Java: What is the difference between == and equals()?

1// String with new keyword (creates separate objects)
2String s1 = new String("hello");
3String s2 = new String("hello");
4
5s1 == s2;      // FALSE — different objects in memory
6s1.equals(s2); // TRUE  — same content
7
8// String literals (stored in string pool)
9String s3 = "hello";
10String s4 = "hello";
11
12s3 == s4;      // TRUE — same object in string pool!
13s3.equals(s4); // TRUE — same content
14
15// Primitives
16int a = 5;
17int b = 5;
18a == b; // TRUE — compares values directly

1class Person {
2    private String name;
3    private int age;
4
5    public Person(String name, int age) {
6        this.name = name;
7        this.age = age;
8    }
9
10    @Override
11    public boolean equals(Object obj) {
12        if (this == obj) return true;                    // Same reference
13        if (obj == null || getClass() != obj.getClass()) return false; // Null or wrong class
14        Person other = (Person) obj;
15        return this.age == other.age
16            && Objects.equals(this.name, other.name); // Null-safe comparison
17    }
18
19    @Override
20    public int hashCode() {
21        return Objects.hash(name, age);
22    }
23}
24
25Person p1 = new Person("Alice", 30);
26Person p2 = new Person("Alice", 30);
27p1 == p2;      // FALSE — different objects
28p1.equals(p2); // TRUE  — content is equal

1// WRONG: overriding equals() without hashCode()
2Set<Person> set = new HashSet<>();
3set.add(new Person("Alice", 30));
4set.add(new Person("Alice", 30));
5set.size(); // 2 — WRONG! hashCode differs, so HashSet treats them as different
6
7// RIGHT: with both equals() and hashCode()
8Set<Person> set2 = new HashSet<>();
9set2.add(new Person("Alice", 30));
10set2.add(new Person("Alice", 30));
11set2.size(); // 1 — correct! hashCode is same, equals() returns true