eval（）解析器和2d数组在Java中的实现

我在两个与Java相关的问题上做了很多工作。一个简单，一个更难

关于2D数组的创建，我初始化了如下表：

private String [][] table_of_classifiers = null;

        String [][] table_of_classifiers = {
            {"x1","x","x","x","x"},
            {"x2","x","x","x","x"},
            {"x3","x","x","x","x"},
            {"x4","x","x","x","x"},
            {"x5","x","x","x","x"},
            {"x6","x","x","x","x"},
        };

private String [][] table_of_classifiers = null;

String [][] table_of_classifiers = {
  {"x1","x","x","x","x"},
  {"x2","x","x","x","x"},
  {"x3","x","x","x","x"},
  {"x4","x","x","x","x"},
  {"x5","x","x","x","x"},
  {"x6","x","x","x","x"},
};

然后，在一个函数中，我像这样填充它的内容：

private String [][] table_of_classifiers = null;

        String [][] table_of_classifiers = {
            {"x1","x","x","x","x"},
            {"x2","x","x","x","x"},
            {"x3","x","x","x","x"},
            {"x4","x","x","x","x"},
            {"x5","x","x","x","x"},
            {"x6","x","x","x","x"},
        };

private String [][] table_of_classifiers = null;

String [][] table_of_classifiers = {
  {"x1","x","x","x","x"},
  {"x2","x","x","x","x"},
  {"x3","x","x","x","x"},
  {"x4","x","x","x","x"},
  {"x5","x","x","x","x"},
  {"x6","x","x","x","x"},
};

但是，正如您可以猜到的，第二个表（本地）覆盖了第一个表，这当然不是我想要做的。我做错了什么？请注意，表格的维度从一开始就未知。

只需执行以下操作：

class Foo {
    private String [][] table_of_classifiers = null;

    void bar() {
        table_of_classifiers = new String[][] {
                    {"x1","x","x","x","x"},
                    {"x2","x","x","x","x"},
                    {"x3","x","x","x","x"},
                    {"x4","x","x","x","x"},
                    {"x5","x","x","x","x"},
                    {"x6","x","x","x","x"},
        };
    }
}

Java没有

eval

（因为它是一种编译语言），但它确实有。但是，这几乎肯定不是你想做的事情的最佳方法。

只要做：

class Foo {
    private String [][] table_of_classifiers = null;

    void bar() {
        table_of_classifiers = new String[][] {
                    {"x1","x","x","x","x"},
                    {"x2","x","x","x","x"},
                    {"x3","x","x","x","x"},
                    {"x4","x","x","x","x"},
                    {"x5","x","x","x","x"},
                    {"x6","x","x","x","x"},
        };
    }
}

---

Java没有

eval

（因为它是一种编译语言），但它确实有。但是，对于你想做的事情，这几乎肯定不是最好的方法。

关于你的第一个问题：要在不重新说明的情况下分配到

分类表，请写：

        table_of_classifiers = new String[][] {
            {"x1","x","x","x","x"},
            {"x2","x","x","x","x"},
            {"x3","x","x","x","x"},
            {"x4","x","x","x","x"},
            {"x5","x","x","x","x"},
            {"x6","x","x","x","x"},
        };

关于
eval
。问题是运行时没有作用域局部变量的名称，尽管它可以获取实例变量的名称，但它必须在对象的上下文中这样做。解决这类问题是可能的，但这不是小事，而且会涉及重大的妥协。我认为，在开始弄清楚
eval
将支持哪些功能之前，您必须彻底了解作用域和反射是如何工作的，因为否则，你只会对你提出的所有不可能的要求感到失望。
关于你的第一个问题：要分配到
分类表，而不重新说明它，请写：

        table_of_classifiers = new String[][] {
            {"x1","x","x","x","x"},
            {"x2","x","x","x","x"},
            {"x3","x","x","x","x"},
            {"x4","x","x","x","x"},
            {"x5","x","x","x","x"},
            {"x6","x","x","x","x"},
        };

关于
eval
。问题是运行时没有作用域局部变量的名称，尽管它可以获取实例变量的名称，但它必须在对象的上下文中这样做。解决这类问题是可能的，但这不是小事，而且会涉及重大的妥协。我认为，在开始弄清楚
eval
将支持哪些功能之前，您必须彻底了解作用域是如何工作的，以及反射是如何工作的，因为否则，您只会对您提供的所有不可能的需求感到失望

关于2D数组的创建，我初始化了如下表：

private String [][] table_of_classifiers = null;

        String [][] table_of_classifiers = {
            {"x1","x","x","x","x"},
            {"x2","x","x","x","x"},
            {"x3","x","x","x","x"},
            {"x4","x","x","x","x"},
            {"x5","x","x","x","x"},
            {"x6","x","x","x","x"},
        };

private String [][] table_of_classifiers = null;

String [][] table_of_classifiers = {
  {"x1","x","x","x","x"},
  {"x2","x","x","x","x"},
  {"x3","x","x","x","x"},
  {"x4","x","x","x","x"},
  {"x5","x","x","x","x"},
  {"x6","x","x","x","x"},
};

不是真的。这是变量的声明和初始化，该变量可以指向字符串的“2d数组”、“表格”或更精确的“数组数组”

除非处理变量可以/将为null的事实，否则将其初始化为
null
通常是个坏主意，因为您需要做额外的工作来检查null。示例：

String[][] a;
// ...
String b = a[0][0];

除非
a
在同一时间没有初始化，否则它不会编译。这是一件好事，因为您可以避免潜在的bug

String[][] a = null;
// ...
String b = a[0][0];

但是，这将被编译，如果您忘记实际为变量分配一个实数组，程序将“崩溃”并出现“空指针异常”，或者您需要添加额外的代码/工作来检查
null

我这样填写它的内容：

private String [][] table_of_classifiers = null;

        String [][] table_of_classifiers = {
            {"x1","x","x","x","x"},
            {"x2","x","x","x","x"},
            {"x3","x","x","x","x"},
            {"x4","x","x","x","x"},
            {"x5","x","x","x","x"},
            {"x6","x","x","x","x"},
        };

private String [][] table_of_classifiers = null;

String [][] table_of_classifiers = {
  {"x1","x","x","x","x"},
  {"x2","x","x","x","x"},
  {"x3","x","x","x","x"},
  {"x4","x","x","x","x"},
  {"x5","x","x","x","x"},
  {"x6","x","x","x","x"},
};

你不是在这里“填充”任何东西。对于要填充的内容，它必须首先存在，但您尚未创建任何内容

这里声明的是同名的第二个变量，只有在与第一个变量处于不同的作用域时才有可能，在这种情况下，如果最初可以从这个新作用域访问原始变量，则“隐藏”（“阴影”）原始变量

但正如您所猜测的，第二个表（本地）覆盖了第一个表
第一，那当然不是我想做的。我做错了什么

哪张“第一”桌？直到现在还没有第一个表，只有第一个变量。其他人已经向您展示了如何将“表”分配给原始变量，方法是不使用行首的“声明”
String[][]

否则就不可能说你“做错了”什么，因为你还没有真正解释你想做什么

请注意，表的维度从一开始就未知

不是吗？那么，您是如何/为什么使用数组文字的？文字数组用于创建具有固定“前缀”的固定大小数组

“开始”到底是什么意思？在编程时（编译时）或程序启动时（运行时），大小不是已知的吗

如果在运行时获得阵列的大小，则可以使用
new
创建普通阵列：

int a = ...; 
int b = ...; // Get the sizes from somewhere, e.g, user input 

String[][] table_of_classifiers = new String[a][b];
// Now you have an "empty" table

如果大小在运行时发生“变化”，那么（取决于您实际尝试的操作），数组是错误的工具，您应该使用
列表
实现，例如
ArrayList


关于“eval”，正如其他人所说，Java是一种编译语言，因此“eval”基本上是不可能的。这是“反射”或使用
类
类型来实现您所暗示的内容，但您确实需要更广泛地解释您试图实现的内容，然后在这里可能会对您有所帮助

然而，反射和类类型是一个复杂的问题，考虑到您显然在与最基本的Java概念作斗争，您还有很长的路要走，直到您能够做您想做的事情

关于2D数组的创建，我初始化了如下表：

private String [][] table_of_classifiers = null;

        String [][] table_of_classifiers = {
            {"x1","x","x","x","x"},
            {"x2","x","x","x","x"},
            {"x3","x","x","x","x"},
            {"x4","x","x","x","x"},
            {"x5","x","x","x","x"},
            {"x6","x","x","x","x"},
        };

private String [][] table_of_classifiers = null;

String [][] table_of_classifiers = {
  {"x1","x","x","x","x"},
  {"x2","x","x","x","x"},
  {"x3","x","x","x","x"},
  {"x4","x","x","x","x"},
  {"x5","x","x","x","x"},
  {"x6","x","x","x","x"},
};

不是真的。这是变量的声明和初始化，该变量可以指向字符串的“2d数组”、“表格”或更精确的“数组数组”

除非处理变量可以/将为null的事实，否则将其初始化为
null
通常是个坏主意，因为您需要做额外的工作来检查null。示例：

String[][] a;
// ...
String b = a[0][0];

这不会编译，除非
a