英文:
why can claim variables in same name?
问题
Java不允许使用相同名称的相同类型变量。
像这样的代码将会产生一个错误消息："重复的局部变量"
public class test123 {
public static void main(String[] args) {
int a=1;
int a=1;
}
}
但是对于这段代码，"int a" 在 while 循环中被声明了三次（因为这个 while 循环运行了3次，所以 "int a" 也被声明了3次），为什么编译器允许它通过编译？
public static void main(String[] args) {
int i=3;
while (i>0) {
int a=1;
System.out.println("1");
i--;
}
}
英文:
Java does not allow same type variable with same names.
Like this code will have an error message: "Duplicate local variable"
public class test123 {
public static void main(String[] args) {
int a=1;
int a=1;
}
}
but for this code, "int a" is claimed in the while loop three times(because this while loop run 3 times, so "int a" is claimed for 3 times also),
why the compiler allow it to pass compile?
public static void main(String[] args) {
int i=3;
while (i>0) {
int a=1;
System.out.println("1");
i--;
}
}
答案1
得分: 2
The scope of the loop resets each iteration, so a is removed from the stack and able to be declared again.
Note: this is valid, and maybe what you wanted.
int a = 1;
a = 1;
英文:
The scope of the loop resets each iteration, so a is removed from the stack and able to be declared again.
Note: this is valid, and maybe what you wanted.
int a = 1;
a = 1;
答案2
得分: 1
"Variable declaration" 在编译时发生，而不是在运行时。即使在循环内声明变量，变量也只声明一次。
在运行时，只存在一个变量 "a" 的副本，它在循环迭代期间既不创建也不销毁。
英文:
"Variable declaration" happens at compile time, not at run time. Even if you declare a variable within a loop, the variable is declared only once.
At run time, only one copy of your variable "a" exists, and it's neither created nor destroyed during the loop iteration.
答案3
得分: 0
类作用域： 被定义为静态的变量称为类级别变量，它们只会为特定类创建一次，然后在该类的整个生命周期中使用。
实例作用域： 这些变量在类级别被定义为非静态的。变量的作用域仅限于特定实例。如果创建了一个类的两个实例，那么每个实例都会有自己的变量和存储空间。
局部作用域： 这些变量在方法内部定义，或者作为方法的参数。这些变量的作用域仅限于该方法。无法在方法外部访问它们。
块作用域： 这些变量在方法中的特定块（if、while、for、switch 等）内定义。该变量只能在该块内访问，无法在该块外部访问。
在你的情况下，变量 i 是局部变量，因此它只在该特定方法中可用。变量 a 是块变量，其作用域仅限于 while 循环的单次迭代。一旦开始执行 while 循环，它将创建变量 a，并且在第一次迭代执行结束后将销毁该变量。因此，在下一次迭代中将重新执行相同的过程。
英文:
There is totally 4 sope of a variable.
Class scope: The variable those defined as static are called class level variables and it will be created only once for a particular class and the same will be used throughout the life of that class.
Instance scope: These the variable which is defined as non-ststic at the class level. The scope of the variable if for that particular instance. If you create 2 instances of a class than each instance will have there own variables and storage.
Local scope: These variables are defined inside the method or as a parameter of a method. The scope of these variables is for that method only. It can't be accessed outside the method.
Block scope: These variables are defined inside a particular block (if, while, for, switch etc) in a method. And the variable is accessed only inside that block, it will be assessable outside that block.
In your case the variable i is a local variable so it will be available for that particular method. variable a is a block variable and the scope of that variable if only for that while loop single iteration. Once one it starts executing the while loop, it will create the variable a and once the execution of the first iteration finished than it will destroy that variable. SO in the next iteration again the same process will be followed.
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