Core Java

Core Java covers the foundational building blocks that everything else rests on. A deep understanding of how primitives and objects differ, how the JVM handles memory, and how OOP concepts apply in Java is expected in any Java engineering interview.

Key Points

• Primitives (byte, short, int, long, float, double, char, boolean) live on the stack; their wrapper classes (Integer, Long, etc.) are objects on the heap
• Autoboxing converts primitives to wrappers automatically — beware NullPointerException when unboxing null and performance overhead in hot loops
• String is immutable; String literals are interned in the String Pool — use StringBuilder for string concatenation in loops
• OOP pillars: Encapsulation (hide state), Inheritance (reuse via extends), Polymorphism (runtime dispatch via overriding), Abstraction (interfaces / abstract classes)
• Interface vs Abstract Class: interfaces support multiple inheritance, default methods since Java 8; abstract classes can have state and constructors
• Generics enforce compile-time type safety — wildcards: ? extends T (covariant/producer), ? super T (contravariant/consumer) — PECS mnemonic
• Exception hierarchy: Throwable → Error (JVM errors, don't catch) / Exception → RuntimeException (unchecked) / checked exceptions
• try-with-resources (Java 7) automatically closes AutoCloseable resources — preferred over finally blocks
• Enums are full classes — can have fields, methods, and implement interfaces

Core Java patterns: generics, enums with behaviour, try-with-resources, equals/hashCode

// Generics + bounded wildcards
public <T extends Comparable<T>> T max(List<T> list) {
    return list.stream().max(Comparator.naturalOrder()).orElseThrow();
}

// Enums with behaviour
enum Planet {
    MERCURY(3.303e+23, 2.4397e6),
    EARTH  (5.976e+24, 6.37814e6);

    private final double mass, radius;
    Planet(double mass, double radius) {
        this.mass = mass; this.radius = radius;
    }
    double surfaceGravity() { return 6.67300E-11 * mass / (radius * radius); }
}

// try-with-resources
try (var reader = new BufferedReader(new FileReader("data.txt"))) {
    String line;
    while ((line = reader.readLine()) != null) process(line);
} // reader.close() called automatically

// equals + hashCode contract (always override together)
@Override public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof Point p)) return false;
    return x == p.x && y == p.y;
}
@Override public int hashCode() { return Objects.hash(x, y); }