CrateSettingsTest.java

package io.crate.metadata.settings;

import com.google.common.collect.Sets;
import org.junit.Test;

import static org.junit.Assert.*;

public class CrateSettingsTest {

    @Test
    public void testStringSettingsValidation() throws Exception {
        StringSetting stringSetting = new StringSetting(
                Sets.newHashSet("foo", "bar", "foobar")
        ) {
            @Override
            public String name() { return "foo_bar_setting"; }

            @Override
            public String defaultValue() { return "foo"; }

        };

        String validation = stringSetting.validate("foo");
        assertEquals(validation, null);
        validation = stringSetting.validate("unknown");
        assertEquals(validation, "'unknown' is not an allowed value. Allowed values are: bar, foo, foobar");
    }
    
    // ... more tests here ...
}

StringSetting.java

package io.crate.metadata.settings;

import com.google.common.base.Joiner;
import org.elasticsearch.common.Nullable;
import org.elasticsearch.common.settings.Settings;

import java.util.Set;

public abstract class StringSetting extends Setting<String> {

    protected Set<String> allowedValues;

    protected StringSetting(Set<String> allowedValues) {
        this.allowedValues = allowedValues;
    }

    @Override
    public String defaultValue() {
        return "";
    }

    @Override
    public String extract(Settings settings) {
        return settings.get(settingName(), defaultValue());
    }

    /**
     * @return Error message if not valid, else null.
     */
    @Nullable
    public String validate(String value) {
        if (allowedValues != null && !allowedValues.contains(value)) {
            return String.format("'%s' is not an allowed value. Allowed values are: %s",
                    value, Joiner.on(", ").join(allowedValues)

            );
        }
        return null;
    }
}