1. Given

class Booby {
class Dooby extends Booby {
class Tooby extends Dooby {

public class TestClass {
  Booby b = new Booby();
  Tooby t = new Tooby();
  public void do1(List<? super Dooby> dataList) {
  public void do2(List<? extends Dooby> dataList) {

and the following four statements:

  1. b = dataList.get(0);
  2. t = dataList.get(0);
  3. dataList.add(b);
  4. dataList.add(t);

What can be inserted in the above code?

  • Statements 1 and 3 can inserted at //1 and Statements 2 and 4 can be inserted at //2.
  • Statement 4 can inserted at //1 and Statement 1 can be inserted at //2.
  • Statements 3 and 4 can inserted at //1 and Statements 1 and 2 can be inserted at //2.
  • Statements 1 and 2 can inserted at //1 and Statements 3 and 4 can be inserted at //2.
  • Statement 1 can inserted at //1 and Statement 4 can be inserted at //2.


  1. addData1(List<? super Dooby> dataList)
    This means that dataList is a List whose elements are of a class that is either Dooby or a super class of Dooby. We don’t know which super class of Dooby. Thus, if you try to add any object to dataList, it has to be a assignable to Dooby.
    Thus, dataList.add(b); will be invalid because b is not assignable to Dooby.
    Further, if you try to take some object out of dataList, that object will be of a class that is either Dooby or a Superclass of Dooby. Only way you can declare a variable that can be assigned the object retrieved from dataList is Object obj. Thus, t = dataList.get(0); and b = dataList.get(0); are both invalid.

  2. addData2(List<? extends Dooby> dataList)
    This means that dataList is a List whose elements are of a class that is either Dooby or a subclass of Dooby. Since we don’t know which subclass of Dooby is the list composed of, there is no way you can add any object to this list.
    If you try to take some object out of dataList, that object will be of a class that is either Dooby or a subclass of Dooby and thus it can be assigned to a variable of class Dooby or its superclass.. Thus, t = dataList.get(0); is invalid.

A type argument $T_1$ is said to contain another type argument $T_2$, written $T_2 <= T_1$, is the set of types denoted by $T_2$ is provably a subset of the set of types denoted by $T_1$ under the reflexive and transitive closure of the following rules(where $<:$ denotes subtyping($\S4.10$)):

  • $?\space extends\space T<=\space ?\space extends \space S$ if $T <: S$
  • $?\space extends \space T<=\space ?$
  • $?\space super \space T<=\space ?\space super \space S$ if $T <: S$
  • $?\space super \space T<=\space ?$
  • $?\space super \space T<=\space ? \space extends \space Object$
  • $ T<=\space T$
  • $T <= \space ? \space extends \space T$
  • $T <= \space ? \space super \space T$


OCP 11-1 Book  2020年3月22日

1. Which of the following is an advantage of the Java Platform Module System? B

A. A central repository of all modules
B. Encapsulating packages
C. Encapsulating objects
D. No defined types
E. Platform independence

2. Which statement is true of the following module? D

|-- staff
|-- Vet.java

A. The directory structure shown is a valid module.
B. The directory structure would be a valid module if module.java were added directly underneath zoo.staff.
C. The directory structure would be a valid module if module.java were added directly underneath zoo.
D. The directory structure would be a valid module if module-info.java were added directly underneath zoo.staff.
E. The directory structure would be a valid module if module-info.java were added directly underneath zoo.
F. None of these changes would make this directory structure a valid module.

解释: Modules are required to have a module-info.java file at the root directory of the module. Option D matches this requirement.

3. B

4. D -> G The -m or –module option is used to specify the module and class name. The -p or -module-path option is used to specify the location of the modules. Option D would be correct if the rest of the command were correct. However, running a program requires specifying the package name with periods (.) instead of slashes. Since the command is incorrect, option G is correct.

5. AF -> AFG Options C and D are incorrect because there is no use keyword. Options A and F are correct because opens is for reflection and uses declares an API that consumes a service. Option G is also correct as the file can be completely empty. This is just something you have to memorize.

6. BDF -> BC Packages inside a module are not exported by default, making option B correct and option A incorrect. Exporting is necessary for other code to use the packages; it is not necessary to call the main() method at the command line, making option C correct and option D incorrect. The module-info.java file has the correct name and compiles, making options E and F incorrect.

7. EF -> DG Options A, B, E, and F are incorrect because they refer to keywords that don’t exist. The requires transitive keyword is used when specifying a module to be used by the requesting module and any other modules that use the requesting module. Therefore, dog needs to specify the transitive relationship, and option G is correct. The module puppy just needs to require dog, and it gets the transitive dependencies, making option D correct.


RSocket与Spring Security简单整合

Spring Tips 学习记录  2020年3月10日


  • greetings-service

    在 start.spring.io 选择 2.3.0 M2 版本 Spring Boot,依赖项如下



    Spring Security

  • greetings-client




    Spring Security




OCP-1Z0-816 模拟测试1回顾

题目比较多  2020年2月26日

4. Given:

Path p1 = Paths.get("c:\\temp\\test1.txt");
Path p2 = Paths.get("c:\\temp\\test2.txt");

Which of the following code fragments moves the file test1.txt to test2.txt, even if test2.txt exists?

  • Files.move(p1, p2);

    This will throw a java.nio.file.FileAlreadyExistsException if the file already exists.

  • Files.move(p1, p2, StandardCopyOption.REPLACE_EXISTING);
  • try(Files.move(p1, p2)) { }

    Files.move returns a Path object (of the destination file), which is not a resource that can be closed because it does not implement AutoCloseable interface. So this will not compile.

  • try(Files.copy(p1, p2, StandardCopyOption.REPLACE_EXISTING)) { Files.delete(p1); }
  • Files.copy(p1, p2, StandardCopyOption.REPLACE_EXISTING); Files.delete(p1);

Files.copy method will copy the test1.txt into test2.txt. If test2.txt doesn’t exist, it will be created. However, Files.isSameFile method doesn’t check the contents of the file. It is meant to check if the two path objects resolve to the same file or not. In this case, they are not, and so, it will return false.
The following is brief JavaDoc description for both the methods:
public static Path copy(Path source, Path target, CopyOption... option) throws IOException



1. Unmodifiable collections using of/copyOf and Collections.unmodifiableXXX methods

java.util.List and java.util.Set have of and copyOf static factory methods that provide a convenient way to create unmodifiable lists/sets.

The of methods accept either an array or multiple individual parameters. If you pass it a collection, it will be treated as a regular object i.e. it will return a list/set containing the same collection object instead of returning a list/set containing the objects that the passed collection contains.

The copyOf, on the other hand, accepts only a Collection. It iterates through the passed Collection and adds all the elements of that Collection in the returned list/set.



1. Which of the following annotations are retained for run time?

  • @SuppressWarnings

    It is defined with @Retention(SOURCE)

  • @Override

    It is defined with @Retention(SOURCE)

  • @SafeVarargs

    It is defined with @Retention(RUNTIME)

  • @FunctionalInterface

    It is defined with @Retention(RUNTIME)

  • @Deprecated

    It is defined with @Retention(RUNTIME)

2. Your application needs to load a set of key value pairs from a database table which never changes. Multiple threads need to access this information but none of them changes it.
Which class would be the most appropriate to store such data if the values need not be kept in a sorted fashion?

  • Hashtable
  • HaspMap
  • Set
  • TreeMap
  • List

You should know that all Hashtable methods are synchronized and this compromises its performance for simultaneous reads.
Since not thread modifies the data, it is not efficient to use a Hashtable.
A HashMap is a perfect choice because its methods are not synchronized and so it allows efficient multiple reads. TreeMap is used to keep the keys sorted which makes it a little bit slower than HashMap.
Set and List can’t be used since we need to store Key-value pairs.


Exercises for Inner class

内部类章节的练习  2020年2月15日

  1. What is an inner class? Differentiate between member, local, and anonymous inner classes.

    An inner class declares inside a top-level class without a static modifier. Inner classes that declared at member level are called member inner classes, local to the method, or instance initialization block are local inner class. And local inner classes that have no name are anonymous inner classes.



学习笔记  2020年2月12日


段落不需要特别的格式,直接写就是了。 然后呢



例如    保留了前边的空格,



Java 内部类

Inner Class  2020年2月4日


  • 内部类是什么
  • 如何声明内部类
  • 如何声明成员、局部,以及匿名内部类
  • 如何创建内部类的对象



// TopLevel.java
package io.zwt.innerclasses;

public class TopLevel {
    private int value = 101;
    public int getValue() {
        return value;
    public void setValue(int value) {
        this.value = value;

上面的 TopLevel 类是 io.zwt.innerclasses 包的成员。 这个里有三个成员:

  • 一个实例变量(instance variable):value
  • 两个方法(method):getValue() and setValue()

一个类也可以声明在另一个类里面。这种类称为内部类。如果定义在另一个类中的类显式或隐式被声明为static,则它被称为嵌套类,而不是内部类。包含内部类的类称为外层类外部类。看一下下面的 OuterInner 类声明:

// Outer.java
pakcage io.zwt.innerclasses;

public class Outer {
	public class Inner {
		// 内部类的成员在这里
	// 外部类的其它成员在这里




Builder Pattern in Depth  2020年2月2日

GoF 定义

Separate the construction of a complex object from its representation so that the same construction processes can create different representations.


Builder 在《设计模式》的中文版里边翻译为“生成器”,那我就按这个译法吧。生成器模式属于创建型模式(Creational patterns),它关注如何创建对象。当需要构建的对象比较复杂,由多个部分组成,也就说它的构造方法会有很多参数,就可以考虑使用这种模式。生成器模式认为对象的构建机制应该独立于它的组成部分(也就是属性),对象的构建过程不关注对象的组成部分。所以同一个构建过程可以构建出不同表示(属性)的对象(通过改变构建步骤)。