Do While Loop

The do-while Statement The do-while statement is like the while statement, except that the associated block always gets executed at least onc Its syntax is as follows:

do {     statement (s) } while (booleanExpression);

Just like the while statement, you can omit the braces if there is only one statement within them. However, always use braces for the sake of clarity.

public class MainClass {   public static void main(String[] args) {     int i = 0;     do {       System.out.println(i);       i++;     } while (i < 3);   } } This prints the following to the console: 0 1 2     The following do-while demonstrates that at least the code in the do block will be executed once even though the initial value of j used to test the expression j < 3 evaluates to false. public class MainClass {   public static void main(String[] args) {     int j = 4;     do {         System.out.println(j);         j++;     } while (j < 3);   } } This prints the following on the console. 4     The do while loop in action do {   // statements } while (expression); public class MainClass {   public static void main(String[] args) {     int limit = 20;     int sum = 0;     int i = 1;     do {       sum += i;       i++;     } while (i <= limit);     System.out.println("sum = " + sum);   } } sum = 210     http://www.java2s.com/Tutorial/Java/ 0080__Statement-Control/Catalog0080__Statement-Control.htm

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While Loop

The while Statement

One way to create a loop is by using the while statement. Another way is to use the for statement

The while statement has the following syntax.

while (booleanExpression) {     statement (s) }

1. statement(s) will be executed as long as booleanExpression evaluates to true.
2. If there is only a single statement inside the braces, you may omit the braces.
3. For clarity, you should always use braces even when there is only one statement.

As an example of the while statement, the following code prints integer numbers that are
less than three.

public class MainClass {   public static void main(String[] args) {     int i = 0;     while (i < 3) {       System.out.println(i);       i++;     }   } }

To produce three beeps with an interval of 500 milliseconds, use this code:

public class MainClass {   public static void main(String[] args) {     int j = 0;     while (j < 3) {       java.awt.Toolkit.getDefaultToolkit().beep();       try {         Thread.currentThread().sleep(500);       } catch (Exception e) {       }       j++;     }   } }

Sometimes, you use an expression that always evaluates to true

(such as the boolean literal true) but relies on the break statement to escape from the loop.

public class MainClass {

  public static void main(String[] args) {
    int k = 0;
    while (true) {
      System.out.println(k);
      k++;
      if (k > 2) {
        break;
      }
    }
  }

}

 

 

Using the while loop to calculate sum







public class MainClass {
  public static void main(String[] args) {
    int limit = 20;
    int sum = 0;
    int i = 1;

    while (i <= limit) {
      sum += i++;
    }
    System.out.println("sum = " + sum);
  }
}

 

Compile

sum = 210




While loop with double value

 

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

 

Compile

0.4377278997305387
0.2688654455422754
0.36392537297385574
0.15254413511361042
0.6688621030424611
0.143156733550304
0.3867695752401421
0.6348496031126075
0.8262243996358971
0.057290108917235516
0.48887104413122007
0.9181002018325309
0.03580026338719999
0.6915245271034702
0.06281957479185263
0.7484170566879976
0.08309381253696246
0.7708638307798187
0.6586973690207539
0.3606321940413979
0.17404632324149583
0.1007512703731156
0.7971297767971545
0.4461084890266951
0.8627269068871244
0.04984737498714209
0.2597921665157922
0.5410470601800582
0.5779541886682865
0.1847556794086489
0.32127067165147705
0.12266607697656651
0.62729812639505
0.12191275953517977
0.72308829958122
0.5766115831015479
0.03444891261877858
0.3034596140603594
0.058872682884219985
0.85618745813345
0.07472991871327217
0.32676415639620604
0.6390549075075913
0.09246846051500635
0.9135703200463736
0.661365290176903
0.834153688687532
0.6340862529502882
0.3240420706550172
0.07246065490188025
0.35761285323105796
0.9964556875646731

Java's 'labeled while' loop 

 

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

 

Compile

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

 




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The switch Statement

An alternative to a series of else if is the switch statement. The switch statement allows you to choose a block of statements to run from a selection of code, based on the return value of an expression. The expression used in the switch statement must return an int or an enumerated value.

The syntax of the switch statement is as follows.

switch (expression) { case value_1 :      statement (s);      break; case value_2 :      statement (s);      break;   .   .   . case value_n :      statement (s);      break; default:      statement (s); }

Failure to add a break statement after a case will not generate a compile error but may have more serious consequences because the statements on the next case will be executed.

Here is an example of the switch statement:

public class MainClass {   public static void main(String[] args) {     int i = 1;     switch (i) {     case 1 :         System.out.println("One.");         break;     case 2 :         System.out.println("Two.");         break;     case 3 :         System.out.println("Three.");         break;     default:         System.out.println("You did not enter a valid value.");     }   } } The switch Statement: a demo  public class MainClass {   public static void main(String[] args) {     int choice = 2;     switch (choice) {     case 1:       System.out.println("Choice 1 selected");       break;     case 2:       System.out.println("Choice 2 selected");       break;     case 3:       System.out.println("Choice 3 selected");       break;     default:       System.out.println("Default");       break;     }   } } compile Choice 2 selected       Execute the same statements for several different case labels public class MainClass {      public static void main(String[] args) {     char yesNo = 'N';     switch(yesNo) {       case 'n': case 'N':            System.out.println("No selected");            break;       case 'y': case 'Y':            System.out.println("Yes selected");            break;     }   } } compile No selected        Free Flowing Switch Statement Example public class Main {   public static void main(String[] args) {     int i = 0;     switch (i) {     case 0:       System.out.println("i is 0");     case 1:       System.out.println("i is 1");     case 2:       System.out.println("i is 2");     default:       System.out.println("Free flowing switch example!");     }   } } /* i is 0 i is 1 i is 2 Free flowing switch example! */         Nested Switch Statements Example public class Main {   public static void main(String[] args) {     int i = 0;     switch (i) {     case 0:       int j = 1;       switch (j) {       case 0:         System.out.println("i is 0, j is 0");         break;       case 1:         System.out.println("i is 0, j is 1");         break;       default:         System.out.println("nested default case!!");       }       break;     default:       System.out.println("No matching case found!!");     }   } } //i is 0, j is 1 Switch statement with enum public class MainClass {   enum Choice { Choice1, Choice2, Choice3 }   public static void main(String[] args) {     Choice ch = Choice.Choice1;     switch(ch) {       case Choice1:         System.out.println("Choice1 selected");         break;      case Choice2:        System.out.println("Choice2 selected");        break;      case Choice3:        System.out.println("Choice3 selected");        break;     }   } } Choice1 selected  http://www.java2s.com/Tutorial/Java/0080 __Statement-Control/Catalog0080__Statement-Control.htm

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If Statement

1. The if statement syntax       The if statement is a conditional branch statement. The syntax of the if statement is either one of these two:

if (booleanExpression) {     statement (s) }

or

if (booleanExpression) {     statement (s) } else {     statement (s) }

For example, in the following if statement, the if block will be executed if x is greater than 4.

public class MainClass {   public static void main(String[] args) {     int x = 9;     if (x > 4) {       // statements     }   } }

In the following example, the if block will be executed if a is greater than 3. Otherwise, the else block will be executed.

public class MainClass {   public static void main(String[] args) {     int a = 3;     if (a > 3) {       // statements     } else {       // statements     }   } } 2. Expression indentation for if statement If the expression is too long, you can use two units of indentation for subsequent lines. public class MainClass {   public static void main(String[] args) {     int numberOfLoginAttempts = 10;     int numberOfMinimumLoginAttempts = 12;     int numberOfMaximumLoginAttempts = 13;     int y = 3;     if (numberOfLoginAttempts < numberOfMaximumLoginAttempts         || numberOfMinimumLoginAttempts > y) {       y++;     }   } } 3. Using braces makes your 'if' statement clearer If there is only one statement in an if or else block, the braces are optional. public class MainClass {   public static void main(String[] args) {     int a = 3;     if (a > 3)       a++;     else       a = 3;   } } Consider the following example: public class MainClass {   public static void main(String[] args) {     int a = 3, b = 1;     if (a > 0 || b < 5)       if (a > 2)         System.out.println("a > 2");       else         System.out.println("a < 2");   } } It is hard to tell which if statement the else statement is associated with. Actually, An else statement is always associated with the immediately preceding if. Using braces makes your code clearer. public class MainClass {   public static void main(String[] args) {     int a = 3, b = 1;     if (a > 0 || b < 5) {       if (a > 2) {         System.out.println("a > 2");       } else {         System.out.println("a < 2");       }     }   } } 4. Multiple selections If there are multiple selections, you can also use if with a series of else statements. if (booleanExpression1) {     // statements } else if (booleanExpression2) {     // statements } ... else {     // statements } For example: public class MainClass {   public static void main(String[] args) {     int a = 0;     if (a == 1) {       System.out.println("one");     } else if (a == 2) {       System.out.println("two");     } else if (a == 3) {       System.out.println("three");     } else {       System.out.println("invalid");     }   } } 5. The if Statement in action public class MainClass {   public static void main(String[] arg) {     int a = 0;     if (a == 0) {       System.out.println("a is 0");     }   } 6. The else Clause public class MainClass {   public static void main(String[] arg) {     int a = 0;     if (a == 0) {       System.out.println("in the block");       System.out.println("in the block");     } else {       System.out.println("a is not 0");     }   } } compile results in the block in the block 7.Nested if Statements public class MainClass {   public static void main(String[] arg) {     int a = 2;     if (a == 0) {       System.out.println("in the block");       if (a == 2) {         System.out.println("a is 0");       } else {         System.out.println("a is not 2");       }     } else {       System.out.println("a is not 0");     }   } } compile results a is not 0 8.Using && in if statement public class MainClass {   public static void main(String[] arg) {     int value = 8;     int count = 10;     int limit = 11;     if (++value % 2 == 0 && ++count < limit) {       System.out.println("here");       System.out.println(value);       System.out.println(count);     }     System.out.println("there");     System.out.println(value);     System.out.println(count);   } } compile results there 9 10   9.Using || (or operator) in if statement    public class MainClass {   public static void main(String[] arg) {     int value = 8;     int count = 10;     int limit = 11;     if (++value % 2 != 0 || ++count < limit) {       System.out.println("here");       System.out.println(value);       System.out.println(count);     }     System.out.println("there");     System.out.println(value);     System.out.println(count);   } } compile results here  9  10  there source is here

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