Java 匿名内部 class 声明

Question

如何与构造函数块分开定义匿名内部class？

例如，在我的代码中，我希望方法 2 做与方法 1 相同的事情，唯一的区别是方法 2 使用工厂 class 创建 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   
    }

}

我不想在工厂中声明匿名内部 class class，是否有任何 lambda 表达式允许我将它放在 method2 中？

Answer 1

是的，你可以这样做：

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);
    }
}

...

如您所见，添加了新界面 ErrorHandller。另外 Factory::createResultSet 现在接收 ErrorHandller 的实例作为第二个参数。然后在 Demo::method2 中，我们将 lambda 作为 Factory.createResultSet 方法的第二个参数传递。

Answer 2

您可以创建另一个 Factory 方法，除了 String sql 之外还采用 BiConsumer。像这样：

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);
            }
        };
    }
}

然后你的 method2 应该是：

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();
    }

}

Java 匿名内部 class 声明

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