String s1 = "Hello";
String s2 = new String("Hello");

char[] chars = {'J', 'a', 'v', 'a'};
String s3 = new String(chars);

String s1 = "abc";
String s2 = "abc";
System.out.println(s1 == s2); // true — обе ссылаются на одну строку из пула

String s1 = new String("abc");
String s2 = "abc";
System.out.println(s1 == s2); // false

String s1 = new String("abc").intern();
String s2 = "abc";
System.out.println(s1 == s2); // true

String s = "Hello, World!";
int length = s.length();
char ch = s.charAt(0);
String lower = s.toLowerCase();
boolean contains = s.contains("World");

String original = "Java";
String modified = original.replace("J", "K");
System.out.println(original); // Java
System.out.println(modified); // Kava

String a = "hello";
String b = new String("hello");

System.out.println(a == b);          // false — разные объекты
System.out.println(a.equals(b));     // true — одинаковое содержимое
Оператор == сравнивает ссылки, а метод equals() — содержимое.

String result = "";
for (int i = 0; i < 1000; i++) {
    result += "a"; // создается 1000 новых строк
}

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 1000; i++) {
    sb.append("a");
}
String result = sb.toString();

String name = null;
if (name.equals("admin")) { // ошибка
}

if ("admin".equals(name)) { // безопасно
}

public class Task130625 {
    public static void main(String[] args) {
        String s1 = "Java";
        String s2 = new String("Java").intern();
        String s3 = new StringBuilder("Ja").append("va").toString();
        String s4 = s3.intern();
        
        System.out.println((s1 == s2) + " " + (s1 == s3) + " " + (s1 == s4));
    }
}

V<Версия>__<Описание>.sql

-- V1__Create_users_table.sql
CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    username VARCHAR(50) NOT NULL
);

R__<Описание>.sql

-- R__Update_user_view.sql
CREATE OR REPLACE VIEW user_view AS
SELECT id, username FROM users;

public class V2__InsertTestData implements JdbcMigration {
    @Override
    public void migrate(Connection connection) throws Exception {
        try (Statement stmt = connection.createStatement()) {
            stmt.execute("INSERT INTO users (username) VALUES ('admin')");
        }
    }
}

public class V3__AddAdminUser implements JavaMigration {
    @Override
    public void migrate(Context context) throws Exception {
        try (Statement stmt = context.getConnection().createStatement()) {
            stmt.execute("INSERT INTO users (username) VALUES ('superuser')");
        }
    }
}

V<Мажор>.<Минор>.<Патч>__<Описание>.sql

-- V1.2.3__Fix_email_constraint.sql
ALTER TABLE users ALTER COLUMN email SET NOT NULL;

-- V3__Drop_users_table.sql
DROP TABLE users;

-- V1__Initial_schema.sql
CREATE TABLE users (
    id INT PRIMARY KEY,
    name VARCHAR(100)
);

-- R__Update_view.sql
CREATE OR REPLACE VIEW active_users AS 
SELECT * FROM users WHERE deleted_at IS NULL;

public class V4__EncryptPasswords implements JavaMigration {
    @Override
    public void migrate(Context context) throws Exception {
        try (PreparedStatement ps = context.getConnection()
            .prepareStatement("UPDATE users SET password = encrypt(password)")) {
            ps.executeUpdate();
        }
    }
}

import java.util.stream.IntStream;

public class Task160625 {
    public static void main(String[] args) {
        IntStream.rangeClosed(1, 5)
                .filter(i -> i % 2 == 0)
                .peek(System.out::print)
                .forEachOrdered(i -> {});
    }
}