Is there a goto statement in Java?_问答_开发者

I'm confused about this. Most of us have been told that there isn't any goto statement in Java.

But I found that it is one 开发者_C百科of the keywords in Java. Where can it be used? If it can not be used, then why was it included in Java as a keyword?

James Gosling created the original JVM with support of goto statements, but then he removed this feature as needless. The main reason goto is unnecessary is that usually it can be replaced with more readable statements (like break/continue) or by extracting a piece of code into a method.

Source: James Gosling, Q&A session

The Java keyword list specifies the goto keyword, but it is marked as "not used".

It was in the original JVM (see answer by @VitaliiFedorenko), but then removed. It was probably kept as a reserved keyword in case it were to be added to a later version of Java.

If goto was not on the list, and it gets added to the language later on, existing code that used the word goto as an identifier (variable name, method name, etc...) would break. But because goto is a keyword, such code will not even compile in the present, and it remains possible to make it actually do something later on, without breaking existing code.

The keyword exists, but it is not implemented.

The only good reason to use goto that I can think of is this:

for (int i = 0; i < MAX_I; i++) {
    for (int j = 0; j < MAX_J; j++) {
        // do stuff
        goto outsideloops; // to break out of both loops
    }
}
outsideloops:

In Java you can do this like this:

loops:
for (int i = 0; i < MAX_I; i++) {
    for (int j = 0; j < MAX_J; j++) {
        // do stuff
        break loops;
    }
}

http://java.sun.com/docs/books/tutorial/java/nutsandbolts/_keywords.html

"The keywords const and goto are reserved, even though they are not currently used. "

So they could be used one day if the language designers felt the need.

Also, if programmers from languages that do have these keywords (eg. C, C++) use them by mistake, then the Java compiler can give a useful error message.

Or maybe it was just to stop programmers using goto :)

They are reserved for future use (see: Java Language Keywords)

The keywords const and goto are reserved, even though they are not currently used.

The reason why there is no goto statement in Java can be found in "The Java Language Environment":

Java has no goto statement. Studies illustrated that goto is (mis)used more often than not simply "because it's there". Eliminating goto led to a simplification of the language--there are no rules about the effects of a goto into the middle of a for statement, for example. Studies on approximately 100,000 lines of C code determined that roughly 90 percent of the goto statements were used purely to obtain the effect of breaking out of nested loops. As mentioned above, multi-level break and continue remove most of the need for goto statements.

An example of how to use "continue" labels in Java is:

public class Label {
    public static void main(String[] args) {
        int temp = 0;
        out: // label
        for (int i = 0; i < 3; ++i) {
            System.out.println("I am here");
            for (int j = 0; j < 20; ++j) {
                if(temp==0) {
                    System.out.println("j: " + j);
                    if (j == 1) {
                        temp = j;
                        continue out; // goto label "out"
                    }
                }
            }
        }
        System.out.println("temp = " + temp);
    }
}

Results:

I am here // i=0
j: 0
j: 1
I am here // i=1
I am here // i=2
temp = 1

It is important to understand that the goto construct is remnant from the days that programmers programmed in machine code and assembly language. Because those languages are so basic (as in, each instruction does only one thing), program control flow is done completely with goto statements (but in assembly language, these are referred to as jump or branch instructions).

Now, although the C language is fairly low-level, it can be thought of as very high-level assembly language - each statement and function in C can easily be broken down into assembly language instructions. Although C is not the prime language to program computers with nowadays, it is still heavily used in low level applications, such as embedded systems. Because C's function so closely mirrors assembly language's function, it only makes sense that goto is included in C.

It is clear that Java is an evolution of C/C++. Java shares a lot of features from C, but abstracts a lot more of the details, and therefore is simply written differently. Java is a very high-level language, so it simply is not necessary to have low-level features like goto when more high-level constructs like functions, for, for each, and while loops do the program control flow. Imagine if you were in one function and did a goto to a label into another function. What would happen when the other function returned? This idea is absurd.

This does not necessarily answer why Java includes the goto statement yet won't let it compile, but it is important to know why goto was ever used in the first place, in lower-level applications, and why it just doesn't make sense to be used in Java.

Because it's not supported and why would you want a goto keyword that did nothing or a variable named goto?

Although you can use break label; and continue label; statements to effectively do what goto does. But I wouldn't recommend it.

public static void main(String [] args) {

     boolean t = true;

     first: {
        second: {
           third: {
               System.out.println("Before the break");

               if (t) {
                  break second;
               }

               System.out.println("Not executed");
           }

           System.out.println("Not executed - end of second block");
        }

        System.out.println("End of third block");
     }
}

No, goto is not used, but you can define labels and leave a loop up to the label. You can use break or continue followed by the label. So you can jump out more than one loop level. Have a look at the tutorial.

No, thankfully, there isn't goto in Java.

The goto keyword is only reserved, but not used (the same goes for const).

No, goto is not used in Java, despite being a reserved word. The same is true for const. Both of these are used in C++, which is probably the reason why they're reserved; the intention was probably to avoid confusing C++ programmers migrating to Java, and perhaps also to keep the option of using them in later revisions of Java.

Yes is it possible, but not as nice as in c# (in my opinion c# is BETTER!). Opinions that goto always obscures software are dull and silly! It's sad java don't have at least goto case xxx.

Jump to forward:

 public static void main(String [] args) {
   myblock: {
     System.out.println("Hello");
     if (some_condition)
       break myblock; 
     System.out.println("Nice day");
   }
   // here code continue after performing break myblock
   System.out.println("And work");
 }

Jump to backward:

 public static void main(String [] args) {
   mystart: //here code continue after performing continue mystart
   do {
     System.out.println("Hello");
     if (some_condition)         
       continue mystart; 
     System.out.println("Nice day");
   } while (false);
   System.out.println("And work");
 }

Note that you can replace most of the benign uses of goto by

return
break
break label
throw inside try-catch-finally

As was pointed out, there is no goto in Java, but the keyword was reserved in case Sun felt like adding goto to Java one day. They wanted to be able to add it without breaking too much code, so they reserved the keyword. Note that with Java 5 they added the enum keyword and it did not break that much code either.

Although Java has no goto, it has some constructs which correspond to some usages of goto, namely being able to break and continue with named loops. Also, finally can be thought of as a kind of twisted goto.

To prohibit declarations of variables with the same name.

e.g. int i = 0, goto;

It's very much considered one of those things you Do Not Do, but was probably listed as a reserved word to avoid confusion for developers.

http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-6.html#jvms-6.5.goto

If you have been told that there is no goto statement in Java you have been fooled. Indeed, Java consists two layers of 'source' code.

See the following link is shows all java reserved words and tells you what versions they where added.

http://java.sun.com/docs/books/tutorial/java/nutsandbolts/_keywords.html

goto is reserved, even though it is not currently used, never say never however :)

I'm not a fan of goto either, as it usually makes code less readable. However I do believe that there are exceptions to that rule (especially when it comes to lexers and parsers!)

Of Course you can always bring your program into Kleene Normalform by translating it to something assembler-like and then write something like

int line = 1;
boolean running = true;
while(running)
{
    switch(line++)
    {
        case 1: /* line 1 */
                break;
        case 2: /* line 2 */
                break;
        ...
        case 42: line = 1337; // goto 1337
                break;
        ...
        default: running = false;
                break;
    }
}

(So you basically write a VM that executes your binary code... where line corresponds to the instruction pointer)

That is so much more readable than code that uses goto, isn't it?

Of course it is keyword, but it is not used on level of source code.

But if you use jasmin or other lower level language, which is transformed to bytecode, then "goto" is there

Because although the Java language doesn't use it, JVM bytecode does.

goto is not in Java

you have to use GOTO But it don't work correctly.in key java word it is not used. http://docs.oracle.com/javase/tutorial/java/nutsandbolts/_keywords.html

   public static void main(String[] args) {            
            GOTO me;
            //code;
            me:
            //code; 
            }   
   }