☕ Java Cheatsheet
A one-page reference for the Java syntax you'll meet first. Press Ctrl + P to save as PDF — the print stylesheet hides everything except the code.
Hello, World
java
public class Main {
public static void main(String[] args) {
System.out.println("Hello, world!");
}
}Variables & Types
java
int n = 42;
long big = 9_000_000_000L;
double pi = 3.14;
String s = "hello";
boolean b = true;
final int MAX = 100; // immutable
var x = 42; // type inferred (Java 10+)Strings
java
String s = "Aisha";
s.length();
s.toUpperCase();
s.contains("is");
s.replace("A", "B");
s.split(",");
String.join(",", new String[]{"a","b"});
s.equals("hi"); // CONTENT compare
// s == "hi" compares references — usually wrongArrays
java
int[] xs = {1, 2, 3};
xs.length; // property, no parens
xs[0]; xs[xs.length-1];
Arrays.sort(xs);
int[] copy = xs.clone();Lists (Resizable)
java
List<Integer> list = new ArrayList<>();
list.add(1);
list.remove(0);
list.set(0, 99);
list.size();
list.contains(1);
for (int n : list) { ... }Maps
java
Map<String, Integer> m = new HashMap<>();
m.put("x", 1);
m.get("x");
m.containsKey("x");
m.remove("x");
for (var e : m.entrySet()) {
e.getKey(); e.getValue();
}Conditionals
java
if (cond) { ... } else if (cond2) { ... } else { ... }
// Ternary
String status = age >= 18 ? "adult" : "minor";
switch (day) {
case "Sat":
case "Sun": System.out.println("weekend"); break;
default: System.out.println("weekday");
}Loops
java
for (int i = 0; i < 5; i++) { ... }
for (int x : xs) { ... }
while (cond) { ... }
do { ... } while (cond);Methods
java
public static int add(int a, int b) {
return a + b;
}
public static void main(String[] args) {
int sum = add(2, 3);
}Classes
java
public class User {
private String name;
public User(String name) { this.name = name; }
public String getName() { return name; }
public String greet() { return "Hi, " + name; }
}
User u = new User("Aisha");📌
Need the concepts?
This is a syntax reference, not a tutorial. Each row maps to a concept lesson — start with the 13 concepts if anything here is unfamiliar.