Simple thread safety testing (think from https://www.ibm.com/developerworks/cn/java/j-jtp06197.html)

NumberRange.java

package org.jonas.test;

public class NumberRange {
    private int lower = 1, upper = 50;

    public int getLower() { return lower; }
    public int getUpper() { return upper; }

    public void setLower(int value) {
        if (value > upper)
            throw new IllegalArgumentException("too large");
        lower = value;
    }

    public void setUpper(int value) {
        if (value < lower)
            throw new IllegalArgumentException("too small");
        upper = value;
    }
}

NumberRangeTest.java

package org.jonas.test;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class NumberRangeTest {

    public static void main(String[] args) throws InterruptedException {
        NumberRange numberRange = new NumberRange();

        ScheduledExecutorService upperSetter = Executors.newSingleThreadScheduledExecutor();
        upperSetter.schedule(() -> numberRange.setUpper(25), 5, TimeUnit.SECONDS);

        ScheduledExecutorService lowerSetter = Executors.newSingleThreadScheduledExecutor();
        lowerSetter.schedule(() -> numberRange.setLower(25), 5, TimeUnit.SECONDS);

        upperSetter.shutdown();
        lowerSetter.shutdown();

        Thread.sleep(6000);

        System.out.println(numberRange.getUpper());
        System.out.println(numberRange.getLower());
    }
}