java 具有最终未初始化字段的抽象类
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Abstract class with final uninitialized field
提问by Cantillon
I was wondering if the below code makes any sense, since the compiler warns that "the blank final field objects may not have been initialized". Is there a better way of doing this?
我想知道下面的代码是否有意义，因为编译器警告说“空白的最终字段对象可能尚未初始化”。有没有更好的方法来做到这一点？
public abstract Test {
protected final ArrayList<Object> objects;
}
public TestSubA extends Test {
public TestSubA() {
objects = new ArrayList<Objects>(20);
// Other stuff
}
}
public TestSubB extends Test {
public TestSubB() {
objects = new ArrayList<Objects>(100);
// Other stuff
}
}
采纳答案by Jon Skeet
I would make the field final and force the constructors to pass the value up:
我会将字段设为 final 并强制构造函数向上传递值：
public abstract class Test {
private final ArrayList<Object> objects;
protected ArrayList<Object> getObjects() {
return objects;
}
protected Test(ArrayList<Object> objects) {
this.objects = objects;
}
}
public class TestSubA extends Test {
public TestSubA() {
super(new ArrayList<Object>(20));
// Other stuff
}
}
public class TestSubB extends Test {
public TestSubB() {
super(new ArrayList<Object>(100));
// Other stuff
}
}
回答by qwertzguy
The problem with initializing the final parameters directly in the constructor of the sub-classes is that you need to do it all in one line since super() must be the first statement of the constructor. So instead, I prefer to make the constructor non-public and make a static build method like this:
直接在子类的构造函数中初始化最终参数的问题在于，您需要在一行中完成所有操作，因为 super() 必须是构造函数的第一条语句。所以相反，我更喜欢将构造函数设为非公开，并创建一个像这样的静态构建方法：
public abstract class Test {
protected final ArrayList<Object> objects;
protected Test(ArrayList<Object> objects) {
this.objects = objects;
}
}
public class TestSubA extends Test {
public static TestSubA build() {
ArrayList<Object> objects = new ArrayList<Object>(20);
objects.put(...);
// Other stuff
return new TestSubA(objects);
}
private TestSubA(ArrayList<Object> objects) {
super(objects);
}
}
回答by ctrlShiftBryan
Instantiate the objects in the abstract class constructor and just pass the difference to the that constructor.
实例化抽象类构造函数中的对象，并将差异传递给该构造函数。
回答by Uri
Generally speaking, it might be better to have a constructor in the base class that always sets the field, and not have a default constructor that doesn't set it. The subclasses can then explicitly pass the parameter in the first line of their constructor using super(value)
一般来说，最好在基类中有一个总是设置字段的构造函数，而不是没有设置它的默认构造函数。然后子类可以使用 super(value) 在其构造函数的第一行中显式传递参数
