Java匿名内部类声明

Question

How to define the anonymous inner class separately from the constructor block?

For example in my codes I want method2 do the same thing as method1 The only difference is method2 uses the Factory class to create the ClosableResultSet.

import java.sql.*;

public class Demo {

    public static void main(String[] args) {
        method1();
        method2();  
    }

    static void method1() {

        Connection conn;
        try {
            conn = DriverManager.getConnection("jdbc:h2:C:/myDB", "sa", "sa");

            String sql = "select * from Customers";

            try (ClosableResultSet rs = new ClosableResultSet(conn, sql) {
                @Override
                public void handleError(SQLException e, String action) {
                    System.out.print(action + ": " + e.getMessage());       
                }               
            }) {
                while (rs.next()) {
                    System.out.print("Name: " + rs.getString("name") + "\n");
                }
            }

        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }               

    }


    static void method2() {

        String sql = "select * from Customers";

        //here has the syntax errors    
        try (ClosableResultSet rs = Factory.createResultSet(sql) {
            @Override
            public void handleError(SQLException e, String action) {
                System.out.print(action + ": " + e.getMessage());           
            }               
        }) {
            while (rs.next()) {
                System.out.print("Name: " + rs.getString("name") + "\n");
            }


        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }           

    }   
}



import java.sql.*;

public class Factory {
    public static ClosableResultSet createResultSet(String sql) throws SQLException {
        Connection conn = DriverManager.getConnection("jdbc:h2:C:/myDB", "sa", "sa");

        return new ClosableResultSet(conn, sql);
    }
}


import java.sql.*;

public class ClosableResultSet implements java.io.Closeable {

    private ResultSet rs;

    public ClosableResultSet(Connection conn, String sql) throws SQLException  {
        Statement stmt = conn.createStatement();
        rs = stmt.executeQuery(sql);    
    }

    public Boolean next() {
        try {
            return rs.next();
        } catch (SQLException e) {
            handleError(e, "next");
            return false;
        }
    }

    public String getString(String columnLabel) {
        try {
            return rs.getString(columnLabel);
        } catch (SQLException e) {
            handleError(e, "get " + columnLabel);
            return "";
        }
    }


    @Override
    public void close() {       
        try {
            rs.close();
        } catch (SQLException e) {
            handleError(e, "close");
        }

    }

    public void handleError(SQLException e, String action) {
        // to be override   
    }

}

I don't want to declare the anonymous inner class in the Factory class, is there any lambda expression allow me to put it in method2?

Answer 1

Yes, you can do something like this:

import java.sql.*;

public class Demo {

    public static void main(String[] args) {
        method1();
        method2();  
    }

    static void method1() {

        Connection conn;
        try {
            conn = DriverManager.getConnection("jdbc:h2:C:/myDB", "sa", "sa");

            String sql = "select * from Customers";

            try (ClosableResultSet rs = new ClosableResultSet(conn, sql) {
                @Override
                public void handleError(SQLException e, String action) {
                    System.out.print(action + ": " + e.getMessage());       
                }               
            }) {
                while (rs.next()) {
                    System.out.print("Name: " + rs.getString("name") + "\n");
                }
            }
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }               
    }

    static void method2() {

        String sql = "select * from Customers";

        //here has the syntax errors
        try (ClosableResultSet rs = Factory.createResultSet(
                sql, (e, action) -> System.out.print(action + ": " + e.getMessage())
        )) {
            while (rs.next()) {
                System.out.print("Name: " + rs.getString("name") + "\n");
            }
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

...

import java.sql.*;

public class Factory {
        public static ClosableResultSet createResultSet(String sql, final ErrorHandller errorHandller) throws SQLException {
        Connection conn = DriverManager.getConnection("jdbc:h2:C:/myDB", "sa", "sa");

        return new ClosableResultSet(conn, sql) {
            @Override
            public void handleError(SQLException e, String action) {
                errorHandller.handleError(e, action);
            }
        };
    }

    interface ErrorHandller {
        void handleError(SQLException e, String action);
    }
}

...

As you can see, new interface ErrorHandller was added. Also Factory::createResultSet receive instance of ErrorHandller as second argument now. And then in Demo::method2 we pass lambda as the second argument of Factory.createResultSet method.

Answer 2

You can create another Factory method that takes a BiConsumer in addition to the String sql . Something like this:

import java.sql.*;
import java.util.function.BiConsumer;

public class Factory {

    public static ClosableResultSet createResultSet(String sql) throws SQLException {
        Connection conn = DriverManager.getConnection("jdbc:h2:C:/myDB", "sa", "sa");

        return new ClosableResultSet(conn, sql);
    }

    public static ClosableResultSet createResultSet(String sql, BiConsumer<SQLException, String> handler) throws SQLException {
        Connection conn = DriverManager.getConnection("jdbc:h2:C:/myDB", "sa", "sa");

        return new ClosableResultSet(conn, sql) {
            @Override
            public void handleError(SQLException e, String action) {
                handler.accept(e, action);
            }
        };
    }
}

And then your method2 should be:

static void method2() {

    String sql = "select * from Customers";

    try (ClosableResultSet rs = Factory.createResultSet(
            sql,
            (e, action) -> System.out.print(action + ": " + e.getMessage())
    )) {
        while (rs.next()) {
            System.out.print("Name: " + rs.getString("name") + "\n");
        }

    } catch (SQLException e) {
        e.printStackTrace();
    }

}