Print Line
System.out.println() can print to the console:
Systemis a class from the core library provided by Javaoutis an object that controls the outputprintln()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 helloWhitespace
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 statementStatements
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?