使用Java匿名函数返回值

Question

Transitioning from C# to Java and attempting to clean up a bunch of Connection leaks in some code.

To prevent leaks in I would have done something like

public class DB{
    public interface StatementUser{
         public void useit(Statement cmd);
    }

    public static void UseCommand(StatementUser usingfunc){
        try(Connection cnn = new Connection(...)){
            cnn.open();
            try(Statement stmt = new Statement(cnn)){
                usingfunc(stmt);
            }
        }
    }

    static void main(string[] args){
        int affected = 0;
        DB.useStatement((stmt) -> {
            // THIS STATEMENT IS DISALLOWED
            affected = ... select/update... whatever
        });
        System.out.println("Records: " + affected);
    }
}

I like this delegate because it handles the cleanup but still leaves most of the interaction with the data to the developer's creativity.

I get that the assignment to affected is considered accessing a variable outside of its scope, and not allowed. So now I'm kind of at a loss as to how to perform a similar operation in Java (I want to maintain the generic usage of the connection/statement object)

Everything I've come up with just makes it uglier, so I suspect I'm simply heading down a completely dead-end path.

What is a Java way of doing something like this? (I get that this may look drastically different than I'm expecting)

Answer 1

Another way to write this code is like this:

package controllers;

import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;

public class DB {

    public interface StatementUser<T> {
        T run(Statement cmd);
    }

    public static <T> T runStatement(StatementUser<T> usingfunc) {
        try(Connection cnn = getConnection()){
            try(Statement stmt = cnn.createStatement()){
                return usingfunc.run(stmt);
            }
        } catch(SQLException ex) {
            throw new RuntimeException(ex);
        }
    }

    private static Connection getConnection() throws SQLException {
        return ...; // someway to acquire a connection
    }

    public static void main(String[] args) {
        int affected = DB.runStatement(cmd -> {
            // do something with the statement
            return 10;
        });
        System.out.println(affected);
    }
}