这可能很简单,但我似乎无法弄清楚错误是什么。我正在尝试创建一个展示Encapsulation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中:
<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>
我得到一个&#34; null对象引用&#34;错误,启动应用程序并通过defaultNumberOfEggs.setText(encapsulation.getNumberOfEggs());
设置文本时,应显示&#34; 10&#34; (不带引号)。
答案 0 :(得分:2)
您忘记初始化encapsulation
对象。默认情况下,它指向null。
所以你应该在课堂上定义它:
Encapsulation encapsulation = new Encapsulation();
或者在构造函数中:
MainActivity() {
encapsulation = new Encapsulation();
}
在调用super:
之后的onCreate
方法内部
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
encapsulation = new Encapsulation();
setContentView(R.layout.activity_main);
....
}
答案 1 :(得分:1)
在onCreate
方法中,将第一行添加为
encapsulation = new Encapsulation();