Thou shalt not introduce side-effects in Java toString()

TestSideEffectInToString.java

import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class TestSideEffectInToString {
	private final class ToStringWithSideEffect {
		private int value = 0;

		public ToStringWithSideEffect() {
			System.out.println("ToStringWithSideEffect() called");
		}

		public ToStringWithSideEffect(int v) {
			value = v;
			System.out.println(String.format("ToStringWithSideEffect(%d) called", v));
		}

		@Override
		public String toString() {
			System.out.println("ToStringWithSideEffect() called");
		    return String.format("ToStringWithSideEffect(v=%d)", ++value);
		}

		public void setValue(int v) {
			value = v;
		}

		public int getValue() { return value; }
	}

	/*
	 * To check the effect of "side effect in toString()",
	 * first you run this test using normal "run" command.
	 * and then, you need to run this test using "debug" command, 
	 * during which you need to watch the value of v1 and v2.
         */
	@Test
	public void testSideEffect() {
		ToStringWithSideEffect v1 = new ToStringWithSideEffect();
		ToStringWithSideEffect v2 = new ToStringWithSideEffect(10);

		v1.setValue(10);

		assertEquals(v1.getValue(), v2.getValue());
	}
}