Java Basics Cheat Sheet

public class Main {                              // file must be Main.java
    public static void main(String[] args) {
        System.out.println("Hello, Java!");
    }
}

javac Main.java
java Main
java Main.java        # run a single file directly (Java 11+)

int count = 42;                 // whole numbers (32-bit)
long big = 9_000_000_000L;      // 64-bit, note the L
double price = 49.99;           // decimals (default choice)
char grade = 'A';               // single quotes
boolean ok = true;
String name = "Asha";           // double quotes, a class
var total = price * count;      // inferred: double

final double TAX = 0.18;        // cannot change
static final int MAX_USERS = 100;   // class constant (field)

int a = 7 / 2;                    // 3
double b = 7.0 / 2;               // 3.5
int c = 7 % 2;                    // 1
double d = (double) 7 / 2;        // 3.5
double p = Math.pow(2, 10);       // 1024.0
long r = Math.round(2.5);         // 3

int n = Integer.parseInt("42");          // String -> int (NumberFormatException if invalid)
double x = Double.parseDouble("3.75");
String s = String.valueOf(99);           // number -> String
int i = (int) 99.99;                     // 99 (truncates)
int digit = '7' - '0';                   // 7

System.out.println("Line");                     // with newline
System.out.print("No newline");
System.out.printf("%s is %d, %.2f%n", "Asha", 21, 92.5);
String text = String.format("%05d", 42);        // "00042"

Scanner in = new Scanner(System.in);            // import java.util.Scanner;
int age = in.nextInt();
in.nextLine();                                  // consume the leftover newline
String line = in.nextLine();

String s = "Hello, Java";
s.length();                  // 11
s.charAt(0);                 // 'H'
s.substring(7);              // "Java"
s.indexOf("Java");           // 7 (-1 if missing)
s.contains("Java");          // true
s.toUpperCase();             // new string: "HELLO, JAVA"
s.replace("Java", "World");  // "Hello, World"
"  hi  ".strip();            // "hi"
"a,b,c".split(",");          // ["a", "b", "c"]
String.join("-", "a", "b");  // "a-b"
s.equals("Hello, Java");     // compare text (never ==)
"ab".repeat(3);              // "ababab"

StringBuilder sb = new StringBuilder();
sb.append("x").append(1).reverse();   // build text in loops

if (score >= 90) {
    grade = 'A';
} else if (score >= 75) {
    grade = 'B';
} else {
    grade = 'C';
}

String kind = switch (day) {          // switch expression (Java 14+)
    case 1, 2, 3, 4, 5 -> "weekday";
    case 6, 7 -> "weekend";
    default -> "invalid";
};

for (int i = 0; i < 5; i++) { }        // 0..4
for (String item : items) { }          // for-each
while (balance < target) { }           // check first
do { } while (choice != 0);            // at least once

break;                                 // leave the loop
continue;                              // next iteration
outer:                                 // labeled break leaves nested loops
for (int r = 0; r < 3; r++) {
    for (int c = 0; c < 3; c++) {
        if (r * c == 2) break outer;
    }
}

int[] nums = {42, 7, 19};
int[] zeros = new int[5];              // 0 0 0 0 0
int len = nums.length;                 // 3 (a field, no parentheses)
int first = nums[0];                   // 42 (index out of range throws)
Arrays.sort(nums);                     // import java.util.Arrays;
String shown = Arrays.toString(nums);  // "[7, 19, 42]"
int[] firstTwo = Arrays.copyOf(nums, 2);   // [7, 19]
int[][] grid = new int[3][4];          // 3 rows, 4 columns

static int square(int n) {             // call from main without an object
    return n * n;
}

static double average(double... values) { return 0; }   // varargs

static int area(int side) { return side * side; }             // overloads:
static int area(int width, int height) { return width * height; }   // same name, different parameters