break and switch appears to execute all case statements

问题

In the latest stable release of Java and Eclipse (Kempler), entering the following code and executing it, assuming the package and class names exist:

package some_package;

public class what_the_heck {

    public static void main(String[] args) {
        int p = 2;
        int x = 1;
        switch(p){  
            case (1):
                x--;
            case (2):
                x = 2;
            case (3):
                x = 3;
            default:
                x++;
        }
        System.out.println(x);
    }
}

This prints the value 4. Originally, I thought it should print 2 because I thought that even if there were no break statements, each piece of code is still held in a case statement. Now I think that the issue lies in how it is compiled. For example, my current belief is that internally a boolean keeps track of whether or not a case statement was equal to the value. If it was, then the boolean is true and all case statements will be treated as true until a break is found. This makes sense, but I am still wondering if there are other reasons behind this, or if I am entirely wrong in my assumption.

回答1:

The reason why switch works as it does is that this:

switch(p){  
        case (1):
            x--;
        case (2):
            x = 2;
        case (3):
            x = 3;
        default:
            x++;
    }

is really just syntactic sugar for this (basically):

if (p == 1)
    goto .L1;
else if (p == 2)
    goto .L2;
else if (p == 3)
    goto .L3;
else
    goto .L4;

.L1:
    x--;
.L2:
    x = 2;
.L3:
    x = 3;
.L4:
    x++;

Java doesn't have a goto statement, but C does, and that's where it comes from. So if p is 2, it jumps to .L2 and executes all the statements following that label.

回答2:

When you don't put a break the switch will execute all other cases that are underneath the entry point

So it actually executes

 x = 2;
 x = 3;
 x++;
 print(x);
 System.out.println(x);

Tks to Pshemo here is a link to the specification of the switch statement

回答3:

If a condition in a case block is true and there is no return or break, all the other case blocks will be executed regardless if the are true or not.

For a rule of thumb always put a break or return at the end of a case block and you will be 90% right.

switch (p) {
    case (1):
        x--;
        break;
    case (2):
        x = 2;
        break;
    case (3):
        x = 3;
        break;
    default:
        x++;
        break;
}

回答4:

If you want to skip all following cases you need to put a break; at last of this case block or all following case blocks are executed, too.

Have a look: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/switch.html

来源：https://stackoverflow.com/questions/19796711/break-and-switch-appears-to-execute-all-case-statements

标签

java

switch-statement

break