java - 这个 Java String 数组声明有什么问题 - 非常奇怪

Question

我有三种不同的方式在 Java 中声明我的字符串数组。其中一个始终有效，但另外两个仅根据写入顺序起作用。

在第一个版本中，方法2不起作用，但方法3起作用。

public class weirdStringArray {

    String [] method1 = new String [] {"one","two","three"}; //<------ Works no matter where it's placed

    String [] method2;       //<------ "Syntax error on token ";", { expected after this token"
    method2 = new String[3]; //  Doesn't work in this postion
    method2[0] = "one";
    method2[1] = "two";
    method2[2] = "three";

    String [] method3 = new String[3]; //<-------  No issue, works fine in this position
    method3[0] = "one";                
    method3[1] = "two";
    method3[2] = "three";
} // <----- Syntax error, insert "}" to complete ClassBody (but it does seem to complete ClassBody...?)

但是，交换位置和工作声明也会交换。看，现在方法2可以工作，但方法3不行。

public class weirdStringArray {

    String [] method1 = new String [] {"one","two","three"};

    String [] method3 = new String[3]; //<------ "Syntax error on token ";", { expected after this token"
    method3[0] = "one";               //  Doesn't work in this postion
    method3[1] = "two";
    method3[2] = "three";

    String [] method2; //<---------- Put it in a different place and it works
    method2 = new String[3]; 
    method2[0] = "one";
    method2[1] = "two";
    method2[2] = "three";

} // <----- Syntax error, insert "}" to complete ClassBody (but it does seem to complete ClassBody...?)

这里可能发生什么？为什么顺序会产生任何影响？位置 2 发生了什么？顺便说一句，如果我删除第一个工作表单，也没有什么区别:

public class weirdStringArray {

    //String [] method1 = new String [] {"one","two","three"};

    String [] method2;        //<------ "Syntax error on token ";", { expected after this token"
    method2 = new String[3]; //  Doesn't work in this postion 
    method2[0] = "one";
    method2[1] = "two";
    method2[2] = "three";

    String [] method3 = new String[3]; //<-------  No issue, works fine in this position
    method3[0] = "one";               
    method3[1] = "two";
    method3[2] = "three";

} // <----- Syntax error, insert "}" to complete ClassBody (but it does seem to complete ClassBody...?)

Answer 1

尝试这样做，让它按照您的预期工作:

public class weirdStringArray {

    String [] method1 = new String [] {"one","two","three"}; //<-- Works no matter where it's placed, because this is a valid one line initialisation

    String [] method2;
    {
        // We gave it the '{' the compiler asked for …
        method2 = new String[3];
        method2[0] = "one";
        method2[1] = "two";
        method2[2] = "three";
    }

    String [] method3 = new String[3];
    {
        method3[0] = "one";                
        method3[1] = "two";
        method3[2] = "three";
    } // <-- … and this is the missing '}'
}

这些“{…}” block 是初始化 block ，更广为人知/更常在其静态变体中使用，如 的“static {…}”静态最终 属性。这是因为这些初始化程序中的代码通常最好放入构造函数中。

使用这些初始化程序的优点是它们在所有构造函数之前执行，缺点是它们在所有构造函数之前执行。当您使用 final 属性时效果最好......

它们适用于所有常量初始化(如此处的示例)以及其他类型(可能)永不失败的初始化。应避免从此处调用非静态方法，并且调用当前类的非静态方法根本不起作用(this 尚未初始化，因为您当前正在初始化它......)。