Java Cheatsheet

Setup & Environment

  • Install Java (macOS)
brew install openjdk
  • Install Java (Linux)
# Ubuntu/Debian
sudo apt update
sudo apt install default-jdk

# CentOS/RHEL
sudo yum install java-11-openjdk-devel
  • Install Java (Windows)
  1. Download JDK from Oracle or OpenJDK
  2. Run the installer and follow the prompts
  3. Set JAVA_HOME environment variable
  • Check Java version
java -version
javac -version
  • Set up Java environment variables (add to ~/.bashrc or ~/.zshrc)
export JAVA_HOME=/usr/lib/jvm/java-11-openjdk
export PATH=$PATH:$JAVA_HOME/bin

Building & Running

  • Compile a Java file
javac HelloWorld.java
  • Run a Java program
java HelloWorld
  • Compile and run with classpath
javac -cp "lib/*" MyApp.java
java -cp ".:lib/*" MyApp
  • Create a JAR file
jar cvf myapp.jar *.class
  • Run a JAR file
java -jar myapp.jar

Maven

  • Create a new Maven project
mvn archetype:generate -DgroupId=com.example -DartifactId=myapp -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false
  • Compile project
mvn compile
  • Run tests
mvn test
  • Package project
mvn package
  • Clean project
mvn clean
  • Install to local repository
mvn install

Gradle

  • Create a new Gradle project
gradle init --type java-application
  • Build project
./gradlew build
  • Run tests
./gradlew test
  • Run application
./gradlew run
  • Clean project
./gradlew clean

Basic Java Structure

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Variables and Data Types

  • Primitive types
int number = 42;
double decimal = 3.14;
boolean flag = true;
char letter = 'A';
byte smallNumber = 127;
short mediumNumber = 32767;
long largeNumber = 123456789L;
float floatNumber = 2.5f;
  • Strings
String text = "Hello";
String name = new String("World");
  • Arrays
int[] numbers = {1, 2, 3, 4, 5};
String[] names = new String[3];
names[0] = "Alice";
  • Constants
final int MAX_SIZE = 100;
static final String CONSTANT = "VALUE";

Control Flow

  • If/else statement
if (x > 0) {
    System.out.println("Positive");
} else if (x < 0) {
    System.out.println("Negative");
} else {
    System.out.println("Zero");
}
  • Switch statement
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    default:
        System.out.println("Other day");
}
  • For loop
for (int i = 0; i < 10; i++) {
    System.out.println(i);
}
  • Enhanced for loop
int[] array = {1, 2, 3, 4, 5};
for (int num : array) {
    System.out.println(num);
}
  • While loop
int i = 0;
while (i < 10) {
    System.out.println(i);
    i++;
}
  • Do-while loop
int i = 0;
do {
    System.out.println(i);
    i++;
} while (i < 10);

Classes and Objects

  • Define a class
public class Person {
    private String name;
    private int age;
    
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String getName() {
        return name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
    
    public int getAge() {
        return age;
    }
    
    public void setAge(int age) {
        this.age = age;
    }
}
  • Create and use objects
Person person = new Person("Alice", 30);
System.out.println(person.getName());
person.setAge(31);

Inheritance

  • Extend a class
public class Student extends Person {
    private String studentId;
    
    public Student(String name, int age, String studentId) {
        super(name, age);
        this.studentId = studentId;
    }
    
    public String getStudentId() {
        return studentId;
    }
}
  • Override methods
@Override
public String toString() {
    return "Student{" +
           "name='" + getName() + '\'' +
           ", age=" + getAge() +
           ", studentId='" + studentId + '\'' +
           '}';
}

Interfaces

  • Define an interface
public interface Drawable {
    void draw();
    default void print() {
        System.out.println("Printing...");
    }
}
  • Implement an interface
public class Circle implements Drawable {
    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }
}

Exception Handling

  • Try-catch block
try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Error: " + e.getMessage());
} finally {
    System.out.println("This always executes");
}
  • Throw an exception
public void checkAge(int age) throws IllegalArgumentException {
    if (age < 0) {
        throw new IllegalArgumentException("Age cannot be negative");
    }
}

Collections

  • ArrayList
import java.util.ArrayList;
import java.util.List;

List<String> names = new ArrayList<>();
names.add("Alice");
names.add("Bob");
names.remove("Alice");
  • HashMap
import java.util.HashMap;
import java.util.Map;

Map<String, Integer> ages = new HashMap<>();
ages.put("Alice", 30);
ages.put("Bob", 25);
int aliceAge = ages.get("Alice");
  • HashSet
import java.util.HashSet;
import java.util.Set;

Set<String> uniqueNames = new HashSet<>();
uniqueNames.add("Alice");
uniqueNames.add("Bob");
uniqueNames.add("Alice"); // Duplicate, won't be added

File I/O

  • Read file
import java.io.*;
import java.util.Scanner;

try (Scanner scanner = new Scanner(new File("file.txt"))) {
    while (scanner.hasNextLine()) {
        String line = scanner.nextLine();
        System.out.println(line);
    }
} catch (FileNotFoundException e) {
    System.out.println("File not found: " + e.getMessage());
}
  • Write file
import java.io.*;

try (PrintWriter writer = new PrintWriter("output.txt")) {
    writer.println("Hello, World!");
    writer.println("Writing to file");
} catch (FileNotFoundException e) {
    System.out.println("Error writing file: " + e.getMessage());
}

Useful Methods

  • String methods
String text = "Hello World";
int length = text.length();
String upper = text.toUpperCase();
String lower = text.toLowerCase();
boolean contains = text.contains("World");
String[] parts = text.split(" ");
String trimmed = text.trim();
  • Math methods
int max = Math.max(10, 20);
int min = Math.min(10, 20);
double sqrt = Math.sqrt(16);
double random = Math.random(); // 0.0 to 1.0
int randomInt = (int) (Math.random() * 100); // 0 to 99
  • Array methods
import java.util.Arrays;

int[] numbers = {3, 1, 4, 1, 5};
Arrays.sort(numbers);
String arrayString = Arrays.toString(numbers);

Common Patterns

  • Singleton pattern
public class Singleton {
    private static Singleton instance;
    
    private Singleton() {}
    
    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}
  • Builder pattern
public class User {
    private String name;
    private int age;
    private String email;
    
    private User(Builder builder) {
        this.name = builder.name;
        this.age = builder.age;
        this.email = builder.email;
    }
    
    public static class Builder {
        private String name;
        private int age;
        private String email;
        
        public Builder setName(String name) {
            this.name = name;
            return this;
        }
        
        public Builder setAge(int age) {
            this.age = age;
            return this;
        }
        
        public Builder setEmail(String email) {
            this.email = email;
            return this;
        }
        
        public User build() {
            return new User(this);
        }
    }
}

// Usage
User user = new User.Builder()
    .setName("Alice")
    .setAge(30)
    .setEmail("alice@example.com")
    .build();

Useful Packages

  • "java.util" — Collections, utilities
  • "java.io" — Input/output
  • "java.nio" — New I/O API
  • "java.lang" — Core language features
  • "java.time" — Date and time API
  • "java.util.stream" — Stream API
  • "java.util.concurrent" — Concurrency utilities

Resources