Branching Statements - Java Tutorial 5.0 英文版

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Trail: Learning the Java Language
Lesson: Language Basics

Branching Statements

The Java programming language supports the following branching statements:

The break statement
The continue statement
The return statement
The break and continue statements, which are covered next, can be used with or without a label. A label is an identifier placed before a statement; it is followed by a colon (:).
statementName: someStatement;
An example of a label within the context of a program is shown in the next section.

The break Statement

The break statement has two forms — unlabeled and labeled. The unlabeled form of the break statement was used with switch earlier. As noted there, an unlabeled break terminates the enclosing switch statement, and flow of control transfers to the statement immediately following the switch. You can also use the unlabeled form of the break statement to terminate a for, while, or do-while loop. The following sample program, BreakDemo, contains a for loop that searches for a particular value within an array.
public class BreakDemo {
    public static void main(String[] args) {

        int[] arrayOfInts = { 32, 87, 3, 589, 12, 1076,
                              2000, 8, 622, 127 };
        int searchfor = 12;

        int i = 0;
        boolean foundIt = false;

        for ( ; i < arrayOfInts.length; i++) {
            if (arrayOfInts[i] == searchfor) {
                foundIt = true;
                break;
            }
        }

        if (foundIt) {
            System.out.println("Found " + searchfor
                               + " at index " + i);
        } else {
            System.out.println(searchfor
                               + "not in the array");
        }
    }
}
The break statement, shown in boldface, terminates the for loop when the value is found. The flow of control transfers to the statement following the enclosing for, which is the print statement at the end of the program.
This is the output of the program.
Found 12 at index 4
The unlabeled form of the break statement is used to terminate the innermost switch, for, while, or do-while statement; the labeled form terminates an outer statement, which is identified by the label specified in the break statement. The following program, BreakWithLabelDemo, is similar to the previous one, but it searches for a value in a two-dimensional array. Two nested for loops traverse the array. When the value is found, a labeled break terminates the statement labeled search, which is the outer for loop.
public class BreakWithLabelDemo {
    public static void main(String[] args) {

        int[][] arrayOfInts = { { 32, 87, 3, 589 },
                                { 12, 1076, 2000, 8 },
                                { 622, 127, 77, 955 }
                              };
        int searchfor = 12;

        int i = 0;
        int j = 0;
        boolean foundIt = false;

    search:
        for ( ; i < arrayOfInts.length; i++) {
            for (j = 0; j < arrayOfInts[i].length; j++) {
                if (arrayOfInts[i][j] == searchfor) {
                    foundIt = true;
                    break search;
                }
            }
        }

        if (foundIt) {
            System.out.println("Found " + searchfor +
                               " at " + i + ", " + j);
        } else {
            System.out.println(searchfor
                               + "not in the array");
        }

    }
}
This is the output of the program.
Found 12 at 1, 0
This syntax can be a little confusing. The break statement terminates the labeled statement; it does not transfer the flow of control to the label. The flow of control is transferred to the statement immediately following the labeled (terminated) statement.

The continue Statement

The continue statement is used to skip the current iteration of a for, while , or do-while loop. The unlabeled form skips to the end of the innermost loop's body and evaluates the boolean expression that controls the loop, basically skipping the remainder of this iteration of the loop. The following program, ContinueDemo, steps through a string buffer, checking each letter. If the current character is not a p, the continue statement skips the rest of the loop and proceeds to the next character. If it is a p, the program increments a counter and converts the p to an uppercase letter.
public class ContinueDemo {
    public static void main(String[] args) {

        StringBuffer searchMe = new StringBuffer(
            "peter piper picked a peck of pickled peppers");
        int max = searchMe.length();
        int numPs = 0;

        for (int i = 0; i < max; i++) {
            //interested only in p's
            if (searchMe.charAt(i) != 'p')
                continue;

            //process p's
            numPs++;
            searchMe.setCharAt(i, 'P');
        }
        System.out.println("Found " + numPs
                           + " p's in the string.");
        System.out.println(searchMe);
    }
}
Here is the output of this program.
Found 9 p's in the string.
Peter PiPer Picked a Peck of Pickled PePPers
The labeled form of the continue statement skips the current iteration of an outer loop marked with the given label. The following example program, ContinueWithLabelDemo, uses nested loops to search for a substring within another string. Two nested loops are required: one to iterate over the substring and one to iterate over the string being searched. This program uses the labeled form of continue to skip an iteration in the outer loop.
public class ContinueWithLabelDemo {
    public static void main(String[] args) {

        String searchMe = "Look for a substring in me";
        String substring = "sub";
        boolean foundIt = false;

        int max = searchMe.length() - substring.length();

    test:
        for (int i = 0; i <= max; i++) {
            int n = substring.length();
            int j = i;
            int k = 0;
            while (n-- != 0) {
                if (searchMe.charAt(j++)
                        != substring.charAt(k++)) {
                    continue test;
                }
            }
            foundIt = true;
                    break test;
        }
        System.out.println(foundIt ? "Found it" :
                                     "Didn't find it");
    }
}
Here is the output from this program.
Found it

The return Statement

The last of the branching statements is the return statement. You use return to exit from the current method; the flow of control returns to the statement that follows the original method call. The return statement has two forms: (1) one that returns a value and (2) one that doesn't. To return a value, simply put the value (or an expression that calculates the value) after the return keyword.
return ++count;
The data type of the value returned by return must match the type of the method's declared return value. When a method is declared void, use the form of return that doesn't return a value.
return;
For information about writing methods for your classes, refer to the Defining Methods section.

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