Replacing if else statement with pattern

Let's assume that we have such code (which is the same as yours):

    public void decide(String someCondition) {
        if(someCondition.equals("conditionOne")) {
            someMethod("someParameter");
        }
        else if(someCondition.equals("conditionTwo")) {
            someMethod("anotherParameter");
        }
        else {
            someMethod("elseParameter");
        }
    }

Assuming that you don't want to refactor other parts of the application and you don't want to change method signature there are possible ways in which it could be refactored:

Warning - You should use generic versions of mentioned patterns.
I showed non generic ones because it is easier to read them.

Strategy + Factory Method
We can use Strategy and Factory Method patterns. We also take advantage of polymorphism.

  private final StrategyConditionFactory strategyConditionFactory = new StrategyConditionFactory();

    public void decide(String someCondition) {
        Strategy strategy = strategyConditionFactory.getStrategy(someCondition)
                .orElseThrow(() -> new IllegalArgumentException("Wrong condition"));
        strategy.apply();
    }

It would be better to design it in a way that else condition is included in the factory, and developer calls it on purpose. In such case we throw exception when condition is not meet. Alternatively we could write it exactly as it was in question. If you want so instead of .orElseThrow(() -> new IllegalArgumentException("Wrong condition")); put .orElse(new ElseStrategy());

StrategyConditionFactory (factory method):

    public class StrategyConditionFactory {
        private Map<String, Strategy> conditions = new HashMap<>();
    
        public StrategyConditionFactory() {
            conditions.put("conditionOne", new ConditionOneStrategy());
            conditions.put("conditionTwo", new ConditionTwoStrategy());
            //It is better to call else condition on purpose than to have it in the conditional method
            conditions.put("conditionElse", new ElseStrategy());
            //...
        }
    
        public Optional<Strategy> getStrategy(String condition) {
            return Optional.ofNullable(conditions.get(condition));
        }
    }

Strategy interface:

public interface Strategy {
    void apply();
}

Implementations:

    public class ConditionOneStrategy implements Strategy {
        @Override
        public void apply() {
            //someMethod("someParameter");
        }
    }  

    public class ConditionTwoStrategy implements Strategy {
        @Override
        public void apply() {
            //someMethod("anotherParameter")
        }
    }

    public class ElseStrategy implements Strategy {
        @Override
        public void apply() {
            //someMethod("elseParameter")
        }
    }

Usage (simplified):

    public void strategyFactoryApp() {
        //...
        decide("conditionOne");
        decide("conditionTwo");
        decide("conditionElse");
        //...
    }

Strategy + Factory Method - this particular case (where only parameter changes)
We can use the fact that in this case we always call the same method, only parameter changes
We change our base strategy interface to abstract class with getParameter() method and we make new implementations of this abstract class. Other code remains the same.

public abstract class Strategy {
    public abstract String getParameter();

    public void apply() {
        someMethod(getParameter());
    }

    private void someMethod(String parameter) {
        //someAction
    }
}

Implementations:

public class CondtionOneStrategy extends Strategy {
    @Override
    public String getParameter() {
        return "someParameter";
    }
}

public class CondtionTwoStrategy extends Strategy {
    @Override
    public String getParameter() {
        return "anotherParameter";
    }
}

public class ElseStrategy extends Strategy {
    @Override
    public String getParameter() {
        return "elseParameter";
    }
}

Enum + enum kinda "factory"
We might use Enum to implement strategy and instead of factory method we can use valueOf() from enum.

    public void decide(String someCondition) {
            ConditionEnum conditionEnum = ConditionEnum.valueOf(someCondition);
            conditionEnum.apply();
        }

Condition enum:

public enum ConditionEnum {
    CONDITION_ONE {
        @Override
        public void apply() {
            //someMethod("someParameter");
        }
    },
    CONDITION_TWO {
        @Override
        public void apply() {
            //someMethod("anotherParameter");
        }
    },
    CONDITION_ELSE {
        @Override
        public void apply() {
            //someMethod("elseParameter");
        }
    };
    //...more conditions

    public abstract void apply();
}

Usage (simplified):

    public void enumFactoryApp() {
        //...
        decide("CONDITION_ONE");
        decide("CONDITION_TWO");
        decide("CONDITION_ELSE");
        //...
    }

Notice that you will get IllegalArgumentException when enum type has no constant with the specified name.

Command + Factory
The difference between strategy and command is that command holds also state, so if you have for example compute(int a, int b, String someCondition) and you want to refactor it with strategy including it's signature change you can reduce it to compute(int a, int b, ComputeStrategy computeStrategy) with command you can reduce it to one argument compute(ComputeCommand computeCommand). In this case we also take advantage of polymorphism similarly to strategy pattern case.

    CommandConditionFactory commandConditionFactory = new CommandConditionFactory();

    public void decide(String someCondition) {
        Command command = commandConditionFactory.getCommand(someCondition)
                .orElseThrow(() -> new IllegalArgumentException("Wrong condition"));
        command.apply();
    }

It would be better to design it in a way that else condition is included in the factory, and developer calls it on purpose. In such case we throw exception when condition is not meet. Alternatively we could write it exactly as it was in question. If you want so instead of .orElseThrow(() -> new IllegalArgumentException("Wrong condition")); put .orElse(new ElseCommand());

CommandConditionFactory (factory method):

public class CommandConditionFactory {
    private Map<String, Command> conditions = new HashMap<>();

    public CommandConditionFactory() {
        conditions.put("conditionOne", new ConditionOneCommand("someParameter"));
        conditions.put("conditionTwo", new ConditionTwoCommand("anotherParameter"));
        //It is better to call else condition on purpose than to have it in the conditional method
        conditions.put("conditionElse", new ElseCommand("elseParameter"));
        //...
    }

    public Optional<Command> getCommand(String condition) {
        return Optional.ofNullable(conditions.get(condition));
    }
}

Command interface:

public interface Command {
    void apply();
}

Implementations (there is some redundancy, but It is there to show how command should look in more general case where instead of someMethod() we have three different methods):

public class ConditionOneCommand implements Command {
    private final String parameter;

    public ConditionOneCommand(String parameter) {
        this.parameter = parameter;
    }

    @Override
    public void apply() {
        //someMethod(parameter);
    }
}

public class ConditionTwoCommand implements Command {
    private final String parameter;

    public ConditionTwoCommand(String parameter) {
        this.parameter = parameter;
    }

    @Override
    public void apply() {
        //someMethod(parameter);
    }
}

public class ElseCommand implements Command {
    private final String parameter;

    public ElseCommand(String parameter) {
        this.parameter = parameter;
    }

    @Override
    public void apply() {
        //someMethod(parameter);
    }
}

Usage (simplified):

    public void commandFactoryApp() {
        //...
        decide("conditionOne");
        decide("conditionTwo");
        decide("conditionElse");
        //...
    }

Command + Factory - This particular case.
This in fact isn't a real command pattern just a derivative. It takes advantage of the fact that in this case we are always calling the same method someMethod(parameter) and only the parameter changes.
Abstract class:

public abstract class Command {
    abstract void apply();

    protected void someMethod(String parameter) {
        //someAction
    }
}

Implementation (the same for all 3 conditional cases):

public class CommandImpl extends Command {
    private final String parameter;

    public CommandImpl (String parameter) {
        this.parameter = parameter;
    }

    @Override
    public void apply(){
        someMethod(parameter);
    }
}

Factory, please notice that there is only one command implementation, only parameter changes:

public class CommandConditionFactory {
    Map<String, Command> conditions = new HashMap<>();

    public CommandConditionFactory() {
        conditions.put("conditionOne", new CommandImpl("someParameter"));
        conditions.put("conditionTwo", new CommandImpl("anotherParameter"));
        //It is better to call else condition on purpose than to have it in the conditional method
        conditions.put("conditionElse", new CommandImpl("elseParameter"));
        //...
    }

    public Optional<Command> getCommand(String condition) {
        return Optional.ofNullable(conditions.get(condition));
    }
}

Nested if's
Note that even if you have nested ifs sometimes it is possible to refactor them and use one of the mentioned techniques. Lets say that we have following code:

    public void decide2(String someCondition, String nestedCondition) {
        if(someCondition.equals("conditionOne")) {
            if(nestedCondition.equals("nestedConditionOne")){
                someLogic1();
            }
            else if(nestedCondition.equals("nestedConditionTwo")){
                someLogic2();
            }
        }
        else if(someCondition.equals("conditionTwo")) {
            if(nestedCondition.equals("nestedConditionThree")){
                someLogic3();
            }
            else if(nestedCondition.equals("nestedConditionFour")){
                someLogic4();
            }
        }
    }

You could refactor it using mathematical logic rules:

    public void decide2(String someCondition, String nestedCondition) {
        if(someCondition.equals("conditionOne")
                && nestedCondition.equals("nestedConditionOne")) {
            someLogic1();
        }
        else if(someCondition.equals("conditionOne")
                && nestedCondition.equals("nestedConditionTwo")) {
            someLogic2();
        }
        else if(someCondition.equals("conditionTwo")
                && nestedCondition.equals("nestedConditionThree")) {
            someLogic3();
        }
        else if(someCondition.equals("conditionTwo")
                && nestedCondition.equals("nestedConditionFour")) {
            someLogic4();
        }
    }

and then you can use strategy, enum or command. You just have a pair of Strings <String, String> instead of single String.

Decision Tables
When you have nested ifs that couldn't be refactored as mentioned you can implement your own decision tables or use some ready to go decision tables solution. I won't give the implementation there.

Rules Engine
When you have nested ifs that couldn't be refactored as mentioned you can also implement your own simple rules engine. You should use it only if you have many nested ifs, otherwise it is triumph of form over content.
For very complicated Business Logic there are professional Rule Engines like Drools.
I won't give the implementation there.

One more thing
In the example that you gave there is a high possibility that someone introduced these ifs, but they are totally redundant. And we can check it by trying to refactor decide method signature to make it take some other argument and to refactor surrounding code that is calling our method. By doing so we are getting rid of our Factory Method. There are examples that present how the code might look when it occurs that these ifs were redundant.

Strategy
Decide method:

    public void decide(Strategy strategy) {
        strategy.apply();
    }

Usage (simplified):

    public void strategyApp() {
        //...
        decide(new ConditionOneStrategy());
        decide(new ConditionTwoStrategy());
        decide(new ElseStrategy());
        //...
    }

Enum
Decide method:

    public void decide(ConditionEnum conditionEnum) {
        conditionEnum.apply();
    }

Usage (simplified):

    public void enumApp() {
        //...
        decide(ConditionEnum.CONDITION_ONE);
        decide(ConditionEnum.CONDITION_TWO);
        decide(ConditionEnum.CONDITION_ELSE);
        //...
    }

Command
Decide method:

    public void decide(Command command) {
        command.apply();
    }

Usage (simplified):

    public void commandApp() {
        //...
        decide(new ConditionOneCommand("someParameter"));
        decide(new ConditionTwoCommand("anotherParameter"));
        decide(new ElseCommand("elseParameter"));
        //...
    }

In fact it is quite specific case, there are cases in which for example we have to use simple type like String, because it comes from the external system or condition is based on integer from input so we can't refactor the code so easily.

The general recommendation by Martin Fowler is to Replace Conditional with Polymorphism.

In terms of design patterns this would often be the Strategy Pattern Replace Conditional Logic with Strategy.

If you have a small, finite set of conditions, I recommend to use an enum to implement the Strategy Pattern (provide an abstract method in the enum and override it for each constant).

public enum SomeCondition{
   CONDITION_ONE{

       public void someMethod(MyClass myClass){
              //...
       }
   },

   CONDITION_TWO{

       public void someMethod(MyClass myClass){
       }

   }

   public abstract void someMethod(MyClass myClass);

}

public class MyClass{
//...
    public void decide(SomeCondition someCondition){
        someCondition.someMethod(this);
    }

}

If it's really just a parameter you want to pick, then you could define the enum like this instead:

public enum SomeCondition{
   CONDITION_ONE("parameterOne"),

   CONDITION_TWO("parameterTwo");

   private final String parameter;

   private SomeCondition(String parameter){
       this.parameter = parameter;
   }

   public String getParameter(){
       return parameter;
   }

}


public class MyClass{
//...
    public void decide(SomeCondition someCondition){
        someMethod(someCondition.getParameter());
    }

}

This is a classic Replace Condition dispatcher with Command in the Refactoring to Patterns book.

Basically you make a Command object for each of the blocks of code in your old if/else group and then make a Map of those commands where the keys are your condition Strings

interface Handler{
    void handle( myObject o);
}


 Map<String, Handler> commandMap = new HashMap<>();
 //feel free to factor these out to their own class or
 //if using Java 8 use the new Lambda syntax
 commandMap.put("conditionOne", new Handler(){
         void handle(MyObject o){
                //get desired parameters from MyObject and do stuff
          }
 });
 ...

Then instead of your if/else code it is instead:

 commandMap.get(someCondition).handle(this);

Now if you need to later add new commands, you just add to the hash.

If you want to handle a default case, you can use the Null Object pattern to handle the case where a condition isn't in the Map.

 Handler defaultHandler = ...

if(commandMap.containsKey(someCondition)){
    commandMap.get(someCondition).handle(this);
}else{
    defaultHandler.handle(this);
}

I guess you must have already considered it, but if you are using JDK 7 or above, you can switch on strings. That way your code can look cleaner than a bunch of if-else statements.