Syntax and types

public class Main {
    public static void main(String[] args) {
        String name = "Ada";
        int age = 28;
        double price = 29.99;
        System.out.println(name + " is " + age + " years old");
    }
}

Every Java file needs a class matching its filename, and execution starts from a main method -- this boilerplate is unavoidable even for the smallest program.

Java's primitive types (int, double, boolean, char, long, ...) are not objects -- they live directly on the stack and have no methods. Each has a corresponding wrapper class (Integer, Double, Boolean, ...) used whenever you need an object, such as inside a generic collection:

int primitive = 42;
Integer boxed = 42;          // autoboxing: primitive -> wrapper, automatic
int unboxed = boxed;         // unboxing: wrapper -> primitive, automatic

List<Integer> scores = new ArrayList<>();
scores.add(95); // autoboxed to Integer automatically

This autoboxing is convenient but has a sharp edge: wrapper objects compare by reference with ==, not by value (except for a small cached range of Integer values from -128 to 127, which makes the bug even sneakier since it "works" for small numbers):

Integer a = 200;
Integer b = 200;
System.out.println(a == b);        // false! different objects
System.out.println(a.equals(b));    // true -- compares value

Integer small1 = 100;
Integer small2 = 100;
System.out.println(small1 == small2); // true, thanks to caching -- don't rely on this

Always use .equals() to compare wrapper objects (and any object type, including String) -- reserve == for primitives, where it correctly compares values.

String concatenation with + is convenient for a couple of values, but building a string across a loop with + creates a new String object on every iteration, since strings are immutable in Java too. For loops, StringBuilder is the efficient choice:

StringBuilder sb = new StringBuilder();
for (Course c : courses) {
    sb.append(c.getTitle()).append(", ");
}
String joined = sb.toString();