Java Programming Glossary: Key Words Explained Simply

CLASS + OBJECT:

public class Car {
    String color; // variable
    int speed;    // variable

    void start() { // method
        System.out.println("Car started");
    }
}

// Object = instance of the class
Car myCar = new Car();
myCar.start();

METHOD + VARIABLE:

public class Calculator {
    int result; // variable

    int add(int a, int b) { // method
        return a + b;
    }
}

Calculator calc = new Calculator();
int sum = calc.add(5, 3);  // 8

ENCAPSULATION:

public class Account {
    private double balance; // hidden

    public double getBalance() {
        return balance;
    }

    public void deposit(double amount) {
        if (amount > 0) balance += amount;
    }
}

// balance is hidden — only accessible via methods

INHERITANCE:

class Animal {
    void eat() { System.out.println("Eating"); }
}

class Dog extends Animal {
    void bark() { System.out.println("Barking"); }
}

Dog d = new Dog();
d.eat();  // inherited
d.bark(); // own method

STREAM:

List nums = List.of(1, 2, 3, 4, 5);

int sum = nums.stream()
    .filter(n -> n % 2 == 0)
    .mapToInt(Integer::intValue)
    .sum();

System.out.println(sum); // 6

LAMBDA:

// Old way: anonymous class
Runnable r = new Runnable() {
    public void run() {
        System.out.println("Running");
    }
};

// New way: lambda
Runnable r = () -> System.out.println("Running");

// Functional interface
Function square = x -> x * x;
System.out.println(square.apply(5)); // 25