Can a LABEL block be used without loop?

问题:

Can a LABEL block be used without loop? Any examples?

回答1:

Here is an example of using labels and break statements without a loop:

        block1: {
            if (a < 0) {
                break block1;
            }
            if (b < 0) {
                break block1;
            }
            return a + b;
        }

回答2:

But why use a label on a code block?

My application is usually a switch on a String variable (or anything other than char, byte, short, int, Enum).

"To make an omelette, you have to break some eggs"

Example:

String key = "scrambled";
eggs: {
    if ("do-nothing".equals(key)) break eggs;
    if ("scrambled".equals(key)) {
        ;//scramble code here
        break eggs;
    }
    if ("fried".equals(key)) {
        ;//fry code here
        break eggs;
    }
    //default behaviour goes here
    //or maybe throw an exception
}

Ok, ok, "Sometimes, to make an omelette, you have to kill a few people"

Alternatives:

• Java 7 allows String as the switch.
• An Enum workaround using MyEnum.valueOf(str) can be made to work.
• A switch on the String (or Object)'s hashcode plus some more if-then-else if is possible but would only make sense for lots of possibilities, in which case the whole thing is probably due for an overhaul.

回答3:

  public static void main(String[] args)
  {
    hello: break hello;
  }

回答4:

certainly:

private boolean isSafe(String data) {

    validation: {

        if (data.contains("voldemort")) {
            break validation;
        }
        if (data.contains("avada")) {
            break validation;
        }
        if (data.contains("kedavra")) {
            break validation;
        }
        return true;
    }
    return false;
}

@DragonBorn: that's not possible. You can only continue or break a label from within it's scope, for example:

    label1: for (int i = 0; i < 4; i++) {
        for (int j = 0; j < 4; j++) {
            System.out.println(i + " " + j);
            if (i == j) {
                continue label1;
            }
        }
    }

produces:

0 0
1 0
1 1
2 0
2 1
2 2
3 0
3 1
3 2
3 3

回答5:

If you want some unreadable code:

int i = 1;
int j = 1;
label: switch (i) {
case 1:
    switch (j) {
    case 1:
        break label;
    }
default:
    System.out.println("end");
}

Without break; will print "end". break label; will skip the print.