Java Type System and Language Fundamentals¶

Core Java type system covering primitives, wrappers, strings, type casting, generics, and exception handling. Foundation for all Java/Spring development.

Key Facts¶

Java has 8 primitive types: byte (1B), short (2B), int (4B), long (8B), float (4B), double (8B), char (2B), boolean (1 bit)
Wrapper classes (Integer, Long, etc.) are required for generics: List<Integer> not List<int>
Strings are immutable objects stored in JVM string pool
== compares references, .equals() compares content - always use .equals() for strings
Widening casts are implicit (int -> long), narrowing requires explicit cast and may lose data
BigDecimal for precise arithmetic (financial) - always use String constructor, never double

Patterns¶

String Operations¶

String s = "hello";
s.length();              // 5
s.substring(1, 4);       // "ell" (endIndex exclusive)
s.contains("ell");       // true
s.indexOf("l");          // 2
s.toUpperCase();         // "HELLO" - returns NEW string
s.replace("l", "r");     // "herro"
s.charAt(0);             // 'h'
s.trim();                // strip whitespace

// Parsing
int parsed = Integer.parseInt("42");
double d = Double.parseDouble("3.14");

Generics¶

public class Box<T> {
    private T content;
    public void set(T item) { this.content = item; }
    public T get() { return content; }
}

// Bounded generics
public <T extends Comparable<T>> T findMax(List<T> list) { ... }

Exception Handling¶

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Division by zero: " + e.getMessage());
} catch (Exception e) {
    System.out.println("General: " + e.getMessage());
} finally {
    // always executes
}

Hierarchy: Throwable -> Exception (+ RuntimeException) / Error. Checked exceptions must be caught or declared in Java.

Comparable and Comparator¶

// Natural ordering - class implements Comparable
public class User implements Comparable<User> {
    @Override
    public int compareTo(User other) {
        return this.name.compareTo(other.name);
    }
}

// Custom ordering - external Comparator
users.sort(Comparator.comparing(User::getAge).reversed());

BigDecimal for Financial Math¶

BigDecimal price = new BigDecimal("19.99");   // String constructor!
BigDecimal tax = new BigDecimal("0.08");
BigDecimal total = price.multiply(BigDecimal.ONE.add(tax));
// NEVER: new BigDecimal(19.99) - imprecise due to floating point

Gotchas¶

String s1 = "hello"; String s2 = new String("hello"); s1 == s2 is false - different references
s.toUpperCase() creates a NEW string, original is unchanged (immutability)
Integer.parseInt() throws NumberFormatException for invalid input
Narrowing cast truncates, does not round: (int) 9.78 = 9
Autoboxing can cause NullPointerException: Integer x = null; int y = x;