Java Final变量是否具有默认值？ [英] Will Java Final variables have default values?
问题描述
我有这样的程序：
class Test {
final int x;
{
printX（）;
}
Test（）{
System.out.println（const called）;
}
void printX（）{
System.out.println（这里x是+ x）;
}
public static void main（String [] args）{
Test t = new Test（）;
}
}
如果我尝试执行它，我收到编译器错误：变量x可能尚未初始化基于java默认值我应该得到以下输出吗？
这里x为0。
最终变量是否具有dafault值？
如果我这样更改我的代码，
class Test {
final int x;
{
printX（）;
x = 7;
printX（）;
}
Test（）{
System.out.println（const called）;
}
void printX（）{
System.out.println（这里x是+ x）;
}
public static void main（String [] args）{
Test t = new Test（）;
}
}
我输出为：
这里x是0
这里x是7
const叫做
任何人都可以解释这种行为..
http：// docs。 oracle.com/javase/tutorial/java/javaOO/initial.html ，初始化实例成员一章：
Java编译器将初始化程序块复制到每个构造函数中。
也就是说：
< pre class =lang-java prettyprint-override>
{
printX（）;
}
Test（）{
System.out.println（const called）;
}
表现如下：
Test（）{
printX（）;
System.out.println（const called）;
}
正如您可以看到的，一旦创建了实例，最后的字段尚未 明确分配 ，而（来自 http://docs.oracle.com/javase/ specs / jls / se7 / html / jls-8.html＃jls-8.3.1.2 ）：
空白决赛实例变量必须在
处明确赋值，该类的每个构造函数的末尾都是
声明的;否则会发生编译时错误。
虽然在文档中似乎没有明确说明（至少我没有能够找到它），一个final字段必须临时在构造函数结束之前取其默认值，以便它有一个可预测值 ，如果您在分配之前阅读它。
默认值： http://docs.oracle.com/javase/specs/jls/ se7 / html / jls-4.html＃jls-4.12.5
在你的第二个片段中，x在实例创建时被初始化，所以编译器会不要抱怨：
Test（）{
printX（）;
x = 7;
printX（）;
System.out.println（const called）;
}
另请注意以下内容方法不起作用。使用final变量的默认值只能通过方法。
Test（）{
System.out.println（这里x是+ x）; //编译时错误：变量'x'可能未初始化
x = 7;
System.out.println（这里x是+ x）;
System.out.println（const called）;
}
I have a program like this:
class Test {
final int x;
{
printX();
}
Test() {
System.out.println("const called");
}
void printX() {
System.out.println("Here x is " + x);
}
public static void main(String[] args) {
Test t = new Test();
}
}
If I try to execute it, i am getting compiler error as : variable x might not have been initialized based on java default values i should get the below output right??
"Here x is 0".
Will final variables have dafault values?
if I change my code like this,
class Test {
final int x;
{
printX();
x = 7;
printX();
}
Test() {
System.out.println("const called");
}
void printX() {
System.out.println("Here x is " + x);
}
public static void main(String[] args) {
Test t = new Test();
}
}
I am getting output as:
Here x is 0
Here x is 7
const called
Can anyone please explain this behavior..
http://docs.oracle.com/javase/tutorial/java/javaOO/initial.html, chapter "Initializing Instance Members":
The Java compiler copies initializer blocks into every constructor.
That is to say:
{
printX();
}
Test() {
System.out.println("const called");
}
behaves exactly like:
Test() {
printX();
System.out.println("const called");
}
As you can thus see, once an instance has been created, the final field has not been definitely assigned, while (from http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.3.1.2):
A blank final instance variable must be definitely assigned at the end of every constructor of the class in which it is declared; otherwise a compile-time error occurs.
While it does not seem to be stated explitely in the docs (at least I have not been able to find it), a final field must temporary take its default value before the end of the constructor, so that it has a predictable value if you read it before its assignment.
Default values: http://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.12.5
On your second snippet, x is initialized on instance creation, so the compiler does not complain:
Test() {
printX();
x = 7;
printX();
System.out.println("const called");
}
Also note that the following approach doesn't work. Using default value of final variable is only allowed through a method.
Test() {
System.out.println("Here x is " + x); // Compile time error : variable 'x' might not be initialized
x = 7;
System.out.println("Here x is " + x);
System.out.println("const called");
}
这篇关于Java Final变量是否具有默认值？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
