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

1String s1 = new String("hello");
2String s2 = new String("hello");
3s1 == s2;      // FALSE
4s1.equals(s2); // TRUE
5
6String s3 = "hello";
7String s4 = "hello";
8s3 == s4;      // TRUE (pool)
9s3.equals(s4); // TRUE
10
11int a = 5, b = 5;
12a == 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 age == other.age && Objects.equals(name, other.name);
11    }
12
13    @Override
14    public int hashCode() {
15        return Objects.hash(name, age);
16    }
17}
18
19Person p1 = new Person("Alice", 30);
20Person p2 = new Person("Alice", 30);
21p1.equals(p2); // TRUE

1Set<Person> set = new HashSet<>();
2set.add(new Person("Alice", 30));
3set.add(new Person("Alice", 30));
4set.size(); // 1 (with hashCode), 2 (without — WRONG)