Objects and interfaces¶

Encapsulation¶

Encapsulation keeps state and the operations that protect it together:

final class Account {
    private long balance;

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

    long balance() {
        return balance;
    }
}

Private fields provide access control; methods preserve the object's invariants.

Inheritance and polymorphism¶

abstract class Shape {
    abstract double area();
}

final class Circle extends Shape {
    private final double radius;

    Circle(double radius) {
        this.radius = radius;
    }

    @Override
    double area() {
        return Math.PI * radius * radius;
    }
}

A class can extend one class and implement multiple interfaces.

Interfaces¶

interface Named {
    String name();

    default String displayName() {
        return name().toUpperCase();
    }
}

Modern interfaces may declare abstract, default, static, and private methods. Interface fields are implicitly public static final. Interfaces cannot be instantiated directly.

An abstract class that implements an interface may leave methods unimplemented. A concrete subclass must provide the remaining implementations.

Creating objects¶

The normal construction syntax is:

Account account = new Account();

Reflection, cloning, deserialization, factories, and dependency-injection frameworks can also produce objects, but they are specialized mechanisms rather than replacements for constructors.

Verified repository programs¶

Runnable Deterministic output Stable Java

Run the polymorphism example:

java --enable-preview \
  -cp build/classes/java/main:build/resources/main \
  nitin.a5object.AnimalDoctor

Cat Checkup
Bird Checkup
Bird Checkup
Bird Checkup
Dog Checkup
Dog Checkup

Run the interface example:

java --enable-preview \
  -cp build/classes/java/main:build/resources/main \
  nitin.a6oops.interfaces.AnotherServiceProvider

From Abstract Class, Not all Methods has to be implemented

Both outputs are verified by scripts/verify-doc-examples.sh.

Repository examples¶

src/main/java/nitin/a5object
src/main/java/nitin/a6oops
src/main/java/nitin/a6oops/interfaces/I1InterfaceMethod.java
src/main/java/nitin/nestedClasses
src/main/java/nitin/cloning