#Control Flow

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What we’ll cover

Block Scope

Conditional Statements

Loops

Switch Statements

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Control Flow

Java, like any programming language, supports both conditional statements and loops to determine control flow

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Block Scope

Before learning about control structures, you need to know more about blocks.

A block or compound statement consists of a number of Java statements, surrounded by a pair of braces. Blocks define the scope of your variables.

Blocks can be nested

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public static void main(String[] args)
{
    int n;
    ...
    {
        int k;
        ...
    } // k is only defined up to here
}

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You may not declare identically named variables in two nested blocks

public static void main(String[] args)
{
    int n;
    ...
    {
        int k;
        int n; // Error--can't redefine n in inner block ...
    }
}

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Conditional Statements

The conditional statement in Java has the form

if(condition) statement
{
    statement1
    statement2
    ...
}

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if/else statement

if (condition)
{
    statement1
}
else
{
    statement2
}

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if/else if

if (condition)
{
    statement1
}
else if (condition2)
{
    statement2
}
else
{
    statement3
}

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Branches in control flow

Control structure code created branches, or different paths that the code can take.

For example,

if(condition) {
  // do something
} else {
  // do something else
}

has two branches. One code path when the condition succeeds and one for when it fails.

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Loops

The while loop executes a statement (which may be a block statement) while a condition is true

while(condition) statement

The while loop will never execute if the condition is false at the outset

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do/while loops

If you want to make sure a block is executed at least once, you need to move the test to the bottom, using the do/while loop.

do statement while (condition);

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Determinate Loops

The for loop is a general construct to support iteration controlled by a counter or similar variable that is updated after every iteration.

for (int i = 1; i <= 10; i++)
    System.out.println(i);

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Nested Loops

You can have loops within loops, but be aware of variable scoping:

for(int i = 0; i < 5; i++) {
  for(int j = 0; j < 5; j++) {
    System.out.println(j);
  }
}

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Multiple Selections: The switch Statement

switch (choice)
{
    case 1:
        ...
        break;
    case 2:
        ...
        break;
    case 3:
        ...
        break;
    default:
        // bad input ...
        break;
}

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Switch statements (continued)

switch statements without breaks can have code “fall through” to the next one.

switch (choice)
{
    case 1:
        ...
        // notice no break
    case 2:
        ...
        break;
    case 3:
        ...
        break;
    default:
        // bad input ...
        break;
}

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Switch statements (continued)

A case label can be

• A constant expression of type char, byte, short, or int
• An enumerated constant
• Starting with Java SE 7, a string literal

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Statements That Break Control Flow

The same break statement that you use to exit a switch can also be used to break out of a loop

while (years <= 100) {
    balance += payment;
    double interest = balance * interestRate / 100; balance += interest;
    if (balance >= goal) break;
    years++;
}

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Continue Statement

The continue statement transfers control to the header of the innermost enclosing loop

Scanner in = new Scanner(System.in);
while (sum < goal)
{
    System.out.print("Enter a number: ");
    n = in.nextInt();
    if (n < 0) continue;
    sum += n; // not executed if n < 0
}