Introduction to Java

public class Main {
    public static void main(String[] args) { // Main method
        System.out.println("Hello World");
    }   
}

A snippet of the Hello World code

Classes

Classes are often referred to as blueprints in programming. Specifically, they represent data abstractions. Classes are used to create, or “instantiate” objects, which can be updated to represent behaviors and change.

Tip

Java is an object-oriented language, meaning that everything must be contained within a class

When thinking about classes, ask yourself, what does it represent? How does it behave?

About Java Execution

Source code becomes Java bytecode (has ending of .class). Bytecode can run on the Java Virtual Machine (JVM). JVM can run on a majority of platforms

Manually Running Java

Compile java files in the command line with javac, then execute the generated .class file with java

Tip

File names must be the same as the class name, otherwise there will be a compilation error

Differences between print() and println()

print() only appends to the standard output, but println() will print on a new line.

Tip

println(str); is equivalent to print(str + "\n");

Strings

Relevant String Methods

Signature	Description	Return Type
substring(startIndex, endIndex?)	Returns a substring from startIndex (inclusive) to endIndex (exclusive). If endIndex is not specified, it returns everything after startIndex	String
toLowerCase()	Returns a String with all lowercase letters	String
toUpperCase()	Returns a String with all uppercase letters	String
replace(target, replacement)	Returns a String which replaces all instances of target with replacement	String

Note

Methods that return a String will not change the original String, as String is immutable

Escape Sequences

Using the backslash character \, special characters can be formed

Sequence	Meaning
\n	New line or line-break character
\t	Tab
\	The backslash character \

String Concatenation

System.out.println("Hello" + " " + "World" + 5")
>>> Hello World5

Using the + operator, different primitive variables can be “added together”

Tip

Concatenation in Java is not dependent on type, unlike Python. This means combining numbers with strings isn’t a problem.

Unique Cases

System.out.println("Hi" + 5 + 5); // Hi55

System.out.println(1 + 2 + "Three"); // 3Three