Java Course

Cheatsheet

A quick reference — the essential syntax of modern Java on a single page. Press Ctrl/Cmd + P to print it.

Variables and types

Declaration and Assignment

int age = 25;
double price = 19.99;
boolean isActive = true;
char grade = 'A';
String name = "Alice";

Java is strongly typed: every variable must explicitly declare its type.

Casting and Conversions

double d = 9.8;
int i = (int) d;               // 9 (narrowing cast)
int num = Integer.parseInt("42");
String s = String.valueOf(123);

Casting from float/double to int truncates decimal values without rounding.

Strings

Common Methods

String s = "Hello";
int len = s.length();          // 5
char c = s.charAt(0);          // 'H'
String sub = s.substring(1, 4); // "ell"
boolean eq = s.equals("hello");// false
boolean eqIgnoreCase = s.equalsIgnoreCase("hello"); // true

Always use .equals() method to compare string values, not the == operator.

Concatenation and Formatting

String name = "Bob";
int age = 30;
String msg = "Name: " + name + ", Age: " + age;
String formatted = String.format("Name: %s, Age: %d", name, age);
System.out.printf("Price: %.2f", 19.99);

Control Flow

Conditional Branching

if (x > 0) {
    System.out.println("Positive");
} else if (x < 0) {
    System.out.println("Negative");
} else {
    System.out.println("Zero");
}

Loops (for, foreach, while)

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

for (String item : items) {
    System.out.println(item);
}

while (x > 0) {
    x--;
}

The foreach loop (for-each) allows easy iteration over arrays or collections.

Classes and Objects (OOP)

Class and Constructor

class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void introduce() {
        System.out.println("Hello, " + name);
    }
}

Private members (private) are not directly accessible from outside the class.

Inheritance

class Dog extends Animal {
    public Dog(String name) {
        super(name); // calls parent constructor
    }

    @Override
    public void makeSound() {
        System.out.println("Woof");
    }
}

Use the @Override annotation to explicitly indicate that a method is overriding a parent method.

Abstract Classes

abstract class Shape {
    abstract double getArea();
}

class Circle extends Shape {
    double radius;
    @Override
    double getArea() {
        return Math.PI * radius * radius;
    }
}

Interfaces and Collections

Interfaces

interface Drivable {
    void drive();
}

class Car implements Drivable {
    @Override
    public void drive() {
        System.out.println("Driving a car");
    }
}

A class can implement multiple interfaces, separated by commas.

ArrayList (Dynamic List)

import java.util.ArrayList;

ArrayList<String> list = new ArrayList<>();
list.add("Java");
list.add("Kotlin");
String first = list.get(0);    // "Java"
int size = list.size();        // 2
list.remove(1);                // Removes "Kotlin"

ArrayLists only support object types (e.g., ArrayList<Integer>, not ArrayList<int>).

Java · Cheatsheet — eLearner.app

Variables and types

Strings

Control Flow

Classes and Objects (OOP)

Interfaces and Collections