多个if语句的设计方法？

Question

I have method which returns sql query as string. This method consider parameter 'level' , based on this parameter there are multiple if else statement like

   if(level.equals("A1")){
    // add some  field in query
   }
   else if (level.equals("A2"))
   {
    // add some  field  and logic in query
   }
   ..

   so on 

In future number of levels are going to increase and I don`t want to write pesky if else statements so I am looking for cleaner and maintainable design approach for this scenario. I am thinking of strategy design pattern but not sure whether it is best for this scenario.

Answer 1

You have several options:

1. if / else if as you've indicated.

2. A switch statement .

3. A Map with the keys being the level string and the values being references of a functional interface type ( Runnable or an appropriate interface from java.util.function , or your own) you can initialize with method references , lambdas , anonymous class instances, or even concrete class instances as required.

Here's an example of that last option, using the most basic functional interface, Runnable , with method references:

import java.util.Map;
import java.util.HashMap;

public class Example {
    private Map<String, Runnable> handlers;

    public Example() {
        this.handlers = new HashMap<String, Runnable>();
        this.handlers.put("A1", this::a1action);
        this.handlers.put("A2", this::a2action);
    }

    private void a1action() {
        System.out.println("Action A1");
    }

    private void a2action() {
        System.out.println("Action A2");
    }

    public void handleAction(String action) {
        Runnable handler = this.handlers.get(action);
        if (handler == null) {
            System.out.println("No handler for '" + action + "'");
        } else {
            handler.run();
        }
    }

    public static void main(String[] args) throws Exception {
        Example ex = new Example();
        ex.handleAction("A1");
        ex.handleAction("A2");
        ex.handleAction("A3");
    }
}

Answer 2

You can use an enum to list the operations and use valueOf to work out which to use.

enum Levels {
    A1 {
        @Override
        void doSomethingToTheQuery(Query q) {
            // add some  field in query
        }
    },
    A2{
        @Override
        void doSomethingToTheQuery(Query q) {
            // add some  field  and logic in query
        }
    };

    abstract void doSomethingToTheQuery(Query q);
}

public void test() {
    // Lookup the level and do hack the query.
    Levels.valueOf("A1").doSomethingToTheQuery(q);
}

You can then add new levels just by adding to the enum .