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

1String s1 = new String("hello");
2String s2 = new String("hello");
3s1 == s2;      // FALSE — different objects
4s1.equals(s2); // TRUE  — same content
5
6String s3 = "hello";
7String s4 = "hello";
8s3 == s4;      // TRUE — same pool object
9s3.equals(s4); // TRUE — same content
10
11// Primitives:
12int a = 5, b = 5;
13a == b; // TRUE

1class Person {
2    private String name;
3    private int age;
4
5    @Override
6    public boolean equals(Object obj) {
7        if (this == obj) return true;
8        if (obj == null || getClass() != obj.getClass()) return false;
9        Person other = (Person) obj;
10        return this.age == other.age
11            && Objects.equals(this.name, other.name);
12    }
13
14    @Override
15    public int hashCode() {
16        return Objects.hash(name, age);
17    }
18}
19
20Person p1 = new Person("Alice", 30);
21Person p2 = new Person("Alice", 30);
22p1 == p2;      // FALSE
23p1.equals(p2); // TRUE

1// WRONG: equals() without hashCode()
2Set<Person> set = new HashSet<>();
3set.add(new Person("Alice", 30));
4set.add(new Person("Alice", 30));
5set.size(); // 2 — WRONG! Should be 1
6
7// RIGHT: with hashCode()
8Set<Person> set2 = new HashSet<>();
9set2.add(new Person("Alice", 30));
10set2.add(new Person("Alice", 30));
11set2.size(); // 1 — correct