Differences between the two main languages felt by those who studied Java after learning Ruby Part1

I started studying Java grammar and there are many complicated rules compared to Ruby, so make a note of it.


System.out.println "Java";  //This is the same role as Puts in Ruby at the end of the code;Hit

Variables and data types

To define variables in Java (1) Specify the data type of the value to be put in the variable, (2) Decide the name of the variable, Two are required.

int number = 7;

String name = "Sato";   //String type s is uppercase

double syosu = 3.14;

Cast (forced type conversion)

 int number1 = 7;
 int number2 = 2;

 System.out.println(number1 / number2);  //The result is 3
 System.out.println((double)number1 / number2);  //The result is 3.5

Boolean boolean type

  System.out.println(12 / 4 == 3);  //Output as true

  System.out.println(12 / 4 != 3); //output as false

if statement

Write the conditional expression in () after if

  if (true) {
    System.out.println("Output because the conditional expression is true");
  if (false) {
    System.out.println("Not output because the conditional expression is false");

switch statement

int number = 13;
    //Processing when it does not match any case using default
    switch (number % 5) {
      case 0:
        System.out.println("I'm Daikichi");
      case 1:
        System.out.println("I'm Nakayoshi");
      case 4:
        System.out.println("It ’s bad.");
        System.out.println("I'm good");


First, specify the type of the elements of the array. The array is {} curly braces.

 String names[] = {"YUI","YUKI","YOU"}; 

Extended for statement (simple for statement)

   String[] names = {"dog", "Cat", "turtle"};
    for (String name : names){
      System.out.println("My pet"+name+"is");

Classes and methods

Java executes classes, not files. The flow that there is a Main method in the Main class and the hello method in it is executed.

class Main {
//Method definition void means no return value
  public static void main(String[] args) { 
  public static void hello() {
    // "Hello World","Hello Java"Please rewrite to
    System.out.println("Hello Java");

Return value (when you want to use the processing result of the method in the caller of the method)

class Main {
  public static void main(String[] args) {
    //Assign the result of the fullName method to the variable name
    String name = fullName("Kate" ,"Jones");
    //Rewrite the argument of printData
    printData(name, 27);
    //Please do not rewrite this
    printData("John Christopher Smith", 65);

  public static void printData(String name, int age) {
    System.out.println("my name is" + name + "is");
    System.out.println("Age is" + age + "I'm old");

  //Define fullName method
  public static String fullName(String firstName ,String lastName){
    return firstName + " " + lastName;

Overload (If the argument type and number are different, you can define a method with the same name)

//Computers can determine as separate methods

hello();        //Hello method with no arguments

hello("Java"); //Method with one argument hello

Boolean return value

Conditional branch by the return value (true / false value) of isHealthy method
Method 1
    if (isHealthy(bmi)){
      System.out.println("I'm healthy");
      System.out.println("Not healthy");

Method 2
  public static boolean isHealthy(double bmi){
    return bmi >= 18.5 && bmi < 25.0;

Scanner You can also "enter" a value into the console and use that value in your program. A library called Scanner is used to receive input to the console.

//import required
import java.util.Scanner;  

class Main {
  public static void main (String[] args) {
    Scanner scanner = new Scanner(System.in);
    //Define the variable name, receive the string from the console and assign it
    String name = scanner.next();
    //Please be output as "Hello ◯◯-san"

Continue to Part 2 ....

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Differences between the two main languages felt by those who studied Java after learning Ruby Part1