[Java] Summary of control syntax

Structured programming elements

• Sequential (advanced)
• Choice
• Iteration

Summary of control syntax to implement these

if instruction

• boolean Do not compare variables with ==
• NG example ʻif (flag == true)` etc.
• Use ʻif (flag)ʻif (! flag)
• Do not abuse denial
• → Use blank sentences for affirmative sentences and use only else to make it easier to read

import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
    var i = 10;
    if (i == 10) {
      System.out.println("The variable i is 10.");
      } else {
      System.out.println("The variable i is not 10.");
      //Same as below
      //System.out.println((i==10)?"Variable is 10":"Not 10");
     }
    }
}

Multi-branch

• Even if multiple conditions are met, only the first block that matches is executed
• ** Describe from a narrow conditional expression **

public static void main(String[] args) {
  var i = 100;
  if (i > 50) {
    System.out.println("The variable i is greater than 50.");
  } else if (i > 30) {
    System.out.println("The variable i is greater than 30 and less than or equal to 50.");
  } else {
    System.out.println("The variable i is 30 or less.");
  }
}

switch instruction

• Easy multi-branch of equality operator
• Conditional branch in case / default clause
• Move processing to the case clause that matches the expression
• Need to explicitly break ** to exit switch glock after finishing the corresponding process **
• If you do not break ** Subsequent case / default will be executed **
• ** Fallthrough **: You can continue processing without intentionally breaking

public static void main(String[] args) {
  var rank = "Instep";
  switch (rank) {
    case "Instep":
      System.out.println("It's very good.");
      break;
    case "B":
      System.out.println("Is good.");
      break;
    case "Hinoe":
      System.out.println("Let's do our best.");
      break;
    default:
      System.out.println("？？？");
      break;
  }
}

//Arrange multiple cases in fallthrough or implement conditions
public static void main(String[] args) {
  var drink = "beer";
  switch (drink) {
    case "Sake":
    case "beer":
    case "wine":
      System.out.println("It is brewed sake.");
      break;
    case "Brandy":
    case "whisky":
      System.out.println("It is distilled liquor.");
      break;
  }
}

while / do instruction

• Execute do while conditional expression is true
• While instruction ** Prefix judgment **: Conditional expression is judged at the beginning
• do… while instruction is ** post-judgment **: judgment at the end → execute loop once regardless of conditions

public class Main {
    public static void main(String[] args) throws Exception {
        var i = 1;
        do {
          System.out.println(i + "This is the second loop.");
          i++;
        } while (i < 6);
    }
}

for instruction

• Loop only a fixed number of times
• Declare and initialize counter variables (i, j, k, etc.) in the initialization expression
• Do not use floating point types for counter variables
• Floating point type cannot express 0.1 exactly
• You can skip the process with continue
• ** Sequential operator ** can list multiple conditions in initialization expression / continuation conditional expression / increment / decrement expression

//NG example: Floating point type for counter variable
public class Main {
    public static void main(String[] args) throws Exception {
    for (var i = 0.1f; i <= 1.0; i += 0.1f) {
      System.out.println(i);
      //0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.70000005, 0.8000001, 0.9000001
    }
  }
}

//Sequential operator
public static void main(String[] args) {
    for (var i = 1; i < 6; System.out.println(i + "This is the second loop."), i++);
 }

Extended for instruction

• Process specified array or collection from the beginning
• ** Access individual elements via formal variables **
• Manipulating formal variables does not affect extended for loops

public class Main {
  public static void main(String[] args) {
    var data = new String[] { "neko", "tama", "mike" };
    //Extended for instruction
    for (var value : data) {
      System.out.println(value);
    }
  }
}

//Example of receiving command line arguments with args and outputting them in order with extended for
public static void main(String[] args) {
  for (var value : args) {
    System.out.println("Hello," + value + "Mr.!");
  }
}

Loop control

• ** break **: Forcibly breaks the current loop and exits itself completely
• ** continue **: ** Only for the current lap ** Skip, loop continues
• For nested loops, skip only inside
• If you want to get out of the whole, break label name;

//Exit from the specified loop
public class Main {
  public static void main(String[] args) {
    limit:
    for (var i = 1; i < 10; i++) {
      for (var j = 1; j < 10; j++) {
        var result = i * j;
        if (result > 50) {
          break limit;
        }
        System.out.printf(" %2d", result);
      }
      System.out.println();
    }
  }
}