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Cheatsheets / Java: Introductions and Variables

Hello World

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Print Line

System.out.println() can print to the console:

  • System is a class from the core library provided by Java
  • out is an object that controls the output
  • println() is a method associated with that object that receives a single argument
System.out.println("Hello, world!");
// Output: Hello, world!

Comments

Comments are bits of text that are ignored by the compiler. They are used to increase the readability of a program.

  • Single line comments are created by using //.
  • Multi-line comments are created by starting with /* and ending with */.
// I am a single line comment!

/*
And I am a 
multi-line comment!
*/

main() Method

In Java, every application must contain a main() method, which is the entry point for the application. All other methods are invoked from the main() method.

The signature of the method is public static void main(String[] args) { }. It accepts a single argument: an array of elements of type String.

public class Person {
  
  public static void main(String[] args) {
    
    System.out.println("Hello, world!");

  }
  
}

Classes

A class represents a single concept.

A Java program must have one class whose name is the same as the program filename.

In the example, the Person class must be declared in a program file named Person.java.

public class Person {
  
  public static void main(String[] args) {
    
    System.out.println("I am a person, not a computer.");
    
  }
  
}

Compiling Java

In Java, when we compile a program, each individual class is converted into a .class file, which is known as byte code.

The JVM (Java virtual machine) is used to run the byte code.

# Compile the class file:
javac hello.java

# Execute the compiled file:
java hello

Whitespace

Whitespace, including spaces and newlines, between statements is ignored.

System.out.println("Example of a statement");

System.out.println("Another statement");

// Output:
// Example of a statement
// Another statement

Statements

In Java, a statement is a line of code that executes a task and is terminated with a ;.

System.out.println("Java Programming ☕️");

Computing Systems & Personal Security

Using a user’s personal data can create unique and personalized online experiences for use; however, there is a risk to personal privacy when collecting and storing personal data in a program.

Computing Systems & Ethical and Social Impacts

Programs have had both positive and negative ethical and social impacts on society. Some examples of this include:

  • Machine learning: While there are positives to pattern identification and automation, there is a problem with data bias and its impact on marginalized communities.
  • AI: For example, should a driverless car make choices that protect the life of the passenger over the life of another driver or a pedestrian?

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