尝试封装时获得 "Null Object Reference"

Question

这可能非常简单，但我似乎无法弄清楚哪里出了问题。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>

启动应用程序并通过 defaultNumberOfEggs.setText(encapsulation.getNumberOfEggs()); 设置文本时出现 "null object reference" 错误，它应该显示“10”（不带引号）。

Answer 1

在onCreate方法中，添加第一行作为

encapsulation = new Encapsulation();

Answer 2

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

所以你应该在 class 级别上定义它：

Encapsulation encapsulation = new Encapsulation();

或者在构造函数中：

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

或调用 super 后在 onCreate 方法内：

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

    ....
}

尝试封装时获得 "Null Object Reference"

Getting "Null Object Reference" While Trying Encapsulation

java

android

encapsulation