GradeConverter.java

package com.example;

import java.util.LinkedHashMap;
import java.util.Map;

public class GradeConverter {

    public String fromScore(Integer score) {

        Map<Integer, String> rules = getConversionRules();

        String grade = "F";

        for( Map.Entry<Integer, String> entry: rules.entrySet()) {
            if ( score >= entry.getKey()){
                grade = entry.getValue();
                break;
            }
        }
        return grade;
    }

    private Map<Integer, String> getConversionRules() {
        Map<Integer, String> rules = new LinkedHashMap<>();

        rules.put(80, "A");
        rules.put(70, "B");
        rules.put(60, "C");
        rules.put(50, "D");

        return rules;
    }
}

GradeConverterTest2nd.java

import com.example.GradeConverter;

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;

public class GradeConverterTest {

    private GradeConverter converter;

    @Before
    public void setUp() throws Exception {
        converter = new GradeConverter();
    }

    @Test
    public void fromScore_given_90_shouldReturn_A() {
        Assert.assertEquals(converter.fromScore(90), "A");
    }

    @Test
    public void fromScore_given_0_shouldReturn_F() {

        Assert.assertEquals(converter.fromScore(0), "F");
    }

    @Test
    public void fromScore_given_50_shouldReturn_D() {
        Assert.assertEquals(converter.fromScore(50), "D");
    }

    @Test
    public void fromScore_given_60_shouldReturn_C() {
        Assert.assertEquals(converter.fromScore(60), "C");
    }

    @Test
    public void fromScore_given_70_shouldReturn_B() {
        Assert.assertEquals(converter.fromScore(70), "B");
    }

    @Test
    public void fromScore_given_80_shouldReturn_A() {
        Assert.assertEquals(converter.fromScore(80), "A");
    }
}