java.sql.SQLException: Io exception: The Network Adapter could not establish the connection
at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:134)
at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:179)
at oracle.jdbc.dbaccess.DBError.throwSqlException(DBError.java:334)
at oracle.jdbc.ttc7.TTC7Protocol.handleIOException(TTC7Protocol.java:3668)
at oracle.jdbc.ttc7.TTC7Protocol.logon(TTC7Protocol.java:353)
at oracle.jdbc.driver.OracleConnection.<init>(OracleConnection.java:371)
at oracle.jdbc.driver.OracleDriver.getConnectionInstance(OracleDriver.java:551)
at oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:351)
at java.sql.DriverManager.getConnection(DriverManager.java:571)
at java.sql.DriverManager.getConnection(DriverManager.java:215)
....

[root@CLIENT ~]# telnet [Oracle Host IP] 1521
Trying [Oracle Host IP]...
Connected to [Oracle Host IP].
Escape character is '^]'.
^C

Connection closed by foreign host.

import java.sql.DriverManager;

public class ConnectOradb {
    public static void main (String [] args) throws Exception {
        String url = "jdbc:oracle:thin:@localhost:1521:orcl";
        // may be unnecessary (but harmless) for newer versions
        Class.forName("oracle.jdbc.driver.OracleDriver").newInstance();
        DriverManager.getConnection(url, "user", "pass");
    }
}