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
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.