minte9
SoftwareDevelopment / JAVA





Classes 

 
/**
 * JAVA CLASSES
 * --------------------------------------
 * In Java, everything goes into a class.
 *      - src/main/Classes.java
 * 
 * Java virtual machine (JVM) compiles the class and creates an archive.
 *      - target/classes/Class.class
 * 
 * Key Points:
 *  - Every java program starts in main() method.
 *  - Return type `void` means there is no returned value.
 *  - Every statement must end in semicolon.
 * 
 * Objects:
 *  - A class is a blueprint for an object.
 *  - Instance variables (fields) is what an object knows.
 *  - Methods are what an object does.
 * -----------------------------------
 */

package com.minte9.basics.classes;

public class Classes {
    public static void main(String[] args) {
        
        A obj = new A();
        obj.setName("John");

        System.out.println("Hello " + obj.name);  // Hello John
    }
}

class A {
    public String name;

    public void setName(String name) { // This is a class method
        this.name = name;
    }
}

Input Stream 

 
/**
 * INPUT STREAM - Scanner
 * ----------------------------------------
 * Read input from stdin (standard input)
 * Write output to stdout (standard output)
 * ----------------------------------------
 */

package com.minte9.basics.classes;

import java.util.Scanner;

public class InputStream {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        int a = scanner.nextInt();  // 10
        int b = scanner.nextInt();  // 20

        System.out.println("b: " + b);  // b: 20
        System.out.println("a: " + a);  // a: 10
        // ---------------------------
        // a: 20
        // a: 10

        int n = scanner.nextInt();                          // 2147483647
        double d = Double.parseDouble(scanner.next());      // 235345345345.234534

        scanner.nextLine();
        String s = scanner.nextLine();                      // Hello World

        System.out.println("String: " + s);                 
        System.out.println("Double: " + d);
        System.out.println("Int: " + n);
        // --------------------------------
        // String: Hello World
        // Double: 2.3534534534523453E11
        // Int: 2147483647

        scanner.close();
    }
}






Questions and answers:
Clink on Option to Answer




1. In Java code source goes into:

  • a) a class
  • b) a file with .class extension

2. JVM Compiler creates a new related file:

  • a) MyClass.class
  • b) MyClass.java

3. Where does every Java program start?

  • a) In the main() method
  • b) In the constructor

4. What does void in main() mean?

  • a) The method returns nothing
  • b) The method returns a String

5. Every statement in Java must end with:

  • a) A semicolon ;
  • b) A colon :

6. What is a class in Java?

  • a) A blueprint for an object
  • b) An object itself

7. What do instance variables represent?

  • a) What an object knows (its data)
  • b) What an object prints

8. What do methods represent?

  • a) What an object does (its behavior)
  • b) What an object stores

9. What does this.name = name; mean?

  • a) Assign a value to the current object’s variable
  • b) Create a new class


References: