What are the core object-oriented principles in Java?

Encapsulation hides internal state behind methods and enforces invariants. Abstraction exposes a simplified model and hides unnecessary detail. Inheritance expresses an is-a relationship. Polymorphism lets one reference invoke behaviour that depends on the runtime type.

public final class Account {
    private long balance;

    public void deposit(long amount) {
        if (amount <= 0) {
            throw new IllegalArgumentException("amount must be positive");
        }
        balance += amount;
    }

    public long balance() {
        return balance;
    }
}

How the principles reinforce each other

• Encapsulation makes abstraction possible because callers depend on behaviour, not fields
• Inheritance can share a contract, but it also creates coupling
• Polymorphism lets new implementations plug in without changing callers
• Composition is often a safer reuse mechanism than inheritance

Encapsulation and abstraction are related but not identical. Encapsulation is about access control and invariants. Abstraction is about choosing the right model and hiding accidental complexity. A class can have private fields and still expose a poor abstraction.