Strings, Object Contract & Memory Internals

public class StringMemoryDemo {
    public static void main(String[] args) {
        // String Literals: Reuses the single instance in String Constant Pool (SCP)
        String s1 = "Java";
        String s2 = "Java";

        // 'new' Operator: FORCES a brand-new object on Heap outside SCP
        String s3 = new String("Java");

        // == compares MEMORY ADDRESS POINTERS
        System.out.println(s1 == s2);      // true  (Both point to same SCP address!)
        System.out.println(s1 == s3);      // false (Different Heap addresses!)

        // .equals() compares CHARACTER CONTENTS
        System.out.println(s1.equals(s3)); // true  (Both hold characters 'J','a','v','a')

        // .intern() moves/finds the reference in the String Constant Pool
        String s4 = s3.intern();
        System.out.println(s1 == s4);      // true  (Now pointing to same SCP instance!)
    }
}

package com.accenture.ltt.oop;

import java.util.*;

/**
 * Standard Production Immutable Class Design:
 * 1. Declare class 'final' to prevent subclasses from adding mutable state.
 * 2. All fields are 'private final'.
 * 3. No setter methods.
 * 4. Perform defensive copying in constructor for mutable objects (Lists, Dates).
 * 5. Perform defensive copying in getters.
 */
public final class ImmutableEmployee {
    private final String empId;
    private final List<String> skills;

    public ImmutableEmployee(String empId, List<String> skills) {
        this.empId = empId;
        // Defensive copy in constructor: Prevents caller from mutating our internal list!
        this.skills = new ArrayList<>(skills);
    }

    public String getEmpId() { return empId; }

    public List<String> getSkills() {
        // Return unmodifiable view: Prevents caller from adding elements to our list!
        return Collections.unmodifiableList(skills);
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        ImmutableEmployee that = (ImmutableEmployee) o;
        return Objects.equals(empId, that.empId);
    }

    @Override
    public int hashCode() {
        return Objects.hash(empId);
    }
}