Intro to Java Programming — Java Cheat Sheets

1 YOUR FIRST JAVA PROGRAM

public class Welcome1 { public static void main(String[] args) { System.out.println("Welcome to Java!"); } }

Anatomy of the program

class Welcome1 — every app needs at least one class
public static void main(String[] args) — the entry point the JVM calls first
System.out.println(...) — prints text + moves to a new line
File name must match the public class name: Welcome1.java
Statements end with a semicolon ;

2 MODIFYING OUTPUT & printf

print vs println vs printf

print() — no newline after
println() — newline after
printf() — formatted output with placeholders

System.out.printf("%s is %d years old.%n", "Amit", 21); // %s = string, %d = int, %n = newline

3 READING INPUT & ARITHMETIC

Scanner input = new Scanner(System.in); System.out.print("Enter two integers: "); int num1 = input.nextInt(); int num2 = input.nextInt(); int sum = num1 + num2;

Arithmetic operators

Op	Meaning
`+ - *`	add, subtract, multiply
`/`	division (int / int = int)
`%`	remainder (modulus)

Precedence: () → * / % → + -, left-to-right.

4 DECISION MAKING — EQUALITY & RELATIONAL OPERATORS

Operator	Meaning
`==`	equal to
`!=`	not equal to
`> <`	greater / less than
`>= <=`	greater/less than or equal

if (num1 == num2) { System.out.println("Numbers are equal"); }

5 OBJECTS-NATURAL CASE STUDY: THE STRING CLASS

Creating & using

String name = "Java";
Strings are objects — immutable once created

Common methods

length() — number of characters
toUpperCase()/toLowerCase()
charAt(i) — character at index
equals() — compare content

String s = "Java"; System.out.println(s.length()); // 4 System.out.println(s.toUpperCase()); // JAVA

6 METHOD REFERENCE TABLE

class String

Method	Returns
`length()`	number of characters
`charAt(i)`	char at index i
`substring(a,b)`	substring from a to b-1
`toUpperCase()/toLowerCase()`	new cased String
`equals(obj)`	true if content matches
`trim()`	String with whitespace removed from ends

class Scanner

Method	Returns
`nextInt()`	next token as int
`nextDouble()`	next token as double
`next()`	next token as String
`nextLine()`	rest of the current line as String
`hasNextInt()`	true if next token is an int