オブジェクトの値の書き換えで副作用が発生するのを観察する

DangerousCopyTest.java

package com.kamatama41.sandbox4j;

import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;

import org.junit.Test;

public class DangerousCopyTest {

    @Test
    public void test_オブジェクトの値の書き換えで副作用が発生する() throws Exception {
        String str = "e1";
        Value v = new Value("v1");
        int integer = 1;
        
        DangerousEntity e1 = new DangerousEntity(str, v, integer);
        DangerousEntity e2 = new DangerousEntity(str, v, integer);
        DangerousEntity e3 = e1.clone();
        e1.str = "e3";
        e1.value.str = "v3";
        e1.integer = 3;

        assertThat(e1.str, is("e3"));
        assertThat(e1.integer, is(3));
        assertThat(e1.value.str, is("v3"));

        assertThat(e2.str, is("e1"));
        assertThat(e2.integer, is(1));
        assertThat(e2.value.str, is("v3")); // 参照元が書き換えられている
        
        assertThat(e3.str, is("e1"));
        assertThat(e3.integer, is(1));
        assertThat(e3.value.str, is("v3")); // 参照元が書き換えられている
    }

    private static class DangerousEntity implements Cloneable {
        public DangerousEntity(String str, Value value, int integer) {
            this.str = str;
            this.value = value;
            this.integer = integer;
        }
        String str;
        Value value;
        int integer;
        
        @Override
        public DangerousEntity clone() {
            try {
                return (DangerousEntity)super.clone();
            } catch (CloneNotSupportedException e) {
                throw new AssertionError();
            }
        }
        
    }

    private static class Value {
        String str;
        public Value(String str) {
            this.str = str;
        }
    }
}