为什么eclipse会让我的return语句出错？

Question

I am trying to make the method selectVMwareServer return a VMwareServer object when the key is is present in the HashMap. But eclipse does not like the where I am placing my return statement. Why is this? Is it because the method HAS TO return an object?

Here is my code;

package configFS;

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

public class Config {

    private Map <String, Object> config;

    public Config() {

        /*
         * This will create a new configuration everytime we create a new instance of Config.class
         * By using the generics of String and Object I can link a name tag to any object needed.
         */
        config = new HashMap <String, Object> ();

    }

    public void addVMwareServer(String par1) {

        config.put(par1, new VMwareServer());

    }

    public void removeVMwareServer(String par1) {

        config.remove(par1);

    }

    public VMwareServer selectVMwareServer(String par1) {

        if (config.containsKey(par1)) {

            return (VMwareServer) config.get(par1);

        }

        return null;

    }

}

NOTE: This is very much a work in progress!

To silence eclipse I let it add the "return null;"

Answer 1

A method should always have a return statement if it has a return type. According to your code, if the "if" condition is not satisfied then it will never get into that condition and that return statement will never get executed.

That is why you have to have return statement outside the "if" condition.

Answer 2

You specify what to return in case your if statement (contains the key). Think about what to do in case when your if statement does not hold true and is ignored. The method has to return something, that is where your "Null" return statement comes in handy. for example:

public boolean selectVMwareServer(String par1) {

    if (config.containsKey(par1)) {

        return true; //(VMwareServer) config.get(par1);

    }

    return false; // If I dont have this in place, Java has no idea what to do hence it HAS to return something.

}

Better solution may be along the lines of:

create new field say X,

public void selectVMwareServer(String par1) {

    if (config.containsKey(par1)) {

        X =  (VMwareServer) config.get(par1);

    }
    else//do nothing, 


}

After this you can use X however you need to solve your problem. Hope its somewhat clear..

Answer 3

Is it because the method HAS TO return an object?

Basically, yes.

Or more precisely, every "normal termination" path for the program has to return a value that can be assigned to the method's return type.

---

Note that it is actually possible to implement a non-void method without a return statement at all; eg

    public int test() {
        throw new SomeUncheckedException(...);
    }

... but this is not generally useful. The compiler's path analysis code can figure out that the test method cannot go past the throw statement ... and hence that a return statement following it is not necessary. (Indeed it would be a compilation error to add one.)