计算机代写|Java代写|Java’s Magic: The Bytecode

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  • Advanced Probability Theory 高等概率论
  • Advanced Mathematical Statistics 高等数理统计学
  • (Generalized) Linear Models 广义线性模型
  • Statistical Machine Learning 统计机器学习
  • Longitudinal Data Analysis 纵向数据分析
  • Foundations of Data Science 数据科学基础
计算机代写|Java代写|Java’s Magic: The Bytecode

计算机代写|Java代写|Java’s Magic: The Bytecode

The key that allowed Java to address both the security and the portability problems just described is that the output of a Java compiler is not executable code. Rather, it is bytecode. Bytecode is a highly optimized set of instructions designed to be executed by what is called the Java Virtual Machine (JVM), which is part of the Java Runtime Environment (JRE). In essence, the original JVM was designed as an interpreter for bytecode. This may come as a bit of a surprise because many modern languages are designed to be compiled into CPU-specific, executable code due to performance concerns. However, the fact that a Java program is executed by the JVM helps solve the major problems associated with web-based programs. Here is why.
Translating a Java program into bytecode makes it much easier to run a program in a wide variety of environments because only the JRE (which includes the JVM) needs to be implemented for each platform. Once a JRE exists for a given system, any Java program can run on it. Remember, although the details of the JRE will differ from platform to platform, all JREs understand the same Java bytecode. If a Java program were compiled to native code, then

different versions of the same program would have to exist for each type of CPU connected to the Internet. This is, of course, not a feasible solution. Thus, the execution of bytecode by the JVM is the easiest way to create truly portable programs.

The fact that a Java program is executed by the JVM also helps to make it secure. Because the JVM is in control, it manages program execution. Thus, it was possible for the JVM to create a restricted execution environment, called the sandbox, that contains the program, preventing unrestricted access to the machine. Safety is also enhanced by certain restrictions that exist in the Java language.

When a program is interpreted, it generally runs slower than the same program would run if compiled to executable code. However, with Java, the differential between the two is not so great. Because bytecode has been highly optimized, the use of bytecode enables the JVM to execute programs much faster than you might expect.

Although Java was designed as an interpreted language, there is nothing about Java that prevents on-the-fly compilation of bytecode into native code in order to boost performance. For this reason, the HotSpot JVM was introduced not long after Java’s initial release. HotSpot includes a just-in-time (JIT) compiler for bytecode. When a JIT compiler is part of the JVM, selected portions of bytecode are compiled into executable code in real time on a piece-bypiece demand basis. That is, a JIT compiler compiles code as it is needed during execution. Furthermore, not all sequences of bytecode are compiled-only those that will benefit from compilation. The remaining code is simply interpreted. However, the just-in-time approach still yields a significant performance boost. Even when dynamic compilation is applied to bytecode, the portability and safety features still apply because the JVM is still in charge of the execution environment.

One other point: There has been experimentation with an ahead-of-time compiler for Java. Such a compiler can be used to compile bytecode into native code prior to execution by the JVM, rather than on-the-fly. Some previous versions of the JDK supplied an experimental ahead-of-time compiler; however, JDK 17 has removed it. Ahead-of-time compilation is a specialized feature and it does not replace Java’s traditional approach just described. Because of the highly sophisticated nature of ahead-of-time compilation, it is not something that you will use when learning Java, and it is not discussed further in this book.

计算机代写|Java代写|Moving Beyond Applets

At the time of this writing, it has been more than two decades since Java’s original release. Over those years, many changes have taken place. At the time of Java’s creation, the Internet was a new and exciting innovation; web browsers were undergoing rapid development and refinement; the modern form of the smartphone had not yet been invented; and the near ubiquitous use of computers was still a few years off. As you would expect, Java has also changed and so, too, has the way that Java is used. Perhaps nothing illustrates the ongoing evolution of Java better than the applet.

As explained previously, in the early years of Java, applets were a crucial part of Java programming. They not only added excitement to a web page, they were a highly visible part of Java, which added to its charisma. However, applets rely on a Java browser plug-in. Thus, for an applet to work, it must be supported by the browser. Over the past few years support for the Java browser plug-in has been waning. Simply put, without browser support, applets are not viable. Because of this, beginning with JDK 9 , the phase-out of applets was begun, with support for applets being deprecated. In the language of Java, deprecated means that a feature is still available but flagged as obsolete. Thus, a deprecated feature should not be used for new code. The phase-out became complete with the release of JDK 11 because run-time support for applets was removed. Beginning with JDK 17, the entire Applet API was deprecated for removal, which means that it will be removed from the JDK at some point in the future.

As a point of interest, a few years after Java’s creation an alternative to applets was added. Called Java Web Start, it enabled an application to be dynamically downloaded from a web page. It was a deployment mechanism that was especially useful for larger Java applications that were not appropriate for applets. The difference between an applet and a Web Start application is that a Web Start application runs on its own, not inside the browser. Thus, it looks much like a “normal” application. It does, however, require that a stand-alone JRE that supports Web Start is available on the host system. Beginning with JDK 11, support for Java Web Start has been removed.

Given that neither applets nor Java Web Start are viable options for modern versions of Java, you might wonder what mechanism should be used to deploy a Java application. At the time of this writing, one part of the answer is to use the jlink tool added by JDK 9. It can create a complete run-time image that includes all necessary support for your program, including the JRE. Another part of the answer is the jpackage tool. Added by JDK 16, it can be used to create a ready-to-install application. As you might guess, deployment is a rather advanced topic that is outside the scope of this book. Fortunately, you won’t need to worry about deployment to use this book because all of the sample programs run directly on your computer. They are not deployed over the Internet.

计算机代写|Java代写|A Faster Release Schedule

Not long ago, another major change occurred in Java, but it does not involve changes to the language or the run-time environment. Rather, it relates to the way that Java releases are scheduled. In the past, major Java releases were typically separated by two or more years. However, subsequent to the release of JDK 9, the time between major Java releases has been decreased. Today, it is anticipated that a major release will occur on a strict time-based schedule, with the expected time between major releases being just six months.

Each major release, now called a feature release, will include those features ready at the time of the release. This increased release cadence enables new features and enhancements to be available to Java programmers in a timely fashion. Furthermore, it allows Java to respond quickly to the demands of an ever-changing programming environment. Simply put, the faster release schedule promises to be a very positive development for Java programmers.
At three-year intervals it is anticipated that a long-term support (LTS) release will take place. An LTS release will be supported (and thus remain viable) for a period of time longer than six months. The first LTS release was JDK 11. The second LTS release was JDK 17, for which this book has been updated. Because of the stability that an LTS release offers, it is likely that its feature-set will define a baseline of functionality for a number of years. Consult Oracle for the latest information concerning long-term support and the LTS release schedule.
Currently, feature releases are scheduled for March and September of each year. As a result, JDK 10 was released in March 2018, which was six months after the release of JDK 9. The next release (JDK 11) was in September 2018. It was an LTS release. This was followed by JDK 12 In March 2019, JDK 13 in September 2019, and so on. At the time of this writing, the latest release is JDK 17, which is an LTS release. Again, it is anticipated that every six months a new feature release will take place. Of course, you will want to consult the latest release schedule information.
At the time of this writing, there are a number of new Java features on the horizon. Because of the faster release schedule, it is very likely that several of them will be added to Java over the next few years. You will want to review the information and release notes provided by each sixmonth release in detail. It is truly an exciting time to be a Java programmer!

计算机代写|Java代写|Java’s Magic: The Bytecode


计算机代写|Java代写|Java’s Magic: The Bytecode

使 Java 能够解决刚才描述的安全性和可移植性问题的关键是 Java 编译器的输出不是可执行代码。相反,它是字节码。字节码是一组高度优化的指令,旨在由所谓的 Java 虚拟机 (JVM) 执行,它是 Java 运行时环境 (JRE) 的一部分。本质上,最初的 JVM 被设计为字节码的解释器。这可能有点令人惊讶,因为出于性能考虑,许多现代语言被设计为编译为特定于 CPU 的可执行代码。然而,Java 程序由 JVM 执行这一事实有助于解决与基于 Web 的程序相关的主要问题。这就是为什么。
将 Java 程序转换为字节码可以更轻松地在各种环境中运行程序,因为每个平台只需要实现 JRE(包括 JVM)。一旦给定系统存在 JRE,任何 Java 程序都可以在其上运行。请记住,尽管 JRE 的详细信息会因平台而异,但所有 JRE 都理解相同的 Java 字节码。如果 Java 程序被编译为本机代码,那么

对于连接到 Internet 的每种类型的 CPU,必须存在同一程序的不同版本。当然,这不是一个可行的解决方案。因此,JVM 执行字节码是创建真正可移植程序的最简单方法。

Java 程序由 JVM 执行的事实也有助于使其安全。因为 JVM 处于控制之中,所以它管理程序执行。因此,JVM 可以创建一个包含程序的受限执行环境(称为沙箱),从而防止对机器的无限制访问。Java 语言中存在的某些限制也增强了安全性。

当一个程序被解释时,它的运行速度通常比同一个程序编译成可执行代码时的运行速度要慢。但是,对于 Java,两者之间的差异并没有那么大。因为字节码已经高度优化,使用字节码使 JVM 执行程序的速度比您预期的要快得多。

尽管 Java 被设计为一种解释性语言,但 Java 并没有阻止将字节码动态编译为本机代码以提高性能。出于这个原因,HotSpot JVM 是在 Java 最初发布后不久推出的。HotSpot 包括一个用于字节码的即时 (JIT) 编译器。当 JIT 编译器是 JVM 的一部分时,字节码的选定部分会根据需要实时编译为可执行代码。也就是说,JIT 编译器在执行期间根据需要编译代码。此外,并非所有字节码序列都被编译——只有那些将从编译中受益的序列。剩下的代码被简单地解释。但是,即时方法仍然可以显着提高性能。即使将动态编译应用于字节码,

还有一点:已经对 Java 的提前编译器进行了试验。这样的编译器可用于在 JVM 执行之前将字节码编译为本机代码,而不是即时编译。JDK 的一些早期版本提供了一个实验性的提前编译器;但是,JDK 17 已将其删除。提前编译是一项专门的功能,它不会取代刚刚描述的 Java 的传统方法。由于提前编译的高度复杂性,它不是你在学习 Java 时会用到的东西,本书也不会进一步讨论它。

计算机代写|Java代写|Moving Beyond Applets

在撰写本文时,距离 Java 最初的发布已经过去了二十多年。这些年,发生了很多变化。在 Java 创建的时候,Internet 是一项新的、令人兴奋的创新。网络浏览器正在快速发展和完善;智能手机的现代形式尚未发明;计算机几乎无处不在的使用还需要几年的时间。正如您所料,Java 也发生了变化,Java 的使用方式也发生了变化。也许没有什么比小程序更能说明 Java 的持续发展了。

如前所述,在 Java 的早期,applet 是 Java 编程的关键部分。它们不仅为网页增添了兴奋感,而且是 Java 的一个高度可见的部分,这增加了它的魅力。但是,applet 依赖于 Java 浏览器插件。因此,要使小程序工作,它必须得到浏览器的支持。在过去几年中,对 Java 浏览器插件的支持一直在减弱。简单地说,没有浏览器支持,applet 是不可行的。正因为如此,从 JDK 9 开始,applet 的逐步淘汰开始,对 applet 的支持被弃用。在 Java 语言中,deprecated 意味着某个功能仍然可用,但被标记为已过时。因此,不推荐使用的功能不应用于新代码。由于删除了对小程序的运行时支持,因此随着 JDK 11 的发布而逐步淘汰。

有趣的是,在 Java 创建几年后,添加了一个小程序的替代方案。称为 Java Web Start,它使应用程序能够从网页动态下载。这是一种部署机制,对于不适合 applet 的大型 Java 应用程序特别有用。applet 和 Web Start 应用程序之间的区别在于,Web Start 应用程序独立运行,而不是在浏览器内运行。因此,它看起来很像一个“正常”的应用程序。但是,它要求主机系统上有一个支持 Web Start 的独立 JRE。从 JDK 11 开始,已删除对 Java Web Start 的支持。

鉴于 applet 和 Java Web Start 都不是现代 Java 版本的可行选项,您可能想知道应该使用什么机制来部署 Java 应用程序。在撰写本文时,部分答案是使用 JDK 9 添加的 jlink 工具。它可以创建一个完整的运行时映像,其中包括对您的程序的所有必要支持,包括 JRE。答案的另一部分是 jpackage 工具。由 JDK 16 添加,可用于创建可立即安装的应用程序。正如您可能猜到的那样,部署是一个相当高级的主题,超出了本书的范围。幸运的是,您无需担心部署即可使用本书,因为所有示例程序都直接在您的计算机上运行。它们没有部署在 Internet 上。

计算机代写|Java代写|A Faster Release Schedule

不久前,Java 发生了另一次重大变化,但它不涉及语言或运行时环境的变化。相反,它与 Java 发布的计划方式有关。过去,主要的 Java 版本通常相隔两年或更长时间。但是,在 JDK 9 发布之后,主要 Java 版本之间的时间缩短了。今天,预计主要版本将按照严格的时间安排发布,预计主要版本之间的时间仅为六个月。

每个主要版本,现在称为功能版本,都将包含在发布时准备好的功能。这种增加的发布节奏使 Java 程序员能够及时地使用新功能和增强功能。此外,它还允许 Java 快速响应不断变化的编程环境的需求。简而言之,更快的发布计划对 Java 程序员来说是一个非常积极的发展。
预计将每隔三年发布一次长期支持 (LTS) 版本。LTS 版本将在超过六个月的时间内得到支持(因此仍然可行)。第一个 LTS 版本是 JDK 11。第二个 LTS 版本是 JDK 17,本书对此进行了更新。由于 LTS 版本提供的稳定性,它的功能集很可能会定义多年的功能基线。有关长期支持和 LTS 发布计划的最新信息,请咨询 Oracle。
目前,功能发布计划在每年的 3 月和 9 月发布。结果,JDK 10 于 2018 年 3 月发布,比 JDK 9 发布时间晚了六个月。下一个版本(JDK 11)是在 2018 年 9 月。它是一个 LTS 版本。紧随其后的是 2019 年 3 月的 JDK 12,2019 年 9 月的 JDK 13,依此类推。在撰写本文时,最新版本是 JDK 17,它是一个 LTS 版本。同样,预计每六个月将发布一次新功能。当然,您需要查阅最新的发布时间表信息。
在撰写本文时,有许多新的 Java 特性即将出现。由于更快的发布计划,很可能在接下来的几年中,其中有几个会被添加到 Java 中。您需要详细查看每个六个月版本提供的信息和版本说明。成为一名 Java 程序员真的是一个激动人心的时刻!

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术语 广义线性模型(GLM)通常是指给定连续和/或分类预测因素的连续响应变量的常规线性回归模型。它包括多元线性回归,以及方差分析和方差分析(仅含固定效应)。



有限元是一种通用的数值方法,用于解决两个或三个空间变量的偏微分方程(即一些边界值问题)。为了解决一个问题,有限元将一个大系统细分为更小、更简单的部分,称为有限元。这是通过在空间维度上的特定空间离散化来实现的,它是通过构建对象的网格来实现的:用于求解的数值域,它有有限数量的点。边界值问题的有限元方法表述最终导致一个代数方程组。该方法在域上对未知函数进行逼近。[1] 然后将模拟这些有限元的简单方程组合成一个更大的方程系统,以模拟整个问题。然后,有限元通过变化微积分使相关的误差函数最小化来逼近一个解决方案。





随机过程,是依赖于参数的一组随机变量的全体,参数通常是时间。 随机变量是随机现象的数量表现,其时间序列是一组按照时间发生先后顺序进行排列的数据点序列。通常一组时间序列的时间间隔为一恒定值(如1秒,5分钟,12小时,7天,1年),因此时间序列可以作为离散时间数据进行分析处理。研究时间序列数据的意义在于现实中,往往需要研究某个事物其随时间发展变化的规律。这就需要通过研究该事物过去发展的历史记录,以得到其自身发展的规律。


多元回归分析渐进(Multiple Regression Analysis Asymptotics)属于计量经济学领域,主要是一种数学上的统计分析方法,可以分析复杂情况下各影响因素的数学关系,在自然科学、社会和经济学等多个领域内应用广泛。


MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中,其中问题和解决方案以熟悉的数学符号表示。典型用途包括:数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发,包括图形用户界面构建MATLAB 是一个交互式系统,其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题,尤其是那些具有矩阵和向量公式的问题,而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问,这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展,得到了许多用户的投入。在大学环境中,它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域,MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要,工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数(M 文件)的综合集合,可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。



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