Thinking in Java,Fourth Edition(Java 编程思想,第四版)学习笔记(五)之Controlling Execution

　　In Java, the keywords include if-else,while,do-while,for,return,break, and a selection statement called switch.

　　Java does not support the much-maligned goto. You can still do a goto-like jump, but it is much more constrained than a typical goto.

true and false

　　All conditional statement use the truth or falsehood of a conditional expression to determine the execution path.

if-else

　　if (Boolean-expression)

　　　　statement

　　or

　　if (Boolean-expression)

　　　　statement

　　else

　　　　statement

　　or

　　if (Boolean-expression)

　　　　statement

　　else if (Boolean-expression)

　　　　statement

　　else

　　　　statement

　　

　　"statement" can be simple statement or a compound statement.

Iteration

　　while, do-while, for

　　while (Boolean-expression)

　　　　statement

　　The Boolean-expression is evaluated once at the beginning of the loop and again before each further iteration of the statement.

　　do

　　　　statement

　　while (Boolean-expression);

　　The sole difference between while and do-while is that the statement of the do-while always executes at least once, even if the expression evaluates to false the first time. In a while, if the conditional is false the first time the statement never executes. In practice, do-while is less common than while.

　　for ( initialization; Boolean-expression; step)

　　　　statement

　　Any of the expressions initialization, Boolean-expression or step can be empty. The expression is tested before each iteration, and as soon as it evaluates to false, execution will continue at the line following the for statement. At the end of each loop, the step executes.

　　The comma operator　

　　In both the initialization and step portions of the for control expression, you can have a number of statements separated by commas, and those statements will be evaluated sequentially.
Using the comma operator, you can define multiple variables within a for statement, but they must be of the same type

　　The ability to define variables in a control expression is limited to the for loop. You cannot use this approach with any of the other selection or iteration statements.

For each syntax

　　Java SE5 introduces a new and more succinct for syntax, for use with arrays and containers, called foreach syntax.

　　foreach will also work with any object that is Iterable.

return

　　Several keywords represent unconditional branching, which simply means that the branch happens without any test. These include return, break, continue, and a way to jump to a labeled statement which is similar to the goto in other languages.

　　The return keyword has two purposes: It specifies what value a method will return (if it doesn’t have a void return value) and it causes the current method to exit, returning that value.

　　If you do not have a return statement in a method that returns void, there’s an implicit return at the end of that method, so it’s not always necessary to include a return statement. However, if your method states it will return anything other than void, you must ensure every code path will return a value.

break and continue

　　You can also control the flow of the loop inside the body of any of the iteration statements by using break and continue. break quits the loop without executing the rest of the statements in the loop. continue stops the execution of the current iteration and goes back to the beginning of the loop to begin the next iteration.

The infamous "goto"

　　Although goto is a reserved word in Java, it is not used in the language; Java has no goto. However, it does have something that looks a bit like a jump tied in with the break and continue keywords. It’s not a jump but rather a way to break out of an iteration statement. The reason it’s often thrown in with discussions of goto is because it uses the same mechanism: a label.　

　　label1:
　　The only place a label is useful in Java is right before an iteration statement. And that means right before—it does no good to put any other statement between the label and the iteration. And the sole reason to put a label before an iteration is if you’re going to nest another iteration or a switch (which you’ll learn about shortly) inside it. That’s because the break and continue keywords will normally interrupt only the current loop, but when used with a label, they’ll interrupt the loops up to where the label exists.　

　　The same rules hold true for while:
　　　　1. A plain continue goes to the top of the innermost loop and continues.
　　　　2. A labeled continue goes to the label and reenters the loop right after that label.
　　　　3. A break “drops out of the bottom” of the loop.
　　　　4. A labeled break drops out of the bottom of the end of the loop denoted by the label.
　　It’s important to remember that the only reason to use labels in Java is when you have nested loops and you want to break or continue through more than one nested level.

switch

　　switch (integral-selector) {

　　　　case integral-value1 : statement;break;

　　　　case integral-value2 : statement;break;

　　　　//....

　　　　default : statement;

　　Integral-selector is an expression that produces an integral value. The switch compares the result of integral-selector to each integral-value. If it finds a match, the corresponding statement (a single statement or multiple statements; braces are not required) executes. If no match occurs, the default statement executes.

　　You will notice in the preceding definition that each case ends with a break, which causes execution to jump to the end of the switch body. This is the conventional way to build a switch statement, but the break is optional. If it is missing, the code for the following case statements executes until a break is encountered. Although you don’t usually want this kind of behavior, it can be useful to an experienced programmer. Note that the last statement, following the default, doesn’t have a break because the execution just falls through to where the break would have taken it anyway. You could put a break at the end of the default statement with no harm if you considered it important for style’s sake.

　　The switch statement is a clean way to implement multiway selection (i.e., selecting from among a number of different execution paths), but it requires a selector that evaluates to an integral value.

　　If you want to use, for example, a string or a floating point number as a selector, it won’t work in a switch statement. For non-integral types, you must use a series of if statements. At the end of the next chapter, you’ll see that Java SE5’s new enum feature helps ease this restriction, as enums are designed to work nicely with switch.