Execution control

Published on: September 19, 2000
Last Updated: September 19, 2000

Execution control

Published on: September 19, 2000
Last Updated: September 19, 2000

Java uses all of C’s execution control statements, so if you’ve programmed with C or C++ then most of what you see will be familiar.

Most procedural programming languages have some kind of control statements, and there is often overlap among languages.

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

Java does not, however, support the much-maligned goto (which can still be the most expedient way to solve certain types of problems).

You can still do a goto-like jump, but it is much more constrained than a typical go to.

True And False

All conditional statements use the truth or falsehood of a conditional expression to determine the execution path. An example of a conditional expression is A == B .

This uses the conditional operator == to see if the value of A is equivalent to the value of B. The expression returns true or false.

Any of the relational operators you’ve seen earlier in this chapter can be used to produce a conditional statement.

Note that Java doesn’t allow you to use a number as a boolean, even though it’s allowed in C and C++ (where truth is nonzero and falsehood is zero).

If you want to use a non- boolean in a boolean test, such as if(a), you must first convert it to a boolean value using a conditional expression, such as if(a != 0) .

if-else

The if-else statement is probably the most basic way to control program flow. The else is optional, so you can use if in two forms:if(Boolean-expression)

statementorif(Boolean-expression)

statement

else

statement The conditional must produce a Boolean result. The statement means either a simple statement terminated by a semicolon or a compound statement, which is a group of simple statements enclosed in braces.

Anytime the word “ statement” is used, it always implies that the statement can be simple or compound.

As an example of if-else, here is a test( ) method that will tell you whether a guess is above, below, or equivalent to a target number:

static int test(int testval) {
  int result = 0;
  if(testval > target)
    result = -1;
  else if(testval < target)
    result = +1;
  else
    result = 0; // match
  return result;
}

It is conventional to indent the body of a control flow statement so the reader might easily determine where it begins and ends.

Return

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 that value to be returned immediately. The test( ) method above can be rewritten to take advantage of this:

static int test2(int testval) {
  if(testval > target)
    return -1;
  if(testval < target)
    return +1;
  return 0; // match
}

There’s no need for else because the method will not continue after executing a return.

Iteration

while, do-while and for control looping and are sometimes classified as iteration statements . A statement repeats until the controlling Booleanexpression evaluates to false. The form for a while loop is 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. Here’s a simple example that generates random numbers until a particular condition is met:

//: WhileTest.java
// Demonstrates the while loop

public class WhileTest {
  public static void main(String[] args) {
    double r = 0;
    while(r < 0.99d) {
      r = Math.random();
      System.out.println(r);
    }
  }
} ///:~ 

This uses the static method random( ) in the Math library, which generates a double value between 0 and 1. (It includes 0, but not 1.) The conditional expression for the while says “keep doing this loop until the number is 0.99 or greater.” Each time you run this program you’ll get a different-sized list of numbers.

Do-while

The form for do-while is 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

A for loop performs initialization before the first iteration. Then it performs conditional testing and, at the end of each iteration, some form of “stepping.” The form of the for loop is: for(initialization; Boolean-expression; step)

statement Any of the expressions initializationBoolean-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. for loops are usually used for “counting” tasks:

//: ListCharacters.java
// Demonstrates "for" loop by listing
// all the ASCII characters.

public class ListCharacters {
  public static void main(String[] args) {
  for( char c = 0; c < 128; c++)
    if (c != 26 )  // ANSI Clear screen
      System.out.println(
        "value: " + (int)c +
        " character: " + c);
  }
} ///:~ 

Note that the variable c is defined at the point where it is used, inside the control expression of the for loop, rather than at the beginning of the block denoted by the open curly brace.

The scope of c is the expression controlled by the for. Traditional procedural languages like C require that all variables be defined at the beginning of a block so when the compiler creates a block it can allocate space for those variables.

In Java and C++ you can spread your variable declarations throughout the block, defining them at the point that you need them.

This allows a more natural coding style and makes code easier to understand. You can define multiple variables within a for statement, but they must be of the same type:

for(int i = 0, j = 1;
    i < 10 && j != 11;
    i++, j++)
 /* body of for loop */; 

The int definition in the for statement covers both i and j. The ability to define variables in the control expression is limited to the for loop. You cannot use this approach with any of the other selection or iteration statements.

The Comma Operator

Earlier in this chapter I stated that the comma operator (not the comma separator, which is used to separate function arguments) has only one use in Java: in the control expression of a for loop.

In both the initialization and step portions of the control expression you can have a number of statements separated by commas, and those statements will be evaluated sequentially. The previous bit of code uses this ability. Here’s another example:

//: CommaOperator.java

public class CommaOperator {
  public static void main(String[] args) {
    for(int i = 1, j = i + 10; i < 5;
        i++, j = i * 2) {
      System.out.println("i= " + i + " j= " + j);
    }
  }
} ///:~ 

Here’s the output:

i= 1 j= 11
i= 2 j= 4
i= 3 j= 6
i= 4 j= 8 

You can see that in both the initialization and step portions the statements are evaluated in sequential order. Also, the initialization portion can have any number of definitions of one type .

Break And Continue

Inside the body of any of the iteration statements you can also control the flow of the loop 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 a new iteration. This program shows examples of break and continue within for and while loops:

//: BreakAndContinue.java
// Demonstrates break and continue keywords

public class BreakAndContinue {
  public static void main(String[] args) {
    for(int i = 0; i < 100; i++) {
      if(i == 74) break; // Out of for loop
      if(i % 9 != 0) continue; // Next iteration
      System.out.println(i);
    }
    int i = 0;
    // An "infinite loop":
    while(true) {
      i++;
      int j = i * 27;
      if(j == 1269) break; // Out of loop
      if(i % 10 != 0) continue; // Top of loop
      System.out.println(i);
    }
  }
} ///:~ 

In the for loop the value of i never gets to 100 because the break statement breaks out of the loop when i is 74.

Normally, you’d use a break like this only if you didn’t know when the terminating condition was going to occur.

The continue statement causes execution to go back to the top of the iteration loop (thus incrementing i) whenever i is not evenly divisible by 9. When it is, the value is printed.

The second portion shows an “infinite loop” that would, in theory, continue forever. However, inside the loop there is a break statement that will break out of the loop.

In addition, you’ll see that the continue moves back to the top of the loop without completing the remainder. (Thus printing happens only when the value of i is divisible by 9.) The output is:

0
9
18
27
36
45
54
63
72
10
20
30
40

The value 0 is printed because 0 % 9 produces 0.A second form of the infinite loop is for(;;). The compiler treats both while(true) and for(;;) in the same way so whichever one you use is a matter of programming taste.

The Infamous “goto”

The goto keyword has been present in programming languages from the beginning. Indeed, goto was the genesis of program control in assembly language: “if condition A, then jump here, otherwise jump there.”

If you read the assembly code that is ultimately generated by virtually any compiler, you’ll see that program control contains many jumps.

However, goto jumps at the source-code level, and that’s what brought it into disrepute. If a program will always jump from one point to another, isn’t there some way to reorganize the code so the flow of control is not so jumpy? 

goto fell into true disfavor with the publication of the famous “Goto considered harmful” paper by Edsger Dijkstra, and since then goto-bashing has been a popular sport, with advocates of the cast-out keyword scurrying for cover.

As is typical in situations like this, the middle ground is the most fruitful. The problem is not the use of goto but the overuse of goto, and in rare situations goto is the best way to structure control flow.

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.

A label is an identifier followed by a colon, like this: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 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:label1:

outer-iteration {

inner-iteration {

//…

break; // 1

//…

continue; // 2

//…

continue label1; // 3

//…

break label1; // 4

}

}In case 1, the break breaks out of the inner iteration and you end up in the outer iteration. In case 2, the continue moves back to the beginning of the inner iteration.

But in case 3, the continue label1 breaks out of the inner iteration and the outer iteration, all the way back to label1.

Then it does in fact continue the iteration, but starting at the outer iteration. In case 4, the break label1 also breaks all the way out to label1, but it does not re-enter the iteration. It actually does break out of both iterations. Here is an example using for loops:

//: LabeledFor.java
// Java’s "labeled for loop"

public class LabeledFor {
  public static void main(String[] args) {
    int i = 0;
    outer: // Can't have statements here
    for(; true 😉 { // infinite loop
      inner: // Can't have statements here
      for(; i < 10; i++) {
        prt("i = " + i);
        if(i == 2) {
          prt("continue");
          continue;
        }
        if(i == 3) {
          prt("break");
          i++; // Otherwise i never
               // gets incremented.
          break;
        }
        if(i == 7) {
          prt("continue outer");
          i++; // Otherwise i never
               // gets incremented.
          continue outer;
        }
        if(i == 8) {
          prt("break outer");
          break outer;
        }
        for(int k = 0; k < 5; k++) {
          if(k == 3) {
            prt("continue inner");
            continue inner;
          }
        }
      }
    }
    // Can't break or continue
    // to labels here
  }
  static void prt(String s) {
    System.out.println(s);
  }
} ///:~ 

This uses the prt( ) method that has been defined in the other examples. Note that break breaks out of the for loop, and that the increment-expression doesn’t occur until the end of the pass through the for loop.

Since break skips the increment expression, the increment is performed directly in the case of i == 3 .

The continue outer statement in the case of I == 7 also goes to the top of the loop and also skips the increment, so it too is incremented directly.Here is the output:

i = 0
continue inner
i = 1
continue inner
i = 2
continue
i = 3
break
i = 4
continue inner
i = 5
continue inner
i = 6
continue inner
i = 7
continue outer
i = 8
break outer 

If not for the break outer statement, there would be no way to get out of the outer loop from within an inner loop, since break by itself can break out of only the innermost loop.

(The same is true for continue.)Of course, in the cases where breaking out of a loop will also exit the method, you can simply use a return. Here is a demonstration of labeled break and continue statements with while loops:

//: LabeledWhile.java
// Java's "labeled while" loop

public class LabeledWhile {
  public static void main(String[] args) {
    int i = 0;
    outer:
    while(true) {
      prt("Outer while loop");
      while(true) {
        i++;
        prt("i = " + i);
        if(i == 1) {
          prt("continue");
          continue;
        }
        if(i == 3) {
          prt("continue outer");
          continue outer;
        }
        if(i == 5) {
          prt("break");
          break;
        }
        if(i == 7) {
          prt("break outer");
          break outer;
        }
      }
    }
  }
  static void prt(String s) {
    System.out.println(s);
  }
} ///:~ 

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 re-enters 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.

The output of this method makes it clear:

Outer while loop
i = 1
continue
i = 2
i = 3
continue outer
Outer while loop
i = 4
i = 5
break
Outer while loop
i = 6
i = 7
break outer 

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.

In Dijkstra’s “goto considered harmful” paper, what he specifically objected to was the labels, not the goto.

He observed that the number of bugs seems to increase with the number of labels in a program.

Labels and gotos make programs difficult to analyze statically, since it introduces cycles in the program execution graph.

Note that Java labels don’t suffer from this problem, since they are constrained in their placement and can’t be used to transfer control in an ad hoc manner.

It’s also interesting to note that this is a case where a language feature is made more useful by restricting the power of the statement.

Switch

The switch is sometimes classified as a selection statement . The switch statement selects from among pieces of code based on the value of an integral expression. Its form is:switch(integral-selector) {

case integral-value1 : statement; break;

case integral-value2 : statement; break;

case integral-value3 : statement; break;

case integral-value4 : statement; break;

case integral-value5 : 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 (simple or compound) executes. If no match occurs, the default statement executes.

You will notice in the above 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 execute until a break is encountered.

Although you don’t usually want this kind of behavior, it can be useful to an experienced programmer.

Note the last statement, for 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 multi-way selection (i.e., selecting from among a number of different execution paths), but it requires a selector that evaluates to an integral value such as int or char.

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. Here’s an example that creates letters randomly and determines whether they’re vowels or consonants:

//: VowelsAndConsonants.java
// Demonstrates the switch statement

public class VowelsAndConsonants {
  public static void main(String[] args) {
    for(int i = 0; i < 100; i++) {
      char c = (char)(Math.random() * 26 + 'a');
      System.out.print(c + ": ");
      switch(c) {
      case 'a':
      case 'e':
      case 'i':
      case 'o':
      case 'u':
                System.out.println("vowel");
                break;
      case 'y':
      case 'w':
                System.out.println(
                  "Sometimes a vowel");
                break;
      default:
                System.out.println("consonant");
      }
    }
  }
} ///:~ 

Since Math.random( ) generates a value between 0 and 1, you need only multiply it by the upper bound of the range of numbers you want to produce (26 for the letters in the alphabet) and add an offset to establish the lower bound.

Although it appears you’re switching on a character here, the switch statement is actually using the integral value of the character.

The singly-quoted characters in the case statements also produce integral values that are used for comparison.

Notice how the cases can be “stacked” on top of each other to provide multiple matches for a particular piece of code.

You should also be aware that it’s essential to put the break statement at the end of a particular case, otherwise control will simply drop through and continue processing on the next case.

Calculation Details

The statement:char c = (char)(Math.random() * 26 + ‘a’);deserves a closer look. Math.random( ) produces a double, so the value 26 is converted to a double to perform the multiplication, which also produces a double.

This means that ‘a’ must be converted to a double to perform the addition. The double result is turned back into a char with a cast.

First, what does the cast to char do? That is, if you have the value 29.7 and you cast it to a char, is the resulting value 30 or 29? The answer to this can be seen in this example:

//: CastingNumbers.java
// What happens when you cast a float or double
// to an integral value?

public class CastingNumbers {
  public static void main(String[] args) {
    double
      above = 0.7,
      below = 0.4;
    System.out.println("above: " + above);
    System.out.println("below: " + below);
    System.out.println(
      "(int)above: " + (int)above);
    System.out.println(
      "(int)below: " + (int)below);
    System.out.println(
      "(char)('a' + above): " +
      (char)('a' + above));
    System.out.println(
      "(char)('a' + below): " +
      (char)('a' + below));
  }
} ///:~ 

The output is:

above: 0.7
below: 0.4
(int)above: 0
(int)below: 0
(char)('a' + above): a
(char)('a' + below): a 

So the answer is that casting from a float or double to an integral value always truncates.The second question has to do with Math.random( ).

Does it produce a value from zero to one, inclusive or exclusive of the value ‘1’? In math lingo, is it (0,1), or [0,1], or (0,1] or [0,1)?

(The square bracket means “includes” whereas the parenthesis means “doesn’t include.”) Again, a test program provides the answer:

//: RandomBounds.java
// Does Math.random() produce 0.0 and 1.0?

public class RandomBounds {
  static void usage() {
    System.err.println("Usage: \n\t" +
      "RandomBounds lower\n\t" +
      "RandomBounds upper");
    System.exit(1);
  }
  public static void main(String[] args) {
    if(args.length != 1) usage();
    if(args[0].equals("lower")) {
      while(Math.random() != 0.0)
        ; // Keep trying
      System.out.println("Produced 0.0!");
    } 
    else if(args[0].equals("upper")) {
      while(Math.random() != 1.0)
        ; // Keep trying
      System.out.println("Produced 1.0!");
    } 
    else 
      usage();
  }
} ///:~ 

To run the program, you type a command line of either:java RandomBounds lowerorjava RandomBounds upperIn both cases you are forced to break out of the program manually, so it would appear that Math.random( ) never produces either 0.0 or 1.0. But this is where such an experiment can be deceiving. If you consider that there are 2 

128 different double fractions between 0 and 1, the likelihood of reaching any one value experimentally might exceed the lifetime of one computer, or even one experimenter.

It turns out that 0.0 is included in the output of Math.random( ). Or, in math lingo, it is [0,1).

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Written by Bobby

Bobby Lawson is a seasoned technology writer with over a decade of experience in the industry. He has written extensively on topics such as cybersecurity, cloud computing, and data analytics. His articles have been featured in several prominent publications, and he is known for his ability to distill complex technical concepts into easily digestible content.