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

1// Example 1: Two different String 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// Example 2: String literals (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// Example 3: Custom class without equals()
16class Person {
17    String name;
18    Person(String name) { this.name = name; }
19}
20
21Person p1 = new Person("Alice");
22Person p2 = new Person("Alice");
23
24p1 == p2;      // FALSE — different objects
25p1.equals(p2); // FALSE — uses default Object.equals() which is ==

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        // Same reference → equal
13        if (this == obj) return true;
14
15        // Null or different class → not equal
16        if (obj == null || getClass() != obj.getClass()) return false;
17
18        // Cast and compare fields
19        Person other = (Person) obj;
20        return this.age == other.age
21            && Objects.equals(this.name, other.name);
22    }
23
24    @Override
25    public int hashCode() {
26        return Objects.hash(name, age);
27    }
28}
29
30Person p1 = new Person("Alice", 30);
31Person p2 = new Person("Alice", 30);
32p1.equals(p2); // TRUE — content is equal

1// WRONG: override equals() but not 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 (hashCode differs)