为什么我在写';'时没有收到任何警告或错误？

Question

I have written these three lines below inside my Java code:

ArrayList<String> QuestionID = new ArrayList<String>();
;
ArrayList<String> QuestionType = new ArrayList<String>();

It compiles and runs perfect without any problems.

Why does it do so?

I cannot understand why I am not getting any warning or error in second line.

Answer 1

Somebody decided that an empty statement is valid. That's all there is to it. Allowing empty statements is sometimes useful, for instance for for loops with empty bodies.

As an example, here's how to find the root node of a tree (assuming there's a parent linkage, and that the root node has no parent):

for (Node parent = anyChildNode; parent.getParent() != null; parent = parent.getParent())
    ; // this for loop has no body, so an empty statement takes its place

Answer 2

Because you are writing an empty statement , which is perfectly valid.

Here you will find some more explanation.

From the above link -

The usefulness of this type of statement is limited. The main use that I can think of is to fulfill the statement required for a loop structure.

Here is an example that I recently used:

while ( sf(++n) != i) ;

This loop will constantly call the method sf with increasing values of n until the return value of sf(n) is equal to i. Each loop in Java must have some code to execute in the loop body. In this case, all necessary work is done in the condition, and so the mandatory loop body is an empty statement.

While there may be other (more clear) ways of writing this bit of code, this is an example of where that empty statement can be used.

Answer 3

Not only is it allowed, but it has its own section in the JLS . As expected:

An empty statement does nothing.

Even more:

Execution of an empty statement always completes normally.

Which means that the following code:

;

will never throw an exception. Good to know ;-)

Answer 4

#if DEBUG
    #define ASSERT(_x) Assert(x)
#else
    #define ASSERT(_x)
#endif

ASSERT(test);    // Results in null statement in non-debug builds

Empty semicolon means there is "empty statement" before that. Thats perfectly valid case.

; means "execute nothing";

Good article about usage of empty statement

Answer 5

";" is considered as a line code without any instructions.

There's no syntax error.

Answer 6

它是一个有效的语句，类似于指示编译器什么都不做。

Answer 7

Empty statement came to Java from C where you could find it very useful for things like

strcpy(char *s, char *t) {
    while(*s++ = *t++);
}

Answer 8

Empty Semi Colon是JAVA中的有效令牌，它表示空语句。