04-controlstatements.md

Control Statements

• Control Statements are to used to advance in the flow of execution and branch based on the flow of the program.

• {

  } Represents block of code.

  If there is a single statement; use of statement block is optional.

• Control statement can be classified in 3 categories.

  • Selection
    • Selection Statements help to chose different path based on the value of a variable or result of an expression.
  • Iteration
    • Iteration Statements are useful for repetitive execution.
  • Jump
    • Jump statements are useful to execute programs in non linear fashion.

Selection

• if-else
  • An if statement consists of a boolean expression followed by one or more statements.
  • The statements inside a if block are executed only of the boolean expression is true.
  • if statement can be followed by an optional else block.
  • The statements inside else is executed only of the boolean expression is false.
  • Java also supports else if which is used in the case of nested statements.

        if (time < 10) {
            System.out.println("Good morning.");
        } else if (time < 20) {
            System.out.println("Good day.");
        } else {
            System.out.println("Good evening.");
        }

• switch:
  • This is extension to if else statement with large possibilities.
  • It provides an easy way to dispatch execution to different parts of your code based on the value of an expression.
  • In Switch, The value of the expression is compared with each of the values in the case statements. If a match is found, the code sequence following that case statement is executed.
  • If none of the case statements match the value of the expression, then the default statement is executed.
  • The break statement is used to terminate a statement sequence.
  • Expression of switch statement can be of data types byte, short, int, char or enum. From JDK 7 String data types are also supported.
  • Duplicate case values are not allowed.

    String sampleExp="day";
    
    switch(sampleExp){
        case "morning":
            System.out.println("Good morning.");
            break;
        case "day":
            System.out.println("Good day.");
            break;
        case "evening":
            System.out.println("Good evening.");
            break;
        default:
            System.out.println("Hello.");
    }

Iteration

• while:
  • This is fundamental looping statement. The body of the loop will be executed as long as the conditional expression is true.
  • When condition becomes false, control passes to the next line of code immediately following the loop.

    int i =0;
    while (i > 11){
        System.out.println(i);
    }

• do while:
  • Each iteration of the do-while loop first executes the body of the loop and then evaluates the conditional expression.
  • This looping construct will allow to execute the loop body at-least once.

    int i =0;
    do {
        System.out.println(i);
    } while (i > 0)

• for:
  • There are two for of for loop
    • Traditional:
      • Supported from original version of JDK.
      • When the loop first starts, java will execute below order. All the below statements are optional. If none provided, java will create infinite loop
        • Initialization: This is used to initialize loop control variables. Variable declared are scoped only w.r.t for loop.
        • Condition: This must be a Boolean expression.
        • Statement: Execution body statements.
        • Iteration: This is usually an expression that increments or decrements the loop control variable.
      • Comma can be used to include multiple Initialization and Iteration statements.

      for (a=1, b=4; a<b; a++, b--){
          System.out.println("a = "+a+" b= "+b);
      }

    • For Each:
      • Supported from JDK 5 and above.
      • This looping style is designed to cycle through a collection of objects, such as an array, in strictly sequential fashion, from start to finish.
      • The iteration variable is “read-only” as it relates to the underlying array.

        int nums[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        int sum = 0;
        
        for(int x: nums) 
          sum += x;

  • Local variable type inference is a feature in Java 10 that allows the developer to skip the type declaration associated with local variables. var keyword is used to declare these variables.

Jump

• break:
  • Used to stop switch or looping flow.
  • It can also be used with labels

    boolean t=true;
    
    first: {
        second: {
            third: {
              System.out.println("Entered third loop"); 
              if (t) 
                break second; 
            }
            System.out.println("Entered second loop");
        }
        System.out.println("Entered first loop");
    }

• continue:
  • Used to skip one looping iteration.
  • Like break, It can also be used with labels.

        outer: for(int i=0; i<10; i++) {
            for(int j=0; j<10; j++){
                if (j>i){
                    System.out.println("j = " + j);
                    break outer;
                }
            }
            System.out.println("i = " + i);
        }

• return:
  • Used to explicitly return from a method to caller method.