

Answer 1

您忘记初始化封装对象。默认情况下，它指向 null。

所以你应该在类级别定义它:

封装 encapsulation = new Encapsulation();

或者在构造函数中:

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

或者在调用 super 之后的 onCreate 方法中:

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

    ....
}