Q: Which of the following subclasses of the InnerClass implementation ChildInnerClassX can be compiled and run?
class OutClass {
class InnerClass {
}
}
class ChildInnerClass1 extends OutClass.InnerClass {
}
class ChildInnerClass2 extends OutClass.InnerClass {
public ChildInnerClass2() {
super();
}
}
class ChildInnerClass3 extends OutClass.InnerClass {
public ChildInnerClass3(OutClass outClass) {
super();
}
}
class ChildInnerClass4 extends OutClass.InnerClass {
public ChildInnerClass3(OutClass outClass) {
outClass.super();
}
}
Copy the code
A: Only the ChildInnerClass4 subclasses will compile and run, the others will not compile. The constructor of an inherited class must have a reference to an object in the outer class and call super() from that reference. The constructor of an inherited class must have a reference to an object in the outer class and call super() from that reference. An external class cannot instantiate itself without first instantiating it.
Q: What is the result of the following program? Why is that?
List list1 = new ArrayList();
List list2 = new ArrayList(){};
List list3 = new ArrayList(){{}};
List list4 = new ArrayList(){{}{}{}};
/ / 1
System.out.println(list1.getClass() == list2.getClass());
/ / 2
System.out.println(list1.getClass() == list3.getClass());
/ / 3
System.out.println(list1.getClass() == list4.getClass());
/ / 4
System.out.println(list2.getClass() == list3.getClass());
/ / 5
System.out.println(list2.getClass() == list4.getClass());
/ / 6
System.out.println(list3.getClass() == list4.getClass());
Copy the codeAnswer: Program run returns 6 false.
First, list1 points to an ArrayList object instance; List2 points to an anonymous class inner class object that inherits from ArrayList; List3 also points to an anonymous inner class (with a pair of parentheses for initialization blocks) that inherits from ArrayList; List4 also points to an anonymous inner class that inherits from ArrayList (with multiple initialization blocks in parentheses); Since these anonymous inner classes are all in the same class, they compile as Demo$1, Demo$2, and Demo$3, so they are all equivalent to each other. You can verify this by using listx.getClass ().getName().
Q: What are the considerations (lessons) for using Java anonymous inner classes in development?
A: Common precautions are as follows.
-
When using anonymous inner classes, you must either inherit a class or implement an interface (you cannot do both and can only inherit a class or implement an interface).
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Constructors cannot be defined in anonymous inner classes and can be initialized by constructing code blocks.
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An anonymous inner class cannot have any static member variables or static methods.
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Anonymous inner classes are local inner classes, so any restrictions that apply to local inner classes also apply to anonymous inner classes.
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An anonymous inner class cannot be an abstract class and must implement either the inherited class or all the abstract methods that implement the interface.
Q: How are Java anonymous inner classes initialized when used?
A: Anonymous inner classes cannot be initialized by constructors, so we can only initialize them by constructing code blocks.
This is the sequel to Part 2 of the last Part of the internal class, please watch tomorrow’s push sequel
Relax with a joke
In the music class, the teacher played a piece of Beethoven. Xiao Ming asked Xiao Hua, “Do you know music?” Xiao Hua: Yes, Xiao Ming: Do you know what the teacher is playing? Xiao Hua: “Piano.”
Only in your smile, I just have to breathe.
Only in your smile, I can breathe.
