HashMap不起作用

Question

Nothing happens when I use the method "frageStellen" - why? I want my computer to return the answer from the HashMap "ratschlagMap" which fits to "frage".

import java.util.HashMap;

public class Kapitän
{
   private HashMap<String, String> ratschlagMap;

public Kapitän()
{
 ratschlagMap = new HashMap<String, String>();
 antwortenGenerieren();  
}

public void antwortenGenerieren()
{
    ratschlagMap.put("hallo", "Guten Tag, mein Herr!");
    ratschlagMap.put("wetter", "Es riecht nach Sturm!");
    ratschlagMap.put("liebe", "Die Liebe ist wie die See: erst stürmisch, dann flaut sie ab...");
    ratschlagMap.put("zukunft", "Erst die Segel setzten, dann übers Ziel nachdenken!");
}

public void frageStellen (String frage)
{
    frage.toLowerCase();

    if (ratschlagMap.containsKey(frage))
    {
       ratschlagMap.get(frage);
    }
}
}

Thanks!!

Answer 1

If you are using hashmap.get you probably want to return this value. Also you don't need to check if map contains this element - it will return null if no value will be found. Take a look at this code:

public String frageStellen (String frage)
{ 
    return ratschlagMap.get(frage.toLowerCase());
}

You also probably should read more about Java language...

Answer 2

There are two problems with this code. The first one iswith this line:

 frage.toLowerCase();

This method returns a string, it doesn't modify the string in place (since String objects in Java are immutable). You can just use frage.toLowerCase() as the key.

Then you have this line:

ratschlagMap.get(frage);

This method returns the value for the key frage . But you don't do anything with this value, not even assign it to a variable. This is a bug.

Answer 3

I could be wrong but in youre method "frageStellen" you only do ratschlagMap.get(frage); but you dont save it into a variable or return it.

Answer 4

There's a couple of issues.

First, String is immutable - any operations on it will create a new String instance. If you don't want it around, then you could only really create it on the fly, where you need it:

 if (ratschlagMap.containsKey(frage.toLowerCase()))

Second, you're not returning anything - you don't declare your method to do so. The thing about a Map#get() is that it will return null if the value doesn't exist at the key passed in, which seems reasonable in your case.

That means it'd be more straightforward to write this, since we don't need to really check for existence:

public String frageStellen(String frage) {
    return ratschlagMap.get(frage.toLowerCase());
}